Arduino Tutorials – Chapter 22 – the AREF pin

Learn how to measure smaller voltages with greater accuracy using your Arduino.

This is chapter twenty-two of our huge Arduino tutorial seriesUpdated 12/12/2013

In this chapter we’ll look at how you can measure smaller voltages with greater accuracy using the analogue input pins on your Arduino or compatible board in conjunction with the AREF pin. However first we’ll do some revision to get you up to speed. Please read this post entirely before working with AREF the first time.

Revision

You may recall from the first few chapters in our tutorial series that we used the analogRead() function to measure the voltage of an electrical current from sensors and so on using one of the analogue input pins. The value returned from analogRead() would be between zero an 1023, with zero representing zero volts and 1023 representing the operating voltage of the Arduino board in use.

And when we say the operating voltage – this is the voltage available to the Arduino after the power supply circuitry. For example, if you have a typical Arduino Uno board and run it from the USB socket – sure, there is 5V available to the board from the USB socket on your computer or hub – but the voltage is reduced slightly as the current winds around the circuit to the microcontroller – or the USB source just isn’t up to scratch.

This can easily be demonstrated by connecting an Arduino Uno to USB and putting a multimeter set to measure voltage across the 5V and GND pins. Some boards will return as low as 4.8 V, some higher but still below 5V. So if you’re gunning for accuracy, power your board from an external power supply via the DC socket or Vin pin – such as 9V DC. Then after that goes through the power regulator circuit you’ll have a nice 5V, for example:

This is important as the accuracy of any analogRead() values will be affected by not having a true 5 V. If you don’t have any option, you can use some maths in your sketch to compensate for the drop in voltage. For example, if your voltage is 4.8V – the analogRead() range of 0~1023 will relate to 0~4.8V and not 0~5V. This may sound trivial, however if you’re using a sensor that returns a value as a voltage (e.g. the TMP36 temperature sensor) – the calculated value will be wrong. So in the interests of accuracy, use an external power supply.

Why does analogRead() return a value between 0 and 1023?

This is due to the resolution of the ADC. The resolution (for this article) is the degree to which something can be represented numerically. The higher the resolution, the greater accuracy with which something can be represented. We measure resolution in the terms of the number of bits of resolution.

For example, a 1-bit resolution would only allow two (two to the power of one) values – zero and one. A 2-bit resolution would allow four (two to the power of two) values – zero, one, two and three. If we tried to measure  a five volt range with a two-bit resolution, and the measured voltage was four volts, our ADC would return a numerical value of 3 – as four volts falls between 3.75 and 5V. It is easier to imagine this with the following image:

So with our example ADC with 2-bit resolution, it can only represent the voltage with four possible resulting values. If the input voltage falls between 0 and 1.25, the ADC returns numerical 0; if the voltage falls between 1.25 and 2.5, the ADC returns a numerical value of 1. And so on. With our Arduino’s ADC range of 0~1023 – we have 1024 possible values – or 2 to the power of 10. So our Arduinos have an ADC with a 10-bit resolution.

So what is AREF?

To cut a long story short, when your Arduino takes an analogue reading, it compares the voltage measured at the analogue pin being used against what is known as the reference voltage. In normal analogRead use, the reference voltage is the operating voltage of the board. For the more popular Arduino boards such as the Uno, Mega, Duemilanove and Leonardo/Yún boards, the operating voltage of 5V. If you have an Arduino Due board, the operating voltage is 3.3V. If you have something else – check the Arduino product page or ask your board supplier.

So if you have a reference voltage of 5V, each unit returned by analogRead() is valued at 0.00488 V. (This is calculated by dividing 1024 into 5V). What if we want to measure voltages between 0 and 2, or 0 and 4.6? How would the ADC know what is 100% of our voltage range?

And therein lies the reason for the AREF pin. AREF means Analogue REFerence. It allows us to feed the Arduino a reference voltage from an external power supply. For example, if we want to measure voltages with a maximum range of 3.3V, we would feed a nice smooth 3.3V into the AREF pin – perhaps from a voltage regulator IC. Then the each step of the ADC would represent around 3.22 millivolts (divide 1024 into 3.3).

Note that the lowest reference voltage you can have is 1.1V. There are two forms of AREF – internal and external, so let’s check them out.

External AREF

An external AREF is where you supply an external reference voltage to the Arduino board. This can come from a regulated power supply, or if you need 3.3V you can get it from the Arduino’s 3.3V pin. If you are using an external power supply, be sure to connect the GND to the Arduino’s GND pin. Or if you’re using the Arduno’s 3.3V source – just run a jumper from the 3.3V pin to the AREF pin.

To activate the external AREF, use the following in void setup():

This sets the reference voltage to whatever you have connected to the AREF pin – which of course will have a voltage between 1.1V and the board’s operation voltage.

Very important note – when using an external voltage reference, you must set the analogue reference to EXTERNAL before using analogRead(). This will prevent you from shorting the active internal reference voltage and the AREF pin, which can damage the microcontroller on the board.

If necessary for your application, you can revert back to the board’s operating voltage for AREF (that is – back to normal) with the following:

Now to demonstrate external AREF at work. Using a 3.3V AREF, the following sketch measures the voltage from A0 and displays the percentage of total AREF and the calculated voltage:

The results of the sketch above are shown in the following video:

Internal AREF

The microcontrollers on our Arduino boards can also generate an internal reference voltage of 1.1V and we can use this for AREF work. Simply use the line:

For Arduino Mega boards, use:

in void setup() and you’re off. If you have an Arduino Mega there is also a 2.56V reference voltage available which is activated with:

Finally – before settling on the results from your AREF pin, always calibrate the readings against a known good multimeter.

Conclusion

The AREF function gives you more flexibility with measuring analogue signals. If you are interested in using specific ADC components, we have tutorials on the ADS1110 16-bit ADC and the NXP PCF 8591 8-bit A/D and D/A IC.

Stay tuned for upcoming Arduino tutorials by subscribing to the blog, RSS feed (top-right), twitter or joining our Google Group. And if you enjoyed the tutorial, or want to introduce someone else to the interesting world of Arduino – check out my book (now in a third printing!) “Arduino Workshop” from No Starch Press.

Kit Review – Altronics 3 Digit Counter Module

Introduction

In this review we examine the three digit counter module kit from Tronixlabs. The purpose of this kit is to allow you to … count things. You feed it a pulse, which it counts on the rising edge of the signal. You can have it count up or down, and each kit includes three digits.

You can add more digits, in groups of three with a maximum of thirty digits. Plus it’s based on simple digital electronics (no microcontrollers here) so there’s some learning afoot as well. Designed by Graham Cattley the kit was first described in the now-defunct (thanks Graham) January 1998 issue of Electronics Australia magazine.

Assembly

The kit arrives in the typical retail fashion:

And includes the magazine article reprint along with an “electronics reference sheet” which covers many useful topics such as resistor colour codes, various formulae, PCB track widths, pinouts and more. There is also a small addendum which uses two extra (and included) diodes for input protection on the clock signal:

The counter is ideally designed to be mounted inside an enclosure of your own choosing, so everything required to build a working counter is included however that’s it:

No IC sockets, however I decided to live dangerously and not use them – the ICs are common and easily found. The PCBs have a good solder mask and silk screen:

With four PCBs (one each for a digit control and one for the displays) the best way to start was to get the common parts out of the way and fitted, such as the current-limiting resistors, links, ICs, capacitors and the display module. The supplied current-limiting resistors are for use with a 9V DC supply, however details for other values are provided in the instructions:

At this point you put one of the control boards aside, and then start fitting the other two to the display board. This involves holding the two at ninety degrees then soldering the PCB pads to the SIL pins on the back of the display board. Starting with the control board for the hundreds digit first:

… at this stage you can power the board for a quick test:

… then fit the other control board for the tens digit and repeat:

Now it’s time to work with the third control board. This one looks after the one’s column and also a few features of the board. Several functions such as display blanking, latch (freeze the display while still counting) and gate (start or stop counting) can be controlled and require resistors fitted to this board which are detailed in the instructions.

