Why are Arduino timer registers not 0?

Question

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
    Serial.begin(9600);
    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).

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);
#endif

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