These notes apply to the [https://www.microchip.com/en-us/product/ATMEGA328P ATMega328P] (Uno etc) using the Arduino IDE and libraries unless otherwise noted.

The 328 has 3 timers:

'''Timer0''' (8 bits)

Used for {{{delay()}}}, {{{millis()}}} and {{{micros()}}}.  Don't mess with it.

'''Timer1''' (16 bits)

Not normally used.  For simple use, see the [https://www.arduino.cc/reference/en/libraries/timerone/ TimerOne] library.

'''Timer2''' (8 bits)

Used by e.g. the {{{tone()}}} function.