Finally, several lengths of wire (included) are soldered to this board so that they can run through the other two to carry signals such as 5V, GND, latch, reset, gate and so on:

These wires can then be pulled through and soldered to the matching pads once the last board has been soldered to the display board:

You also need to run separate wires between the carry-out and clock-in pins between the digit control boards (the curved ones between the PCBs):

For real-life use you also need some robust connections for the power, clock, reset lines, etc., however for demonstration use I just used alligator clips. Once completed a quick power-up showed the LEDs all working:

How it works

Each digit is driven by a common IC pairing – the  4029 (data sheet) is a presettable up/down counter with a BCD (binary-coded decimal) output which feeds a 4511 (data sheet) that converts the BCD signal into outputs for a 7-segment LED display. You can count at any readable speed, and I threw a 2 kHz square-wave at the counter and it didn’t miss a beat. By default the units count upwards, however by setting one pin on the board LOW you can count downwards.

Operation

Using the counters is a simple matter of connecting power, the signal to count and deciding upon display blanking and the direction of counting. Here’s a quick video of counting up, and here it is counting back down.

Conclusion

This is a neat kit that can be used to count pulses from almost anything. Although some care needs to be taken when soldering, this isn’t anything that cannot be overcome without a little patience and diligence. So if you need to count something, get one or more of these kits from Tronixlabs Australia. Full-sized images are available on flickr. And while you’re here – are you interested in Arduino? Check out my book “Arduino Workshop” from No Starch Press – also available from Tronixlabs.

In the meanwhile have fun and keep checking into tronixstuff.com. Why not follow things on twitterGoogle+, subscribe  for email updates or RSS using the links on the right-hand column? And join our friendly Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other –  and we can all learn something.

Various 1 Hz Oscillator Methods

Introduction

During the fun and enjoyment of experimenting with electronics there will come a time when you need a nice 1 Hz oscillator to generate a square-wave signal to drive something in the circuit. On… off… on… off… for all sorts of things. Perhaps a metronome, to drive a TTL clock, blink some LEDs, or for more nefarious purposes. No matter what you need that magic 1 Hz for – there’s a variety of methods to generate it – some more expensive than others – and some more accurate than others.

A few of you may be thinking “pull out the Arduino” and yes, you could knock out a reasonable 1 Hz – however that’s fine for the bench, but wild overkill for embedding a project as a single purpose. So in this article we’ll run through three oscillator methods that can generate a 1 Hz signal (and other frequencies) using methods that vary in cost, accuracy and difficulty – and don’t rely on mains AC. That will be a topic for another day.

Using a 555 timer IC

You can solve this problem quite well for under a dollar with the 555, however the accuracy is going to heavily rely on having the correct values for the passive components. We’ll use the 555 in astable mode, and from a previous article here’s the circuit:

And with a 5V power supply, here’s the result:

As you can see the cycle time isn’t the best, which can be attributed to the tolerance of the resistors and capacitor C1. A method to increase the accuracy would be to add small trimpots in series with the resistors (and reduce their value accordingly by the trimpot value) – then measure the output with a frequency counter (etc). whilst adjusting the trimpots. If you’re curious about not using C2, the result of doing so introduces some noise on the rising edge, for example:

So if you’ve no other option, or have the right values for the passives – the 555 can do the job. Or get yourself a 555 and experiment with it, there’s lots of fun to be had with it.

Using a GPS receiver module

A variety of GPS modules have a one pulse per second output (PPS) and this includes my well-worn EM406A module (as used in the Arduino tutorials):

With a little work you can turn that PPS output into a usable and incredibly accurate source of 1 Hz. As long as your GPS can receive a signal. In fact, this has been demonstrated in the April 2013 edition of Silicon Chip magazine, in their frequency counter timebase project. But I digress.

If you have an EM406A you most likely have the cable and if not, get one to save your sanity as the connector is quite non-standard. If you’re experimenting a breakout board will also be quite convenient, however you can make your own by just chopping off one end of the cable and soldering the required pins – for example:

You will need access to pins 6, 5, 2 and 1. Looking at the socket on the GPS module, they are numbered 6 to 1 from left to right. Pin 6 is the PPS output, 5 is GND, 2 is for 5V and 1 is GND. Both the GNDs need to be connected together.

Before moving forward you’re probably curious about the pulse, and want to see it. Good idea! However the PPS signal is incredibly quick and has an amplitude of about 2.85 V. If you put a DSO on the PPS and GND output, you can see the pulses as shown below:

To find the length of the pulse, we had to really zoom in to a 2 uS timebase:

Wow, that’s small. So a little external circuitry is required to convert that minuscule pulse into something more useful and friendly. We’ll increase the pulse length by using a “pulse stretcher”. To do this we make a monostable timer (“one shot”) with a 555. For around a half-second pulse we’ll use 47k0 for R1 and 10uF for C1. However this triggers on a low signal, so we first pass the PPS signal through a 74HC14 Schmitt inverter – a handy part which turns irregular signals into more sharply defined ones – and also inverts it which can then be used to trigger the monostable. Our circuit:

and here’s the result – the PPS signal is shown with the matching “stretched” signal on the DSO:

So if you’re a stickley for accuracy, or just want something different for portable or battery-powered applications, using the GPS is a relatively simple solution.

Using a Maxim DS1307/DS3232 real-time clock IC

Those of you with a microcontroller bent may have a Maxim DS1307 or DS3232. Apart from being pretty easy to use as a real-time clock, both of them have a programmable square wave output. Connection via your MCU’s I2C bus is quite easy, for example with the DS1307:

Using a DS3232 is equally as simple. We use a pre-built module with a similar schematic. Once you have either of them connected, the code is quite simple. For the DS1307 (bus address 0x68), write 0x07 then 0x11 to the I2C bus – or for the DS3232 (bus address is also 0x68) write 0x0E then 0x00. Finally, let’s see the 1 Hz on the DSO:

Certainly not the cheapest method, however it gives you an excellent level of accuracy without the GPS.

Conclusion

By no means is this list exhaustive, however hopefully it was interesting and useful. If there’s any other methods you’d like to see demonstrated, leave a comment below and we’ll see what’s possible. And if you made it this far – check out my new book “Arduino Workshop” from No Starch Press.

In the meanwhile have fun and keep checking into tronixstuff.com. Why not follow things on twitterGoogle+, subscribe  for email updates or RSS using the links on the right-hand column? And join our friendly Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other –  and we can all learn something.

Australian Electronics Nostalgia – Talking Electronics Kits

Introduction

From 1981, Australian electrical engineer Colin Mitchell started publishing his home-grown electronics magazine “Talking Electronics”. His goal was to get people interested and learning about electronics, and more so with a focus on digital electronics. It was (and still is) a lofty goal – in which he succeeded. From a couple of rooms in his home the magazine flourished, and many projects described within were sold as kits. At one stage there were over 150 Talking Electronics kits on the market. You could find the books and kits in retail outlets such as Dick Smith Electronics, and for a short while there was a TE store in Moorabbin (Victoria). Colin and the team’s style of writing was easy to read and very understandable – but don’t take my word for it, you can download the magazines from his website (they’re near the bottom of the left column). Dave Jones recently interviewed Colin, and you can watch those for much more background information.

Over fifteen issues you could learn about blinking LEDs all the way to making your own expandable Z80 board computer, and some of the kits may still be available. Colin also published a series of tutorial books on electronics, and also single-magazine projects. And thus the subjects of our review … we came across the first of these single-issue projects from 1981 – the Mini Frequency Counter (then afterwards we have another kit):

How great is that? The PCB comes with the magazine. This is what set TE apart from the rest, and helped people learn by actually making it easy to build what was described in the magazine instead of just reading about it. For 1981 the PCB was quite good – they were silk-screened which was quite rare at the time:

And if you weren’t quite ready, the magazine also included details of a square-wave oscillator to make and a 52-page short course in digital electronics. However back to the kit…

Assembly

The kit uses common parts and I hoard CMOS ICs so building wasn’t a problem. This (original) version of the kit used LEDs instead of 7-segment displays (which were expensive at the time) so there was plenty of  careful soldering to do:

And after a while the counter started to come together. I used IC sockets just in case:

The rest was straight-forward, and before long 9 V was supplied, and we found success:

To be honest progress floundered for about an hour at this point – the display wouldn’t budge off zero. After checking the multi-vibrator output, calibrating the RC circuits and finally tracing out the circuit with a continuity tester, it turned out one of the links just wasn’t soldered in far enough – and the IC socket for the 4047 was broken So a new link and directly fitting the 4047 fixed it. You live and learn.

