Arduino/libraries/Firmata/examples/SimpleAnalogFirmata/SimpleAnalogFirmata.ino

/* Supports as many analog inputs and analog PWM outputs as possible.
 *
 * This example code is in the public domain.
 */
#include <Firmata.h>

byte analogPin = 0;

void analogWriteCallback(byte pin, int value)
{
    if (IS_PIN_PWM(pin)) {
        pinMode(PIN_TO_DIGITAL(pin), OUTPUT);
        analogWrite(PIN_TO_PWM(pin), value);
    }
}

void setup()
{
    Firmata.setFirmwareVersion(0, 1);
    Firmata.attach(ANALOG_MESSAGE, analogWriteCallback);
    Firmata.begin(57600);
}

void loop()
{
    while(Firmata.available()) {
        Firmata.processInput();
    }
    // do one analogRead per loop, so if PC is sending a lot of
    // analog write messages, we will only delay 1 analogRead
    Firmata.sendAnalog(analogPin, analogRead(analogPin)); 
    analogPin = analogPin + 1;
    if (analogPin >= TOTAL_ANALOG_PINS) analogPin = 0;
}