analogRead()

Used to perform ADC conversion.

Library Documentation

uint32 analogRead(uint8 pin)

Read an analog value from pin.

This function blocks during ADC conversion, and has 12 bits of resolution. The pin must have its mode set to INPUT_ANALOG. Ignoring function call overhead, conversion time is 55.5 cycles.

Parameters

• pin -

  Pin to read from. One of: 0, 1, 2, 3, 10, 11, 12, 13, 15, 16, 17, 18, 19, 20, 27, 28.

Return

ADC-converted voltage, in the range 0–4095, inclusive (i.e. a 12-bit ADC conversion).

See

pinMode()

Discussion

Reads the value from the specified analog pin. The Maple board contains a 16-channel, 12-bit analog to digital converter. This means that it will map input voltages between 0 and 3.3 volts into integer values between 0 and 4095. This yields a resolution between readings of 3.3V / 4096 units, or 0.8 millivolts. However, a number of factors interfere with getting full accuracy and precision. For more information, see ADC.

Before calling analogRead() on a pin, that pin must first be configured for analog input, using pinMode() (you only have to do this once, so it’s usually done in setup()).

Parameter Discussion

The pin parameter is the number of the analog input pin to read from. Header pins on the Maple with ADC functionality (marked as “AIN” on the silkscreen) are:

0, 1, 2, 3, 10, 11, 12, 13, 15, 16, 17, 18, 19, 20, 27, 28

Note that pins 3, 27, and 28 are not marked AIN on the silkscreen for Maple revisions through Rev 5, however, they do work as analog input pins.

Note

If the analog input pin is not connected to anything, the value returned by analogRead() will fluctuate based on a number of factors (e.g. the values of the other analog inputs, how close your hand is to the board, etc.) in a seemingly random way.

Example

int analogPin = 3;     // potentiometer wiper (middle terminal) connected
                       // to analog pin 3. outside leads to ground and +3.3V
int val = 0;           // variable to store the value read

void setup() {
  pinMode(analogPin, INPUT_ANALOG); // set up pin for analog input
  SerialUSB.begin();                // set up usb virtual COM port
}

void loop() {
  val = analogRead(analogPin);    // read the input pin
  SerialUSB.println(val);         // print the value, for debugging with
                                  // a serial monitor
}

Arduino Compatibility

The Arduino board contains a 6 channel (8 channels on the Mini and Nano, 16 on the Mega), 10-bit analog to digital converter with an input voltage range of 0V–5V. This means that it will map input voltages between 0 and 5 volts (which is larger than Maple’s range of 0V-3.3V) into integer values between 0 and 1023 (which is smaller than the Maple’s range of 0–4095).

This yields a theoretical resolution between readings of: 5 volts / 1024 units or .0049 volts (4.9 mV) per unit on Arduino boards, which is larger, and thus less precise, than Maple’s 0.0008 volts (0.8 mV).

If your program expects Arduino-style 10-bit ADC, you can right shift the value of a Maple readout by 2, like so:

// right shift means that the result will be between 0 and 1023;
// be aware that you're losing a lot of precision if you do this
int adc_reading = analogRead(pin) >> 2;

On the Arduino, the input range and resolution can be changed using their implementation of analogReference(). Because of the way its hardware (as of Rev 5) was designed, it’s not possible to implement analogReference on the Maple, so this function doesn’t exist. If your inputs lie in a different voltage range than 0V–3.3V, you’ll need to bring them into that range before using analogRead. Some basic tools to accomplish this are resistor dividers and Zener diodes. However, opamps and other powered components can also be used if greater precision is required.

See also

• ADC note
• (Arduino) Tutorial: Analog Input Pins

License and Attribution

This documentation page was adapted from the Arduino Reference Documentation, which is released under a Creative Commons Attribution-ShareAlike 3.0 License.