Operation

So – we now have a frequency counter that’s good for 100 Hz to the megahertz range, with a minimum of parts. Younger, non-microcontroller people may wonder how that is possible – so here’s the schematic:

The counter works by using a multi-vibrator using a CD4047 to generate a square-wave at 50, 500 and 5 kHz, and the three trimpots are adjusted to calibrate the output. The incoming pulses to measure are fed to the 4026 decade counter/divider ICs. Three of these operate in tandem and each divide the incoming count by ten – and display or reset by the alternating signal from the 4047. However for larger frequencies (above 900 Hz) you need to change the frequency fed to the display circuit in order to display the higher (left-most) digits of the result. A jumper wire is used to select the required level (however if you mounted the kit in a case, a knob or switch could be used).

For example, if you’re measuring 3.456 MHz you start with the jumper on H and the display reads 345 – then you switch to M to read 456 – then you switch to the L jumper and read 560, giving you 3456000 Hz. If desired, you can extend the kit with another PCB to create a 5-digit display. The counter won’t be winning any precision contests – however it has two purposes, which are fulfilled very well. It gives the reader an inexpensive piece of test equipment that works reasonably well, and a fully-documented project so the reader can understand how it works (and more).

And for the curious –  here it is in action:

[Update 20/07/2013] Siren Kit

Found another kit last week, the Talking Electronics “DIY Kit #31 – 9V siren”. It’s an effective and loud siren with true rise and fall, unlike other kits of the era that alternated between two fixed tones. The packaging was quite strong and idea for mail-order at the time:

The label sells the product (and shows the age):

The kit included every part required to work, apart from a PP3 battery, and a single instruction sheet with a good explanation of how the circuit works, and some data about the LM358:

… and as usual the PCB was ahead of its’ time with full silk-screen and solder mask:

Assembly was quite straight-forward. The design is quite compact, so a lot of vertical resistor mounting was necessary due to the lack of space. However it was refreshing to not have any links to fit. After around twenty minutes of relaxed construction, it was ready to test:

It’s a 1/2 watt speaker, however much louder than originally anticipated:

Once again, another complete and well-produced kit.

Conclusion

That was a lot of fun, and I’m off to make the matching square-wave oscillator for the frequency counter. Kudos to Colin for all those years of publication and helping people learn. Lots of companies bang on about offering tutorials and information on the Internet for free, but Colin has been doing it for over ten years. Check out his Talking Electronics website for a huge variety of knowledge, an excellent electronics course you can get on CD – and go easy on him if you have any questions.

Full-sized images available on flickr. This kit was purchased without notifying the supplier.

And if you made it this far – check out my new book “Arduino Workshop” from No Starch Press.

In the meanwhile have fun and keep checking into tronixstuff.com. Why not follow things on twitterGoogle+, subscribe  for email updates or RSS using the links on the right-hand column? And join our friendly Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other –  and we can all learn something.

Tutorial – Arduino and the TI ADS1110 16-bit ADC

Learn how to use the TI ADS1110 16-bit ADC with Arduino in chapter fifty-three of my Arduino Tutorials. The first chapter is here, the complete series is detailed here.

Updated 02/07/2013

Introduction

Moving on from the last chapter where we explained an 8-bit ADC, in this instalment we have the Texas Instruments ADS1110 – an incredibly tiny but useful 16-bit analogue-to-digital converter IC.  It can operate between 2.7 and 5.5 V so it’s also fine for Arduino Due and other lower-voltage development boards. This is a quick guide to get you going with the ADS1110 ready for further applications. Before continuing any further, please download the data sheet (pdf) as it will be useful and referred to during this tutorial. The ADS1110 gives you the option of a more accurate ADC than offered by the Arduino’s 10-bit ADCs – and it’s relatively easy to use. The only block for some is the package type – it’s only available in SOT23-6:

So unless you’re making a customised PCB, some sort of breakout will be required. One useful example is the Schmartboard we reviewed earlier:

The ADS1110 uses the I2C bus for communication, so if this is new to you – please review the I2C tutorials before continuing. And as there’s only six pins you can’t set the bus address – instead, you can select from six variants of the ADS1110 – each with their own address (see page two of the data sheet). As you can see the in the photo above, ours is marked “EDO” which matches to the bus address 1001000 or 0x48h. And with the example circuits we’ve used 10kΩ pull-up resistors on the I2C bus. You can use the ADS1110 as either a single-ended or differential ADC –  But first we need to examine the configuration register which is used to control various attributes, and the data register.

Configuration register

Turn to page eleven of the data sheet. The configuration register is one byte in size, and as the ADS1110 resets on a power-cycle – you need to reset the register if your needs are different to the defaults. The data sheet spells it out quite neatly… bits 0 and 1 determine the gain setting for the PGA (programmable gain amplifier). If you’re just measuring voltages or experimenting, leave these as zero for a gain of 1V/V. Next, the data rate for the ADS1110 is controlled with bits 2 and 3. If you have continuous sampling turned on, this determines the number of samples per second taken by the ADC.

After some experimenting with an Arduino Uno we found the values returned from the ADC were a bit off when using the fastest rate, so leave it as 15 SPS unless required otherwise. Bit 4 sets either continuous sampling (0) or one-off sampling (1). Ignore bits 5 and 6, however they’re always set as 0. Finally bit 7 – if you’re in one-off sampling mode, setting it to 1 requests a sample – and reading it will tell you if the returned data is new (0) or old (1). You can check that the value measured is a new value – if the first bit of the configuration byte that comes after the data is 0, it’s new. If it returns 1 the ADC conversion hasn’t finished.

Data register

As the ADS1110 is a 16-bit ADC, it returns the data over two bytes – and then follows with the value of the configuration register. So if you request three bytes the whole lot comes back. The data is in “two’s complement” form, which is a method of using signed numbers with binary. Converting those two bytes is done by some simple maths. When sampling at 15 SPS, the value returned by the ADS1110 (not the voltage)  falls between -32768 and 32767. The higher byte of the value is multiplied by 256, then added to the lower byte – which is then multiplied by 2.048 and finally divided by 32768. Don’t panic, as we do this in the example sketch below.

In this mode you can read a voltage that falls between zero and 2.048 V (which also happens to be the inbuilt reference voltage for the ADS1110). The example circuit is simple (from the data sheet):

Don’t forget the 10kΩ pull-up resistors on the I2C bus. The following sketch uses the ADS1110 in the default mode, and simply returns the voltage measured:

Once uploaded, connect the signal to measure and open the serial monitor – you’ll be presented with something similar to:

If you need to alter the gain of the internal programmable gain amplifier of the ADC – you’ll need to write a new byte into the configuration register using:

before requesting the ADC data. This would be 0x8D, 0x8E or 0x8F for gain values of 2, 4 and 8 respectively – and use 0x8C to reset the ADS1110 back to default.

In this mode you can read the difference between two voltages that each fall between zero and 5 V. The example circuit is simple (from the data sheet):

We must note here (and in the data sheet) that the ADS1110 can’t accept negative voltages on either of the inputs. You can use the previous sketch for the same results – and the resulting voltage will be the value of Vin- subtracted from Vin+. For example, if you had 2 V on Vin+ and 1 V on Vin- the resulting voltage would be 1 V (with the gain set to 1).

