I'm trying to use the timer registers to do some timing on my Arduino Mega 2560.

The output of the following code confuses me:

void setup ()
    // Set up serial connection
    while (!Serial) {}

    // Print control registers of timer 1
    Serial.println(TCCR1A); // Outputs 1
    Serial.println(TCCR1B); // Outputs 3

void loop () {}

Why is TCCR1A equal to 1 and TCCR1B equal to 3? The documentation of the chip says the initial values of the registers are zero. I could of course set them to zero, but I want to understand why it works like this. The values are the same for the other 16-bit timers (3, 4 and 5).

1 Answer 1


The Arduino core does things at startup. One of those things is to configure the timers ready for PWM operation.

Here's the relevant bits from init() in wiring.c:

    // set timer 1 prescale factor to 64
    sbi(TCCR1B, CS11);
#if F_CPU >= 8000000L
    sbi(TCCR1B, CS10);

#if defined(TCCR1A) && defined(WGM10)
    sbi(TCCR1A, WGM10);

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