Tutorial: Arduino and the I2C bus – Part One

This is part one of several tutorials on how to use the I2C bus with Arduino, and chapter twenty of a series originally titled “Getting Started/Moving Forward with Arduino!” by John Boxall – A tutorial on the Arduino universe. The first chapter is here, the complete series is detailed here.

[Updated 10/01/2013]

In this first of several tutorials we are going to investigate the I2C data bus, and how we can control devices using it with our Arduino systems. The I2C bus can be a complex interface to master, so I will do my best to simplify it for you. In this article we will learn the necessary theory, and then apply it by controlling a variety of devices. Furthermore it would be in your interest to have an understanding of the binary, binary-coded decimal and hexadecimal number systems.

But first of all, what is it?

I2C is an acronym for “Inter-Integrated Circuit”. In the late 1970s, Philips’ semiconductor division (now NXP) saw the need for simplifying and standardising the data lines that travel between various integrated circuits in their products. Their solution was the I2C bus. This reduced the number of wires to two (SDA – data, and SCL – clock). Here is a nice introductory video from NXP:

Why would we want to use I2C devices?

As there are literally thousands of components that use the I2C interface! And our Arduino boards can control them all. There are many applications, such a real-time clocks, digital potentiometers, temperature sensors, digital compasses, memory chips, FM radio circuits, I/O expanders, LCD controllers, amplifiers, and so on. And you can have more than one on the bus at any time, in fact the maximum number of I2C devices used at any one time is 112.

From a hardware perspective, the wiring is very easy. Those of you with an Arduino Uno or 100% compatible board, you will be using pins A4 for SDA (data) and A5 for SCL (clock):

i2carduinopinsss

If you are using an Arduino Mega, SDA is pin 20 and SCL is 21, so note that shields with I2C need to be specifically for the Mega. If you have another type of board, check your data sheet or try the Arduino team’s hardware website.  And finally, if you are using a bare DIP ATmega328-PU microcontroller, you will use pins 27 for SDA and 28 for SCL. The bus wiring is simple:

nxpi2cbussmall

If you are only using one I2C device, the pull-up resistors are (normally) not required, as the ATmega328 microcontroller in our Arduino has them built-in.  However if you are running a string of devices, use two 10 kilo ohm resistors. Like anything, some testing on a breadboard or prototype circuit will determine their necessity. Sometimes you may see in a particular device’s data sheet the use of different value pull-up resistors – for example 4.7k ohm. If so, heed that advice. The maximum length of an I2C bus is around one metre, and is a function of the capacitance of the bus. This distance can be extended with the use of a special IC, which we will examine during the next I2C chapter.

Each device can be connected to the bus in any order, and devices can be masters or slaves. In our Arduino situation, the board is the master and the devices on the I2C bus are the slaves. We can write data to a device, or read data from a device. By now you should be thinking “how do we differentiate each device on the bus?”… Each device has a unique address. We use that address in the functions described later on to direct our read or write requests to the correct device. It is possible to use two devices with identical addresses on an I2C bus, but that will be discussed in a later article.

As like most devices, we make use of an Arduino library, in this case <wire.h>. Then use the function Wire.begin(); inside of void setup() and we’re ready to go.

Sending data from our Arduino to the I2C devices requires two things: the unique device address (we need this in hexadecimal) and at least one byte of data to send. For example, the address of the part in example 20.1 (below) is 00101111 (binary) which is 0X2F in hexadecimal. Then we want to set the wiper value, which is a value between 0 and 127, or 0x00 and 0x7F in hexadecimal. So to set the wiper to zero, we would use the following three functions:

This sends the device address down the SDA (data) line of the bus. It travels along the bus, and “notifies” the matching device that it has some data coming…

This sends the byte of data to the device – into the device register (or memory of sorts), which is waiting for it with open arms. Any other devices on the bus will ignore this. Note that you can only perform one I2C operation at a time! Then when we have finished sending data to the device, we “end transmission”. This tells the device that we’re finished, and frees up the I2C bus for the next operation:

Some devices may have more than one register, and require more bytes of data in each transmission. For example, the DS1307 real-time clock IC has eight registers to store timing data, each requiring eight bits of data (one byte):

ds1307registers

However with the DS1307  – the entire lot need to be rewritten every time. So in this case we would use eight wire.send(); functions every time. Each device will interpret the byte of data sent to it, so you need the data sheet for your device to understand how to use it.