Conclusion

Once again I hope you found this of interest, and possibly useful. And if you enjoy my tutorials, or want to introduce someone else to the interesting world of Arduino – check out my new book “Arduino Workshop” from No Starch Press.

In the meanwhile have fun and keep checking into tronixstuff.com. Why not follow things on twitterGoogle+, subscribe  for email updates or RSS using the links on the right-hand column? And join our friendly Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other –  and we can all learn something.

Tutorial – Arduino and PCF8591 ADC DAC IC

Learn how to use the NXP PCF 8591 8-bit A/D and D/A IC with Arduino in chapter fifty-two of my Arduino Tutorials. The first chapter is here, the complete series is detailed here.

Updated 17/06/2013

Introduction

Have you ever wanted more analogue input pins on your Arduino project, but not wanted to fork out for a Mega? Or would you like to generate analogue signals? Then check out the subject of our tutorial – the NXP PCF8591 IC. It solves both these problems as it has a single DAC (digital to analogue) converter as well as four ADCs (analogue to digital converters) – all accessible via the I2C bus. If the I2C bus is new to you, please familiarise yourself with the readings here before moving forward.

The PCF8591 is available in DIP form, which makes it easy to experiment with:

You can get them from the usual retailers. Before moving on, download the data sheet. The PCF8591 can operate on both 5V and 3.3V so if you’re using an Arduino Due, Raspberry Pi or other 3.3 V development board you’re fine. Now we’ll first explain the DAC, then the ADCs.

Using the DAC (digital-to-analogue converter)

The DAC on the PCF8591 has a resolution of 8-bits – so it can generate a theoretical signal of between zero volts and the reference voltage (Vref) in 255 steps. For demonstration purposes we’ll use a Vref of 5V, and you can use a lower Vref such as 3.3V or whatever you wish the maximum value to be … however it must be less than the supply voltage. Note that when there is a load on the analogue output (a real-world situation), the maximum output voltage will drop – the data sheet (which you downloaded) shows a 10% drop for a 10kΩ load. Now for our demonstration circuit:

Note the use of 10kΩ pull-up resistors on the I2C bus, and the 10μF capacitor between 5V and GND. The I2C bus address is set by a combination of pins A0~A2, and with them all to GND the address is 0x90. The analogue output can be taken from pin 15 (and there’s a seperate analogue GND on pin 13. Also, connect pin 13 to GND, and circuit GND to Arduino GND.

To control the DAC we need to send two bytes of data. The first is the control byte, which simply activates the DAC and is 1000000 (or 0x40) and the next byte is the value between 0 and 255 (the output level). This is demonstrated in the following sketch:

Did you notice the bit shift of the bus address in the #define statement? Arduino sends 7-bit addresses but the PCF8591 wants an 8-bit, so we shift the byte over by one bit.

The results of the sketch are shown below, we’ve connected the Vref to 5V and the oscilloscope probe and GND to the analogue output and GND respectively:

If you like curves you can generate sine waves with the sketch below. It uses a lookup table in an array which contains the necessary pre-calculated data points:

And the results:

For the following DSO image dump, we changed the Vref to 3.3V – note the change in the maxima on the sine wave:

Now you can experiment with the DAC to make sound effects, signals or control other analogue circuits.

Using the ADCs (analogue-to-digital converters)

If you’ve used the analogRead() function on your Arduino (way back in Chapter One) then you’re already familiar with an ADC. With out PCF8591 we can read a voltage between zero and the Vref and it will return a value of between zero and 255 which is directly proportional to zero and the Vref. For example, measuring 3.3V should return 168. The resolution (8-bit) of the ADC is lower than the onboard Arduino (10-bit) however the PCF8591 can do something the Arduino’s ADC cannot. But we’ll get to that in a moment.

First, to simply read the values of each ADC pin we send a control byte to tell the PCF8591 which ADC we want to read. For ADCs zero to three the control byte is 0x00, 0x01, ox02 and 0x03 respectively. Then we ask for two bytes of data back from the ADC, and store the second byte for use. Why two bytes? The PCF8591 returns the previously measured value first – then the current byte. (See Figure 8 in the data sheet). Finally, if you’re not using all the ADC pins, connect the unused ones to GND.

The following example sketch simply retrieves values from each ADC pin one at a time, then displays them in the serial monitor:

Upon running the sketch you’ll be presented with the values of each ADC in the serial monitor. Although it was a simple demonstration to show you how to individually read each ADC, it is a cumbersome method of getting more than one byte at a time from a particular ADC.

To do this, change the control byte to request auto-increment, which is done by setting bit 2 of the control byte to 1. So to start from ADC0 we use a new control byte of binary 00000100 or hexadecimal 0x04. Then request five bytes of data (once again we ignore the first byte) which will cause the PCF8591 to return all values in one chain of bytes. This process is demonstrated in the following sketch:

Previously we mentioned that the PCF8591 can do something that the Arduino’s ADC cannot, and this is offer a differential ADC. As opposed to the Arduino’s single-ended (i.e. it returns the difference between the positive signal voltage and GND, the differential ADC accepts two signals (that don’t necessarily have to be referenced to ground), and returns the difference between the two signals. This can be convenient for measuring small changes in voltages for load cells and so on.

Setting up the PCF8591 for differential ADC is a simple matter of changing the control byte. If you turn to page seven of the data sheet, then consider the different types of analogue input programming. Previously we used mode ’00’ for four inputs, however you can select the others which are clearly illustrated, for example:

So to set the control byte for two differential inputs, use binary 00110000 or 0x30. Then it’s a simple matter of requesting the bytes of data and working with them. As you can see there’s also combination single/differential and a complex three-differential input. However we’ll leave them for the time being.

Conclusion

Hopefully you found this of interest, whether adding a DAC to your experiments or learning a bit more about ADCs. We’ll have some more analogue to digital articles coming up soon, so stay tuned. And if you enjoy my tutorials, or want to introduce someone else to the interesting world of Arduino – check out my new book “Arduino Workshop” from No Starch Press.

In the meanwhile have fun and keep checking into tronixstuff.com. Why not follow things on twitterGoogle+, subscribe  for email updates or RSS using the links on the right-hand column? And join our friendly Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other –  and we can all learn something.

Kit Review – JYE Tech FG085 DDS Function Generator

Introduction

There has been a lot of talk lately about inexpensive DDS (direct digital synthesis) function generators, and I always enjoy a kit – so it was time to check out the subject of this review. It’s the “FG085 miniDDS function generator” from JYE Tech. JYE is a small company in China that makes inexpensive test equipment kits, for example their capacitance meter (my first kit review!) and DSO. The capacitance meter was good, the DSO not so good – so let’s hope this is better than their last efforts.

Assembly

The instructions (AssemblyGuide_085G) are much better than previous efforts, and if you have bought the kit – read them. The kit arrives in a large zip-lock bag, with the following bundle of parts:

The AC adaptor is 100~240V in, 15V DC out. Everything is included with the kit including a short BNC to alligator clips lead for output. The PCBs are very good, with a nice solder mask and silk screen:

and back:

At this point we realise that most of the work is already done. There’s two microcontrollers ATmega48 and ATmega168- one for display and user-interface control, and the other for function generation. It takes only a few minutes to solder in the through-hole parts, headers and sockets:

… then you flip over the PCB and add the LCD:

… followed by the buttons and rotary encoder. From previous research this is the part that causes people a lot of trouble – so read carefully. There’s a lot of buttons – and if they aren’t inserted into the PCB correctly your life will become very difficult. The buttons must be inserted a certain way – they’re “polarised” – for example:

As you can see above, one side has a double-vertical line and the other side has a single. When you fit the buttons to the PCB – the side with the double-vertical must face the left-hand side of the PCB – the side with the DC socket. For example:

Furthermore, don’t be in a rush and put all the buttons in then try to solder them all at once.  Do them one at a time, and hold them tight to the PCB with some blu-tac or similar. If they don’t sit flush with the PCB the front panel won’t fit properly and the buttons will stick when in use. So exercise some patience, and you’ll be rewarded with an easy to use function generator. Rush them in and you’ll be very unhappy. I warned you! After fitting each button, test fit the front panel to check the alignment, for example:

