A description of the analog input pins on an Atmega168 (Arduino chip).
The Atmega168 contains an onboard 6 channel analog-to-digital (A/D) converter. The converter has 10 bit resolution, returning integers from 0 to 1023. While the main function of the analog pins for most Arduino users is to read analog sensors, the analog pins also have all the functionality of general purpose input/output (GPIO) pins (the same as digital pins 0 - 13).
Consequently, if a user needs more general purpose input output pins, and all the analog pins are not in use, the analog pins may be used for GPIO.
The Arduino pin numbers corresponding to the analog pins are 14 through 19. Note that these are Arduino pin numbers, and do not correspond to the physical pin numbers on the Atmega168 chip. The analog pins can be used identically to the digital pins, so for example, to set analog pin 0 to an output, and to set it HIGH, the code would look like this:
pinMode(14, OUTPUT); digitalWrite(14, HIGH);
The analog pins also have pullup resistors, which work identically to pullup resistors on the digital pins. They are enabled by issuing a command such as
digitalWrite(14, HIGH); // set pullup on analog pin 0
while the pin is an input.
Be aware however that turning on a pullup will affect the value reported by analogRead() when using some sensors if done inadvertently. Most users will want to use the pullup resistors only when using an analog pin in its digital mode.
The analogRead command will not work correctly if a pin has been previously set to an output, so if this is the case, set it back to an input before using analogRead. Similarly if the pin has been set to HIGH as an output.
The Atmega168 datasheet also cautions against switching digital pins in close temporal proximity to making A/D readings (analogRead) on other analog pins. This can cause electrical noise and introduce jitter in the analog system. It may be desirable, after manipulating analog pins (in digital mode), to add a short delay before using analogRead() to read other analog pins.