Receiving data from an I2C device into our Arduino requires two things: the unique device address (we need this in hexadecimal) and the number of bytes of data to accept from the device. Receiving data at this point is a two stage process. If you review the table above from the DS1307 data sheet, note that there is eight registers, or bytes of data in there. The first thing we need to do is have the I2C device start reading from the first register, which is done by sending a zero to the device:

Now the I2C device will send data from the first register when requested. We now need to ask the device for the data, and how many bytes we want. For example, if a device held three bytes of data, we would ask for three, and store each byte in its own variable (for example, we have three variables of type byte: a, b, and c. The first function to execute is:

Which tells the device to send three bytes of data back to the Arduino. We then immediately follow this with:

We do not need to use Wire.endTransmission() when reading data. Now that the requested data is in their respective variables, you can treat them like any ordinary byte variable. For a more detailed explanation of the I2C bus, read this explanatory document by NXP. Now let’s use our I2C knowledge by controlling a range of devices…

The Microchip MCP4018T digital linear potentiometer. The value of this model is 10 kilo ohms. Inside this tiny, tiny SMD part is a resistor array consisting of 127 elements and a wiper that we control by sending a value of between 0 and 127 (in hexadecimal) down the I2C bus. This is a volatile digital potentiometer, it forgets the wiper position when the power is removed. However naturally there is a compromise with using such a small part, it is only rated for 2.5 milliamps – but used in conjunction with op amps and so on. For more information, please consult the data sheet. As this is an SMD part, for breadboard prototyping purposes it needed to be mounted on a breakout board. Here it is in raw form:

mcp4018raw

Above the IC is a breakout board. Consider that the graph paper is 5mm square! It is the incorrect size, but all I have. However soldering was bearable. Put a drop of solder on one pad of the breakout board, then hold the IC with tweezers in one hand, and reheat the solder with the other hand – then push the IC into place. A few more tiny blobs of solder over the remaining pins, and remove the excess with solder wick. Well … it worked for me:

mcp4018cooked

Our example schematic is as follows:

mcp4018sch

As you can see, the part is simple to use, your signal enters pin 6 and the result of the voltage division is found on pin 5. Please note that this is not a replacement for a typical mechanical potentiometer, we can’t just hook this up as a volume or motor-speed control! Again, please read the data sheet.

Control is very simple, we only need to send one byte of data down, the hexadecimal reference point for the wiper, e.g.:

Here is a quick demonstration that moves the wiper across all points:

 and a video demonstration:


Now we will read some data from an I2C device. Our test subject is the ST Microelectronics CN75 temperature sensor. Again, we have another SMD component, but the CN75 is the next stage larger than the part from example 20.1. Thankfully this makes the soldering process much easier, however still requiring some delicate handiwork:

cn75solder1

First, a small blob of solder, then slide the IC into it. Once that has cooled, you can complete the rest and solder the header pins into the breakout board:

cn75solder2ss

Our example schematic is as follows:

cn75schem

Pins 5, 6 and 7 determine the final three bits of the device address – in this case they are all set to GND, which sets the address to 1001000. This allows you to use multiple sensors on the same bus. Pin 3 is not used for basic temperature use, however it is an output for the thermostat functions, which we will examine in the next chapter.

As a thermometer it can return temperatures down to the nearest half of a degree Celsius. Although that may not be accurate enough, it was designed for automotive and thermostat use. For more details please read the data sheet. The CN75 stores the temperature data in two bytes, let’s call them A and B. So we use

with the second parameter as 2, as we want two bytes of data. Which we then store using the following functions:

where *a and *b are variables of the type byte. And as always, there is a twist to decoding the temperature from these bytes. Here are two example pieces of sample data:

The bits in each byte note particular values… the most significant bit (leftmost) of byte A determines whether it is below or above zero degrees – 1 for below zero. The remaining seven bits are the binary representation of the integer part of the temperature; if it is below zero, we subtract 128 from the value of the whole byte and multiply by -1. The most significant bit of byte B determines the fraction, either zero or half a degree. So as you will see in the following example sketch, there is some decision making done in showCN75data():

And here is the result from the serial monitor:

Now that we know how to read and write data to devices on the I2C bus – here is an example of doing both, with a very popular device – the Maxim DS1307 real-time clock IC. Before moving on, consider reading their good data sheet. For those of you new to the world of tronixstuff, we use this part quite often, for example with our Arduino RTC shield and modifications, or blinky – the one-eyed clock. It is an 8-pin DIP IC that allows timing with accuracy down to a few seconds a day:

ds1307small

Furthermore, it also has a programmable square-wave generator. Connection and use is quite simple:

ds1307schem

However some external components are required: a 32.768 kHz crystal, a 3V battery for time retention when the power is off, and a 10k ohm pullup resistor is required if using as a square-wave generator, and 10k ohm pull-up resistors on the SCL and SDA lines. You can use the SQW and timing simultaneously. If we have a more detailed look at the register map for the DS1307:

ds1307registers

We see that the first seven registers are for timing data, the eighth is the square-wave control, and then another eight RAM registers. In this chapter we will look at the first eight only. Hopefully you have noticed that various time parameters are represented by less than eight bits of data – the DS1307 uses binary-coded decimal. But don’t panic, we have some functions to do the conversions for us.

However, in general  – remember that each bit in each register can only be zero or one – so how do we represent a register’s contents in hexadecimal? First, we need to find the binary representation, then convert that to hexadecimal. So, using the third register of the DS1307 as an example, and a time of 12:34 pm – we will read from left to right. Bit 7 is unused, so it is 0. Bit 6 determines whether the time kept is 12- or 24-hour time. So we’ll choose 1 for 12-hour time. Bit 5 (when bit 6 is 0) is the AM/PM indicator – choose 1 for PM. Bit 4 represents the left-most digit of the time, that is the 1 in 12:34 pm. So we’ll choose 1. Bits 3 to 0 represent the BCD version of 2 which is 0010.

So to store 12pm as hours we need to write 00110010 as hexadecimal into the hours register – which is 0x32. Reading data from the DS1307 should be easy for you now, reset the register pointed, then request seven bytes of data and receive them into seven variables. The device address is 0x68.  For example:

At which point the time data will need to be converted to decimal numbers, which we will take care of in the example sketch later. Setting the time, or controlling the square-wave output is another long operation – you need to write seven variables to set the time or eight to change the square-wave output. For example, the time:

The decToBcd is a function defined in our example to convert the decimal numbers to BCD suitable for the DS1307.

You can also address each register individually. We will demonstrate doing this with an explanation of how to control the DS1037’s in built square-wave generator:

Here is the SQW output in action – we measure the frequency using my very old Tek CFC-250:

For further DS1307 examples, I will not repeat myself and instead direct you to the list of many tronixstuff articles that make use of the DS1307.

So there you have it – hopefully an easy to understand introduction to the world of the I2C bus and how to control the devices within. Part two of the I2C tutorial has now been published, as well as an article about the NXP SAA1064 LED display driver IC and the Microchip MC23017 16-bit port expander IC.

LEDborder

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.

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John Boxall

Founder, owner and managing editor of tronixstuff.com.

27 Responses to “Tutorial: Arduino and the I2C bus – Part One”

  1. Jerrar Bukhari says:

    Thank You a Hundred Times … Was Very Very Useful …
    It was both .. very simple and very compact … :)

  2. luis says:

    Hello data sheets show the following addresses,

    MCP4017 ‘0101111’
    MCP4018 ‘0101111’
    MCP4019 ‘0101111′

    Can you connect more than one ? like + = 1 address pulling pin to ground = 2 address ?

    not sure I’ve seeing that in the data sheets

  3. Bela Cseke says:

    Thanks a mill. for this understandable description. That was what I’m looking for. I not seen before, what are the limitations of IIC, for example the number of devices, it’s very helpful for me in my project(s) at work, and I’m definitely happy for the interfacing a digipot to uController part.And of course I’ll sign up for your Google-group for sharing my experience.

    Cheers,
    Bela

  4. cmp says:

    Very well written article – thanks a lot for all your work here!

  5. Tyl says:

    This is a wonderful writeup! Easy to understand, plus you include great examples to show the value. Thank you so much for sharing this!

  6. L’ha ribloggato su Le avventure di TecnoGeppettoe ha commentato:
    Non li conoscevo prima, ma sono componenti molto interessanti. Sto gia’ pensando ad un progettino che utilizzi i potenziomentri digitali….
    TecnoGeppetto!

  7. Ciprian says:

    Hello,

    Nice tutorial. You said something that would be possible to connect more slave devices with the same address to one microcontroller. I tried to look for that in another article, but i couldn’t find a solution.