Then you end up with nicely-aligned buttons:

… which all operate smoothly when the panel is fitted:

After the buttons comes the rotary encoder. Be very careful when fitting it to the PCB – the data legs are really weak, and bend without much effort. If you push in the encoder, be mindful of the legs not going through the holes and bending upwards. Furthermore, when soldering in the encoder note that you’re really close to an electrolytic – you don’t want to stab it with a hot iron:

The CP2012 chip in the image above is for the USB interface. More on that later. Now the next stage is the power-test. Connect DC power and turn it on – you should be greeted by a short copyright message followed by the operation display:

If you didn’t – remove the power and check your soldering –  including the capacitor polarities and look for bridges, especially around the USB socket. Now it’s time to fit the output BNC socket. For some reason only known to the designers, they have this poking out the front of the panel for the kit – however previous revisions have used a simple side-entry socket. Thus you need to do some modifications to the supplied socket. First, chop the tag from the sprocket washer:

… then remove the paper from the front panel:

Now solder a link to the washer in a vertical position:

… then fit the BNC socket to the panel, with the washer aligned as such:

Finally, align the top panel with the PCB so the BNC socket pin and washer link drop into the PCB and solder them in:

If you want to use the servo mode, solder three short wires that can attach to a servo form the three “output” pads between the BNC and USB socket.

Finally, screw in the panels and you’re finished!

Using the function generator

Operation is quite simple, and your first reference should be the manual (manual.pdf). The display defaults to normal function generator mode at power-up – where you can adjust the frequency, offset, amplitude and type of output – sine, square, triangle, ramp up, ramp down, staircase up and down:

The ranges for all functions is 0~10 khz, except for sine which can hit 200 kHz. You can enter higher frequencies, such as up to 250 kHz for sine – but the results aren’t so good.

Instead of filling this review with lots of screen dumps from an oscilloscope to demonstrate the output – I’ve made the following video where you can see various functions being displayed on a DSO:

You can also create signals to test servos, with adjustable pulse-width, amplitude and cycle times. However you’ll need to solder three wires onto the PCB (next to the BNC socket area) to attach to the servo.

According to the user manual and various retailers’ websites – the FG085 can generate frequency sweeping signals. These are signals that sweep from a start to as finish frequency over a period of time. However the firmware on the supplied unit is old and needs updating to enable this function. You can download the firmware in .hex file format from here. Then go and dig up an AVR programmer and avrdudeAt the time of writing we had some issues with the signature not being recognised when updating the firmware, and solidly bricked the FG085. Our fault – so when that’s sorted out we’ll update the review – stay tuned.

There is also a USB port on the side – after installing CP2102 drivers in Windows we could connect at 115200 bps with terminal, however all the FG085 returned was the firmware version number. Perhaps later on the designers will update the firmware to allow for PC control. Somehow I wouldn’t bank on it.

Oh – if you’re wondering what DDS is – click here!

Conclusion

It’s an interesting piece of equipment. Putting the firmware upgrade issues to one side, the FG085 does what it sets out to do. During testing it worked well, and we didn’t come across any obvious inaccuracies during use.  The price varies between US\$43 and \$50 – so for that money it’s  a good kit. Just take care during construction and you’ll be fine.

The function generator is available in kit form or assembled, with or without panels from China. The kit version with panels is also available from Sparkfun (KIT-11394) and their resellers. Full-sized images available on flickr. This kit was purchased and reviewed without notifying the supplier.

In the meanwhile have fun and keep checking into tronixstuff.com. Why not follow things on twitterGoogle+, subscribe  for email updates or RSS using the links on the right-hand column? And join our friendly Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other –  and we can all learn something.

Project: Clock Two – Single digit clock

Let’s hack an Ikea lamp into a single-digit clock! How? Read on…

Updated 18/03/2013

Time for another instalment in my irregular series of clock projects. (Or should that be “Time for another instalment in the series of irregular clock projects”?) In contrast with the extreme “blinkiness” of Clock One, in this article we describe how to build this single-digit digital clock:

Once again the electronics of the clock will be based from an Arduino-compatible board with a DS1307 real-time clock IC added to the board. On top of this we add a shield with some extra circuitry and two buttons – but more on this later. The inspiration for this clock came from a product that was recently acquired at Ikea – the “Kvart” work lamp, for example:

If you are shopping for one, here are the Ikea stock details:

The goal is to place the electronics of the clock in the base, and have one single-digit LED display at the top of the neck which will blink out the digits. There will be two buttons under the base that are used to set the time. It will be powered by a 9V battery or an AC adaptor which is suitable for a typical Arduino board.

Construction

This article is a diary of my construction, and you can always use your own knowledge and initiative. It is assumed that you have a solid knowledge of the basics of the Arduino system.  If not, review my series of tutorials available from here. Furthermore, feel free to modify the design to work with what you have available – I hope this article can be of some inspiration to you.

Software

It is much easier to prototype the clock and get the Arduino sketch working how you like it before breaking down the lamp and building up the clock. To do this involves some jumper wires and a solderless breadboard, for example:

Although there are four buttons on the board we only use two. They are connected to digital pins eight and nine (with 10k pull-down resistors). The LED display segments a~g are connected to Arduino digital pins 0~6 respectively. The decimal point is connected to the pulse output pin of the DS1307 – which will be set to a 1Hz output to have a nice constant blinking to show the clock is alive and well.

If you are unfamiliar with operating the DS1307 real-time clock IC please review this tutorial. Operation of the clock has been made as simple for the user as possible. To set the time, they press button A (on digital eight) while the current time is being displayed, after which point the user can select the first digit (0~2) of the time by pressing button A. Then they press button B (on digital nine) to lock it in and move to the second digit (0~9) which is again chosen with button A and selected with button B. Then they move onto the digits in the same manner.

After this process the new time is checked for validity (so the user cannot enter invalid times such as 2534h) – and is ok, the clock will blink the hyphen twice and then carry on with the new time. If the entered time is invalid, the clock reverts back to the current time. This process is demonstrated in the following video clip:

You can download the Arduino sketch from here.

Hardware

The parts required to replicate the Clock Two in this article are:

• One Arduino-compatible board with DS1307 real-time clock IC as described in this article
• One Arduino protoshield and header pins
• One common-cathode 7-segment LED display of your choosing
• Seven current-limiting resistors to reduce the output current from Arduino digital outputs going to the LED segments. In our example we use a 560 ohm resistor network to save time
• Two buttons and two 10k ohm pull-down resistors
• One meter of nine-core wire that will fit inside the neck and stand of the Kvart lamp – an external diameter of less than 6mm will be fine
• And of course – the lamp

The protoshield is used to hold the buttons, resistor network and the terminus for the wires between the LED display and the Arduino digital outputs, for example:

At this stage you will need to do some heavy deconstruction on the lamp. Cut off the mains lead at the base and remove the plastic grommet from the stand that surrounded the AC lead. Next,  with some elbow grease you can twist off the lamp-shade unit from the end of the flexible neck. You could always reuse the lamp head and AC lead if wired by a licensed electrician.

Now you need to feed the multicore wire through the neck and down to the base of the lamp. You can pull it through the hole near the base, and then will need to drill a hole in the base to feed it through to the electronics as such:

Take care when feeding the cable though so you don’t nick the insulation as shown above. Leave yourself a fair bit of slack at the top which will make life easier when soldering on the LED display, for example:

The next step is to solder the wires at the top to the LED display. Make notes to help recall which wires are soldered to the pins of the display. If your soldering skills (like mine) aren’t so good, use heatshrink to cover the soldering:

Most displays will have two GND pins, so bridge them so you only need to use one wire in the multicore back to base:

At this point use the continuity function of a multimeter or a low-voltage power source to test each LED segment using the other end of the cable protruding from the base. Once you are satisfied the segments have been soldered correctly, carefully draw the cable back through the neck and base in order to reduce the slack between the display and the top of the lamp neck. Then solder the individual LED segment wires to the protoshield.

