9.1: Introduction

In this exercise we shall examine a method of obtaining analog input signals such as those generated by various continuous sensors. Continuously variable sensors might be used to measure temperature, force, acceleration, and so forth. This ability shall be combined with the digital input and output techniques already examined to produce a system that indicates an applied force via a string of LEDs; The greater the applied force to the sensor, the greater the LED indication.

Microcontrollers rely on analog to digital converters (ADCs) to obtain continuously variable input data. The ATmega 328P contains a 10 bit ADC along with a six channel multiplexer. These six lines are brought out to the Arduino Uno’s analog input header; pins A0 through A5. By default, the reference voltage for the converter is the on-board five volt DC source. This represents the maximum input level that may be digitized. The minimum level is ground or zero volts (if negative signals are needed, some form of offset bias will be required). A 10 bit converter yields \(2^{10}\) or 1024 discrete levels. This means that each step will be slightly less than five millivolts.

To use the ADC, a reference voltage must be set during the initialization routine. After this, whenever a value is needed, simply call the analogRead() function. The only argument to the function is the input channel desired (zero through five for the Uno). The function returns the digitized value as an integer in the range of 0 to 1023. While it is possible to go direct to the hardware registers to access the ADC, the Arduino library is fairly efficient and we will save only a few bytes, if any, by avoiding it¹.

We shall start with something straightforward to illustrate how to read an analog input pin. Note that unlike the digital ports, initially setting pinMode() or a data direction register (DDRx) is not required when using the analogRead() function.

Connect the outer pins of a 10 k\(\Omega\) potentiometer to the +5V and ground terminals on the Uno’s header. Connect the wiper to analog pin 0 (A0). Enter the code below, compile and transfer it to the board.

/* Read analog V1. Reads voltage off of a pot and prints the digitized value 
to the Serial Monitor. Range = 0->1024 */

#define ANALOG_IN_PIN 0

int prior = 0; // remembers prior value of analog input

void setup()
{
  Serial.begin(9600);
  analogReference( DEFAULT );
}
void loop()
{
      int a;

      a = analogRead( ANALOG_IN_PIN );

      if( a != prior )
      {
            Serial.println(a);
            prior = a;
      }
}

The code is self-explanatory. The interesting twist is that instead of simply printing the analog value, the value is compared to the previous value and only if they are different is the result printed. This saves some processing effort and speeds things up.

Open the Serial Monitor. Turn the pot shaft clockwise and counterclockwise. The values on the Serial Monitor should change as the shaft is adjusted. The values should track from 0 up to 1023 (you might be off by a count due to noise). Quite literally, you could determine the actual potentiometer wiper voltage by multiplying the printed value by the step size (5V/1024 or roughly 4.883 millivolts per step), thus achieving a very rudimentary DC voltmeter.

References

¹See “Bits & Pieces: analogRead” in the text for details.