Arduino Tutorials – Chapter 22 – the AREF pin

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

In this tutorial 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 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():

`analogReference(EXTERNAL); // use AREF for reference voltage`

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:

`analogReference(DEFAULT);`

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:

```#include <LiquidCrystal.h>
LiquidCrystal lcd(8,9,4,5,6,7);

int analoginput = 0; // our analog pin
int analogamount = 0; // stores incoming value
float percentage = 0; // used to store our percentage value
float voltage =0; // used to store voltage value

void setup()
{
lcd.begin(16, 2);
analogReference(EXTERNAL); // use AREF for reference voltage
}

void loop()
{
lcd.clear();
analogamount=analogRead(analoginput);
percentage=(analogamount/1024.00)*100;
voltage=analogamount*3.222; // in millivolts
lcd.setCursor(0,0);
lcd.print("% of AREF: ");
lcd.print(percentage,2);
lcd.setCursor(0,1);
lcd.print("A0 (mV): ");
lcd.println(voltage,2);
delay(250);
}```

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:

`analogReference(INTERNAL);`

For Arduino Mega boards, use:

`analogReference(INTERNAL1V1);`

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:

`analogReference(INTERNAL2V56);`

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.

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Tutorial: Your Arduino’s inbuilt EEPROM

In this article we are going to examine the internal EEPROM in our Arduino boards. What is an EEPROM some of you may be saying? An EEPROM is an Electrically Erasable Programmable Read-Only Memory.

It is a form of non-volatile memory that can remember things with the power being turned off, or after resetting the Arduino. The beauty of this kind of memory is that we can store data generated within a sketch on a more permanent basis.

Why would you use the internal EEPROM?

For situations where data that is unique to a situation needs a more permanent home. For example, storing the unique serial number and manufacturing date of a commercial Arduino-based project – a function of the sketch could display the serial number on an LCD, or the data could be read by uploading a ‘service sketch’. Or you may need to count certain events and not allow the user to reset them – such as an odometer or operation cycle-counter.

What sort of data can be stored?

Anything that can be represented as bytes of data. One byte of data is made up of eight bits of data. A bit can be either on (value 1) or off (value 0), and are perfect for representing numbers in binary form. In other words, a binary number can only uses zeros and ones to represent a value. Thus binary is also known as “base-2″, as it can only use two digits.

How can a binary number with only the use of two digits represent a larger number? It uses a lot of ones and zeros. Let’s examine a binary number, say 10101010. As this is a base-2 number, each digit represents 2 to the power of x, from x=0 onwards:

See how each digit of the binary number can represent a base-10 number. So the binary number above represents 85 in base-10 – the value 85 is the sum of the base-10 values. Another example – 11111111 in binary equals 255 in base 10.

Now each digit in that binary number uses one ‘bit’ of memory, and eight bits make a byte. Due to internal limitations of the microcontrollers in our Arduino boards, we can only store 8-bit numbers (one byte) in the EEPROM.

This limits the decimal value of the number to fall between zero and 255. It is then up to you to decide how your data can be represented with that number range. Don’t let that put you off – numbers arranged in the correct way can represent almost anything!

There is one limitation to take heed of – the number of times we can read or write to the EEPROM. According to the manufacturer Atmel, the EEPROM is good for 100,000 read/write cycles (see the data sheet).

One would suspect this to be a conservative estimate, however you should plan accordingly. *Update* After some experimentation, the life proved to be a lot longer

Now we know our bits and and bytes, how many bytes can be store in our Arduino’s microcontroller? The answer varies depending on the model of microcontroller. For example:

• Boards with an Atmel ATmega328, such as Arduino Uno, Uno SMD, Nano, Lilypad, etc. – 1024 bytes (1 kilobyte)
• Boards with an Atmel ATmega1280 or 2560, such as the Arduino Mega series – 4096 bytes (4 kilobytes)
• Boards with an Atmel ATmega168, such as the original Arduino Lilypad, old Nano, Diecimila etc – 512 bytes.

If you are unsure have a look at the Arduino hardware index or ask your board supplier.

If you need more EEPROM storage than what is available with your microcontroller, consider using an external I2C EEPROM as described in the Arduino and I2C tutorial part two.

At this point we now understand what sort of data and how much can be stored in our Arduino’s EEPROM. Now it is time to put this into action. As discussed earlier, there is a finite amount of space for our data. In the following examples, we will use a typical Arduino board with the ATmega328 with 1024 bytes of EEPROM storage.

To use the EEPROM, a library is required, so use the following library in your sketches:

`#include "EEPROM.h"`

The rest is very simple. To store a piece of data, we use the following function:

`EEPROM.write(a,b);`

The parameter a is the position in the EEPROM to store the integer (0~255) of data b. In this example, we have 1024 bytes of memory storage, so the value of a is between 0 and 1023. To retrieve a piece of data is equally as simple, use:

`z = EEPROM.read(a);`

Where z is an integer to store the data from the EEPROM position a. Now to see an example.

This sketch will create random numbers between 0 and 255, store them in the EEPROM, then retrieve and display them on the serial monitor. The variable EEsize is the upper limit of your EEPROM size, so (for example) this would be 1024 for an Arduino Uno, or 4096 for a Mega.

```// Arduino internal EEPROM demonstration

#include <EEPROM.h>
int zz;
int EEsize = 1024; // size in bytes of your board's EEPROM

void setup()
{
Serial.begin(9600);
randomSeed(analogRead(0));
}
void loop()
{
Serial.println("Writing random numbers...");
for (int i = 0; i < EEsize; i++)
{
zz=random(255);
EEPROM.write(i, zz);
}
Serial.println();
for (int a=0; a<EEsize; a++)
{
zz = EEPROM.read(a);
Serial.print("EEPROM position: ");
Serial.print(a);
Serial.print(" contains ");
Serial.println(zz);
delay(25);
}
}```

The output from the serial monitor will appear as such:

So there you have it, another useful way to store data with our Arduino systems. Although not the most exciting tutorial, it is certainly a useful.

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