Now if you have not already done so, upload the sketch into the Arduino board – especially if you are going to permanently mount the circuitry into the base. A simple method of mounting would be using  a hot glue gun, but for the purpose of demonstration we have just used blu-tac:

Although this does look a little rough, we are using existing stock which kept the cost down. If you are going to power the clock with an AC adaptor, you will also need to cut out small opening to allow the lead to protrude from the side of the base. And now for the resulting clock – our Clock Two:

So there you have it, the second of many clocks we plan to describe in the future.

In the meanwhile have fun and keep checking into tronixstuff.com. Why not follow things on twitterGoogle+, subscribe  for email updates or RSS using the links on the right-hand column? And join our friendly Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other –  and we can all learn something.

Using an ATtiny as an Arduino

Learn how to use ATtiny45 and ATtiny85 microcontrollers with Arduino in chapter forty-four of a series originally titled “Getting Started/Moving Forward with Arduino!” by John Boxall – a series of articles on the Arduino universe. The first chapter is here, the complete series is detailed here.

Updated 07/10/2014

Did you know you can use an Atmel ATtiny45 or ATtiny85 microcontroller with Arduino software? Well you do now. The team at the High-Low Tech Group at MIT have published the information and examples on how to do this, and it looked like fun – so the purpose of this article is to document my experience with the ATtiny and Arduino and share the instructions with you in my own words. All credit goes to the interesting people at the MIT HLT Group for their article and of course to Alessandro Saporetti for his work on making all this possible.

Introduction

Before anyone gets too excited – there are a few limitations to doing this…

Limitation one – the ATtiny has “tiny” in the name for a reason:

it’s the one on the left

Therefore we have less I/O pins to play with. Consider the pinout for the ATtiny from the data sheet:

So as you can see we have thee analogue inputs (pins 7, 3 and 2) and two digital outputs with PWM (pins 5 and 6). Pin 4 is GND, and pin 8 is 5V.

Limitation two – memory. The ATtiny45 has 4096 bytes of flash memory available, the -85 has 8192. So you may not be controlling your home-built R2D2 with it.

Limitation three – available Arduino functions. As stated by the HLT article, the following commands are supported:

Other functions may work or become available over time.

Limitation four – You need Arduino IDE v1.0.1 or higher, except for v1.0.2. So v1.0.3 and higher is fine.

So please keep these limitations in mind when planning your ATtiny project.

Getting Started

You can use an existing Arduino-compatible board as a programmer with some external wiring. Before wiring it all up – plug in your Arduino board, load the IDE and upload the ArduinoISP sketch which is in the File>Examples menu. Whenever you want to upload a sketch to your ATtiny, you need to upload the ArduinoISP sketch to your Arduino first. Consider this sketch the “bridge” between the IDE and the ATtiny.

Next, build the circuit as shown below:

Depending on the Arduino board you’re using, you may or may not need the 10uF capacitor between Arduino RST and GND. Follow the schematic above each time you want to program the ATtiny.

Software

From a software perspective, to use the ATtinys you need to add some files to your Arduino IDE. First, download this zip file. Then extract the”attiny” folder and copy it to the “hardware” folder which sits under your main Arduino IDE folder, for example:

Now restart the Arduino IDE. As you’re using the Arduino as a programmer, you need select “Arduino as ISP” – which is found in the Tools>Programmer menu. Next – select the board type using the Tools>Board  menu. Select the appropriate ATtiny that you’re using – with the 1 MHz internal clock option. Now you can enter and upload your ATtiny sketch. When uploading sketches you may see error messages as shown below:

The message is “normal” in this situation, so nothing to worry about.

Creating Arduino sketches for ATtinys

When creating your sketches, note that the pin number allocations are different for ATtinys in the IDE. Note the following pin number allocations:

• digital pin zero is physical pin five (also PWM)
• digital pin one is physical pin six (also PWM)
• analogue input two is physical pin seven
• analogue input three is physical pin two
• analogue input four is physical pin three

For a quick demonstration, load the Blink example sketch – File>Examples>1. Basics>Blink. Change the pin number for the digital output from 13 to 0. For example:

Upload the sketch using the methods described earlier. If you’re using programmer method one, your matching circuit is:

If you’re using programmer method two, this will blink the on-board LED.

Final example

We test the digital outputs with digital and PWM outputs using two LEDs instead of one:

And the sketch:

And a quick demonstration video:

So there you have it – another interesting derivative of the Arduino system. Once again, thanks and credit to Alesssandro Saporetti and the MIT HLT Group for their published information. And if you enjoyed this article, or want to introduce someone else to the interesting world of Arduino – check out my book “Arduino Workshop”.

Have fun and keep checking into tronixstuff.com. Why not follow things on twitterGoogle+, subscribe  for email updates or RSS using the links on the right-hand column, or join our forum – dedicated to the projects and related items on this website.

Review – Tenma 72-7222 Digital Clamp Multimeter

The purpose of this article is to examine the Tenma 72-7222 Digital Clamp Multimeter supplied for review by element-14/Farnell/Newark. The Tenma is a strongly featured yet inexpensive piece of test equipment – and considerably good value when you consider there is a current clamp for measuring high AC currents. So let’s have a look and see what we have.

Initial Impression

The Tenma arrives in a retail box, and generally nicely packaged. Naturally this has nothing to do with the performance of the meter at all, but at least they made an effort:

Opening up we find a nicely rounded group of items: the meter itself, some no-name AAA cells, test leads, a thermocouple for temperature measurement, a surprisingly articulate and well-written user manual, and the unit itself – all within a nice pouch. Wow – a pouch. Agilent? Fluke? All that money for a DMM and you don’t include a pouch?

Recent test equipment reviewers have made pulling apart the unit part of the review – so here goes… the back comes off easily:

No user-replaceable fuses… instead a PTC. A closer look at the PCB:

A very neat and organised PCB layout. There are plastic tabs that hold the PCB in along with a screw, however the case flexed too much for me to warrant removing the PCB completely. The spring for the clamp meter is locked in nicely and very strong, it won’t give up for a long time. Pulling the clamp base out reveals the rest of the PCB:

Installation of the battery is two stage procedure, first you need to remove a screw and then slide out the rear door:

… then insert the AAA cells into a frame, which is then inserted inside the unit:

The physical feel of the unit is relative to the purchase price, the plastic is simple and could be quite brittle if the unit was dropped from a height. The user manual claims the unit can be dropped from up to a height of one metre. Onto carpet? Yes. Concrete? Perhaps not. However like all test equipment one would hope the user would take care of it whenever possible. The clamp meter is very strong due to the large spring inside the handle, which can be opened up to around 28mm. The included leads are just on one meter long including the length of the probe:

The leads are rated to Category I 1000V (overkill – the meter can’t go that high) and 600 V Category II – “This category refers to local-level electrical distribution, such as that provided by a standard wall outlet or plug in loads (for example, 115 AC voltage for U.S. or 200 AC voltage for Europe). Examples of Measurement Category II are measurements performed on household appliances, portable tools, and similar modules” – definition from from National Instruments.  Unlike discount DMMs from unknown suppliers you can trust the rating to be true – otherwise element-14 wouldn’t be selling it.

Unit Specifications

• Voltage Measuring Range DC:200mV, 2V, 20V, 200V, 600V
• Voltage Measuring Range AC:2V, 20V, 200V, 600V
• Current Measuring Range AC:2A, 20A, 200A, 400A
• Resistance Measuring Range:200ohm, 2kohm, 20kohm, 200kohm, 2Mohm, 20Mohm
• Temperature Measuring Range:-40°C to +1000°C
• DMM Response Type:True RMS
• DMM Functions:AC Current, AC/DC Voltage, Resistance, Temperature
• Ranging:Auto
• Display Count:1999
• AC Current Range Accuracy:± (1.5% + 5d)
• AC Voltage Range Accuracy:± (1.2% + 5d)
• Accuracy:± (1.0% + 3d)
• Current AC Max:400A
• Current Range AC:2A, 20A, 200A, 400A
• DC Voltage Range Accuracy1:± (0.8% + 1d)
• Resistance Range Accuracy:± (1.0% + 2d)
• Temperature Measuring Range:-40°C to +1000°C

The only measurement missed out on is DC current, however there is the Tenma 72-7224 which has DC current and frequency ranges. Finally, all the modes and buttons can be selected while holding the meter with one hand – for both left- and right-handed folk.