    My problem is that I would like to connect more I2C Color Sensors based on TCS3414CS. Their address is fixed. Is there any way i can connect like 20 sensors like this?

  8. Doug says:

    Hey John. I’m having trouble understanding a part of your book Arduino Workshop, so I came here looking for some insight. You’ve used the same piece of code in the very similar RTC project above.

    The function is reading from the RTC and uses an asterisk (*) preceding a variable name. I *think* this has to do with a pointer, but I couldn’t find any further understanding in your book, the Arduino Reference (talk about a short wiki page!), or here. Can you explain, or point me to an explanation – no pun intended ;)

    Wire.beginTransmission(0x68); //I get all of this part…
    Wire.write(0);
    Wire.endTransmission();

    Wire.requestFrom(DS1307_I2C_ADDRESS, 7); // grab 7 bytes from the serial interface and load them into these next variables..

    *second = bcdToDec(Wire.read();
    *minute = bcdToDec(Wire.read();
    *hour = bcdToDec(Wire.read();
    *dayOfWeek = bcdToDec(Wire.read());
    *dayOfMonth = bcdToDec(Wire.read());
    *month = bcdToDec(Wire.read());
    *year = bcdToDec(Wire.read());

    // But what are the asterisks doing? What would removing them do?

    Thanks for any help. I’ve really enjoyed your book as it reads the way that my learning works – through the project, discovering one thing after another that I want to learn…
    thanks so much for that.

  9. Electrotwelve says:

    This is a wonderful tutorial. I’m fairly new to the embedded programming world and I was really intimidated by all these programming interfaces. You’ve really made it easy to understand. Thanks!

  10. **SJ** says:

    Hi John,
    i’ve been learning many things from your awesome site… i’m working on Interfacing a Capacitance to Digital converter AD7150
    ( http://www.analog.com/static/imported-files/data_sheets/AD7150.pdf)… i’ve tried from arduino i2c lib Master reader program and some others program from web….. Can u guide me on this…i’ve tried many times on this.. Or how to interface this…Thanks in Advance…

    • John Boxall says:

      Seems pretty straight forward. It’s a 3.3V part so be sure to have a logic level converter on the I2C bus if you have a typical Arduino board, e.g.
      http://www.freetronics.com/collections/modules/products/logic-level-converter-module

      • **SJ** says:

        Thanks for the Quick reply John… i’ve connected the Module 3.3 Vcc directly to 3.3V of Arduino and Gnd,SDA,SCL to Arduino’s Gnd,SDA,SCL.. When running i2c scanner,it finds a device in the Address 0x48… After that i can’t get data from CDC by Master Reader Code even after re wrote that in many ways…

      • **SJ** says:

        Hi John,
        Previously i wanted to using 3.3V logic level converter.Since Arduino Has 3.3V power supply and some of the Sensors i previously worked HMC 6352 compass sensor worked fine with 3.3V ,I’ve connected without using logic level converter.

      • John Boxall says:

        Although the Arduino has a 3.3V supply, the MCU is a 5V unit and this alters the definition of high/low for the I2C bus. Thus my earlier suggestion.

  11. Stephen G. says:

    I’ve run across an oddball item.. It’s a joystick pad that connects to the Wii remote, through the socket for the nunchuck.. It has the same device address, BUT…. none of the values from reading the nunchuck examples I’ve found from the net, return anything. (all 7 values come back 255, trying to read the address for the data from the nunchuck.) Is there a way to ferret-out where the data would be stored? The device is the dreamGEAR Arcade Fighter Micro Mini Arcade Style Joystick for Nintendo Wii … The device does show as device # 0x52, but… the 7 values at memory address 0x42 all stay 0xFF.. I’m still trying to figure how to check other memory addresses, to see if they return anything.. As you can pretty much guess, I’m a major newbie when it comes to I2C.. Thanks,

  12. Steve says:

    In the first example if you want to set wiper value to 0, why do you send 69 down the line?

    • John Boxall says:

      After three and a half years you’re the first person to pick up on that error. Yes, it’s zero. Thanks :)

      • Steve says:

        No problem. I’m new to this and your tutorials are much easier to understand than most. I really appreciate all the effort that must go into making these. I actually got my module working without an RTC library! (I prefer to know what’s actually going on in the chip).
        Thanks again,
        Steve

      • John Boxall says:

        Thank you very much for your notes. It’s great to hear that you understand the benefit of not relying on libraries blindly to get things done.

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