Measurement experience

Normally I would compare the measurements against my Agilent U1272A, however it’s out to lunch. Instead, a Fluke 233. First, AC voltage from the mains:

Next, a few DC voltage measurements:

Now for some resistance measurements. Higher values near the maximum of 20M Ohm can take around four seconds to measure:

Forward voltage of a 1N4004 diode:

Now off to the kitchen for some more measurements – first with the thermocouple:

The boiling water test – 100 degrees Celsius (you can also select Fahrenheit if so inclined):

And now to test out the AC current clamp meter function with a 10A kettle at boiling point. First, using the 20A current range:

And then again on the 400A current range:

As always, it’s best to use the multimeter range that more closely corresponds with the current under test. The meter also has a continuity test with a beeper, however it was somewhat slow and would often take around one second to register – so nothing too impressive on that front. The meter can record the maximum value with the grey button, or hold a reading using the yellow button.

Conclusion

The Tenma 72-7222 works as advertised, and as expected. It is a solid little unit that if looked after should last a few years at a minimum. It certainly has a few limitations, such as the 1999 count display, lack of backlight, and the average continuity function. But don’t let that put you off. For the price – under Au\$30 – it is a certified deal. If you need a clamp current meter for odd jobs or a casual-use multimeter and you are on a limited budget, the Tenma will certainly prove a worthwhile purchase. Full-size images are available on Flickr.

You can purchase a Tenma 72-7222 from element-14Farnell and Newark.

[Note – The Tenma 72-7222 Digital Clamp Multimeter was a promotional consideration from element-14/Farnell/Newark]

Tutorial: Arduino timing methods with millis()

This is chapter thirty-seven of a series originally titled “Getting Started/Moving Forward with Arduino!” by John Boxall – in what feels like an endless series of articles on the Arduino universe. The first chapter is here, the complete series is detailed here. Any files from tutorials will be found here.

[Updated 20/01/2013]

In this article we introduce the millis(); function and put it to use to create various timing examples.

Millis? Nothing to do with lip-syncers… hopefully you recognised milli as being the numerical prefix for one-thousandths; that is multiplying a unit of measure by 0.001 (or ten to the power of negative 3). Interestingly our Arduino systems will count the number of milliseconds (thousands of a second) from the start of a sketch running until the count reaches the maximum number capable of being stored in the variable type unsigned long (a 32-bit [four byte] integer – that ranges from zero to (2^32)-1.

(2^32)-1, or 4294967295 milliseconds converts to 49.71027-odd days. The counter resets when the Arduino is reset, it reaches the maximum value or a new sketch is uploaded. To get the value of the counter at a particular juncture, just call the function – for example:

Where start is an unsigned long variable. Here is a very simple example to show you millis() in action:

The sketch stores the current millis count in start, then waits one second, then stores the value of millis again in finished. Finally it calculates the elapsed time of the delay.  In the following screen dump of the serial monitor, you can see that the duration was not always exactly 1000 milliseconds:

To put it simply, the millis function makes use of an internal counter within the ATmega microcontroller at the heart of your Arduino. This counter increments every clock cycle – which happens (in standard Arduino and compatibles) at a clock speed of 16 Mhz. This speed is controlled by the crystal on the Arduino board (the silver thing with T16.000 stamped on it):

Crystal accuracy can vary depending on external temperature, and the tolerance of the crystal itself. This in turn will affect the accuracy of your millis result. Anecdotal experience has reported the drift in timing accuracy can be around three or four seconds per twenty-four hour period. If you are using a board or your own version that is using a ceramic resonator instead of a crystal, note that they are not as accurate and will introduce the possibility of higher drift levels. If you need a much higher level of timing accuracy, consider specific timer ICs such as the Maxim DS3232.

Now we can make use of the millis  for various timing functions. As demonstrated in the previous example sketch, we can calculate elapsed time. To take this idea forward, let’s make a simple stopwatch. Doing so can be as simple or as complex as necessary, but for this case we will veer towards simple. On the hardware perspective, we will have two buttons – Start and Stop – with the 10k ohm pull-down resistors connected to digital pins 2 and 3 respectively.

When the user presses start the sketch will note the value for millis – then after stop is pressed, the sketch will again note the value for millis, calculate and display the elapsed time. The user can then press start to repeat the process, or stop for updated data. Here is the sketch:

The calls to delay() are used to debounce the switches – these are optional and their use will depend on your hardware. Below is an example of the sketch’s serial monitor output – the stopwatch has started, and then button two pressed six times across periods of time:

If you had a sensor at the start and end of a fixed distance, speed could be calculated: speed = distance ÷ time.

You can also make a speedometer for a wheeled form of motion, for example a bicycle. At the present time I do not have a bicycle to mess about with, however we can describe the process to do so – it is quite simple. (Disclaimer – do so at your own risk etc.)  First of all, let’s review the necessary maths. You will need to know the circumference of the wheel. Hardware – you will need a sensor. For example – a reed switch and magnet. Consider the reed switch to be a normally-open button, and connect as usual with a 10k ohm pull-down resistor. Others may use a hall-effect sensor – each to their own). Remember from maths class:

To calculate the circumference – use the formula:

circumference = 2πr

where r is the radius of the circle. Now that you have the wheel circumference, this value can be considered as our ‘fixed distance’, and therefore the speed can be calculated by measuring the elapsed time between of a full rotation.

Your sensor – once fitted – should act in the same method as a normally-open button that is pushed every rotation. Our sketch will measure the time elapsed between every pulse from the sensor. To do this, our example will have the sensor output connected to digital pin 2 – as it will trigger an interrupt to calculate the speed. (Interrupts? See chapter three). The sketch will otherwise be displaying the speed on a normal I2C-interface LCD module. The I2C interface is suggested as this requires only 4 wires from the Arduino board to the LCD – the less wires the better.

Here is the sketch for your perusal:

There isn’t that much going on – every time the wheel completes one revolution the signal from the sensor will go from low to high – triggering an interrupt which calls the function speedCalc(). This takes a reading of millis() and then calculates the difference between the current reading and the previous reading – this value becomes the time to cover the distance (which is the circumference of the wheel relative to the sensor – stored in

and is measured in metres). It finally calculates the speed in km/h and MPH. Between interrupts the sketch displays the updated speed data on the LCD as well as the raw time value for each revolution for curiosity’s sake. In real life I don’t think anyone would mount an LCD on a bicycle, perhaps an LED display would be more relevant.

In the meanwhile, you can see how this example works in the following short video clip. Instead of a bike wheel and reed switch/magnet combination, I have connected the square-wave output from a function generator to the interrupt pin to simulate the pulses from the sensor, so you can get an idea of how it works:

That just about sums up the use of millis() for the time being. There is also the micros(); function which counts microseconds. So there you have it – another practical function that can allow more problems to be solved via the world of Arduino. As always, now it is up to you and your imagination to find something to control or get up to other shenanigans.

Have fun and keep checking into tronixstuff.com. Why not follow things on twitterGoogle+, subscribe  for email updates or RSS using the links on the right-hand column, or join our Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other –  and we can all learn something.

Tutorial: Arduino and the SPI bus part II

This is chapter thirty-six of a series originally titled “Getting Started/Moving Forward with Arduino!” by John Boxall – A seemingly endless series of articles on the Arduino universe. The first chapter is here, the complete series is detailed here

[Updated 10/01/2013]

This is the second of several chapters in which we are investigating the SPI data bus, and how we can control devices using it with our Arduino systems. If you have not done so already, please read part one of the SPI articles. Again we will learn the necessary theory, and then apply it by controlling a variety of devices. As always things will be kept as simple as possible.

First on our list today is the use of multiple SPI devices on the single bus. We briefly touched on this in part one, by showing how multiple devices are wired, for example:

Notice how the slave devices share the clock, MOSI and MISO lines – however they both have their own chip select line back to the master device. At this point a limitation of the SPI bus becomes prevalent – for each slave device we need another digital pin to control chip select for that device. If you were looking to control many devices, it would be better to consider finding I2C solutions to the problem. To implement multiple devices is very easy. Consider the example 34.1 from part one – we controlled a digital rheostat. Now we will repeat the example, but instead control four instead of one. For reference, here is the pinout diagram:

Doing so may sound complex, but it is not. We connect the SCK, MOSI and  MISO pins together, then to Arduino pins D13, D11, D12 respectively. Each CS pin is wired to a separate Arduino digital pin. In our example rheostats 1 to 4 connect to D10 through to D7 respectively. To show the resistance is changing on each rheostat, there is an LED between pin 5 and GND and a 470 ohm resistor between 5V and pin 6. Next, here is the sketch:

Although the example sketch may be longer than necessary, it is quite simple. We have four SPI devices each controlling one LED, so to keep things easy to track we have defined led1~led4 to match the chip select digital out pins used for each SPI device. Then see the first four lines in void setup(); these pins are set to output in order to function as required. Next – this is very important – we set the pins’ state to HIGH. You must do this to every chip select line! Otherwise more than one CS pins may be initially low in some instances and cause the first data sent from MOSI to travel along to two or more SPI devices. With LEDs this may not be an issue, but for motor controllers … well it could be.

The other point of interest is the function

We pass the value for the SPI device we want to control, and the value to send to the device. The value for l is the chip select value for the SPI device to control, and ranges from 10~7 – or as defined earlier, led1~4. The rest of the sketch is involved in controlling the LED’s brightness by varying the resistance of the rheostats. Now to see example 36.1 in action via the following video clip:

(If you are wondering what I have done to the Freetronics board in that video, it was to add a DS1307 real-time clock IC in the prototyping section).

Next on the agenda is a digital-to-analogue converter, to be referred to using the acronym DAC. What is a DAC? In simple terms, it accepts a numerical value between zero and a maximum value (digital) and outputs a voltage between the range of zero and a maximum relative to the input value (analogue). One could consider this to be the opposite of the what we use the function analogRead(); for. For our example we will use a Microchip MCP4921 (data sheet.pdf):

(Please note that this is a beginners’ tutorial and is somewhat simplified). This DAC has a 12-bit resolution. This means that it can accept a decimal number between 0 and 4095 – in binary this is 0 to 1111 1111 1111 (see why it is called 12-bit) – and the outpout voltage is divided into 4096 steps. The output voltage for this particular DAC can fall between 0 and just under the supply voltage (5V). So for each increase of 1 in the decimal input value, the DAC will output around 1.221 millivolts.

It is also possible to reduce the size of the voltage output steps by using a lower reference voltage. Then the DAC will consider the reference voltage to be the maximum output with a value of 4095. So (for example) if the reference voltage was 2.5V, each increase of 1 in the decimal input value, the DAC will output around 0.6105 millivolts. The minimum reference voltage possible is 0.8V, which offers a step of 200 microvolts (uV).

The output of a DAC can be used for many things, such as a function generator or the playback of audio recorded in a digital form. For now we will examine how to use the hardware, and monitoring output on an oscilloscope. First we need the pinouts:

By now these sorts of diagrams shouldn’t present any problems. In this example, we keep pin 5 permanently set to GND; pin 6 is where you feed in the reference voltage – we will set this to +5V; AVss is GND; and Vouta is the output signal pin – where the magic comes from 🙂 The next thing to investigate is the MCP4921’s write command register:

Bits 0 to 11 are the 12 bits of the output value; bit 15 is an output selector (unused on the MPC4921); bit 14 controls the input buffer; bit 13 controls an inbuilt output amplifier; and bit 12 can shutdown the DAC. Unlike previous devices, the input data is spread across two bytes (or a word of data). Therefore a small amount of work needs to be done to format the data ready for the DAC. Let’s explain this through looking at the sketch for example 36.2 that follows. The purpose of the sketch is to go through all possible DAC values, from 0 to 4095, then back to 0 and so on.

First. note the variable outputvalue – it is a word, a 16-bit unsigned variable. This is perfect as we will be sending a word of data to the DAC. We put the increasing/decreasing value for a into outputValue. However as we can only send bytes of data at a time down the SPI bus, we will use the function highbyte() to separate the high side of the word (bits 15~8) into a byte variable called data.

We then use the bitwise AND and OR operators to set the parameter bits 15~12. Then this byte is sent to the SPI bus. Finally, the function lowbyte() is used to send the low side of the word (bits 7~0) into data and thence down the SPI bus as well.

Now for our demonstration sketch:

And a quick look at the DAC in action via an oscilloscope:

By now we have covered in detail how to send data to a device on the SPI bus. But how do we receive data from a device?

Doing so is quite simple, but some information is required about the particular device. For the rest of this chapter, we will use the Maxim DS3234 “extremely accurate” real-time clock. Please download the data sheet (.pdf) now, as it will be referred to many times.

The DS3234 is not available in through-hole packaging, so we will be using one that comes pre-soldered onto a very convenient breakout board:

It only takes a few moments to solder in some header pins for breadboard use. The battery type is CR1220 (12 x 2.0mm, 3V); if you don’t have a battery you will need to short out the battery holder with some wire otherwise the IC will not work. Readers have reported that the IC doesn’t keep time if the USB and external power are both applied to the Arduino at the same time.

A device will have one or more registers where information is read from and written to. Look at page twelve of the DS3234 data sheet, there are twenty-three registers, each containing eight bits (one byte) of data. Please take note that each register has a read and write address. An example – to retrieve the contents of the register at location 08h (alarm minutes) and place it into the byte data we need to do the following:

Don’t forget to take note of  the function SPI.setBitOrder(MSBFIRST); in your sketch, as this also determines the bit order of the data coming from the device. To write data to a specific address is also quite simple, for example:

Up to this point, we have not concerned ourselves with what is called the SPI data mode. The mode determines how the SPI device interprets the ‘pulses’ of data going in and out of the device. For a well-defined explanation, please read this article. With some devices (and in our forthcoming example) the data mode needs to be defined. So we use:

to set the data mode, within void(setup);. To determine a device’s data mode, as always – consult the data sheet. With our DS3234 example, the mode is mentioned on page 1 under Features List.

Finally, let’s delve a little deeper into SPI via the DS3234. The interesting people at Sparkfun have already written a good demonstration sketch for the DS3234, so let’s have a look at that and deconstruct it a little to see what is going on. You can download the sketch below from here, then change the file extension from .c to .pde.

Don’t let the use of custom functions and loops put you off, they are there to save time. Looking in the function SetTimeDate();, you can see that the data is written to the registers 80h through to 86h (skipping 83h – day of week) in the way as described earlier (set CS low, send out address to write to, send out data, set CS high). You will also notice some bitwise arithmetic going on as well. This is done to convert data between binary-coded decimal and decimal numbers.

Why? Go back to page twelve of the DS3234 data sheet and look at (e.g.) register 00h/80h – seconds. The bits 7~4 are used to represent the ‘tens’ column of the value, and bits 3~0 represent the ‘ones’ column of the value. So some bit shifting is necessary to isolate the digit for each column in order to convert the data to decimal. For other ways to convert between BCD and decimal, see the examples using the Maxim DS1307 in chapter seven.

Finally here is another example of reading the time data from the DS3234:

So there you have it – more about the world of the SPI bus and how to control the devices within.

In the meanwhile have fun and keep checking into tronixstuff.com. Why not follow things on twitterGoogle+, subscribe  for email updates or RSS using the links on the right-hand column? And join our friendly Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other –  and we can all learn something.