Generate a Clock Signal with AVR ATmega

AVR-generated clock signal

Several times lately my MCU needed to generate a clock signal to interface with a device, such as a camera, or a serial interface on an analog to digital converter (ADC).

Rather than bit-banging the clock signal, let the MCU's timer hardware do the work, freeing up cycles for real code.

Here's how to generate a simple, 50% duty cycle pulse train, aka clock signal, using an AVR MCU. For this experiment I used an ATmega328P but any of the AVR chips that support a 16-bit Timer1 should do.

I wanted a 500kHz clock signal. To generate it, the MCU must toggle an output pin, PB1 aka OC1A at 1MHz, or 1/16 the MCU's 16MHz clock, for a total period of 2us (500kHz).

Timer1 provides a mode called Clear Timer on Compare Match (CTC). The timer register, TCNT1 counts from 0 up to the value in the Output Compare Register, in this case OCR1A (to go along with the OC1A output). When TCNT1 == OCR1A, the MCU resets TCNT1 and starts counting again.

Ok, let's get started with the code.

Though we probably don't need to, why not initialize the counter:

TCNT1=0;

To run the timer at 1MHz, we need to divide the MCU clock by 16. Using a prescaler value of 1, we simply set the output compare register to 15, since the timer counts from 0-15 or 16 ticks.

So we come up with:

OCR1A = 15;

Note that we could also have used a prescaler value of 8 and an OCR1A of 1.

Now it's time to set some mode bits. The AVR has three Timer/Counter Control Registers for timer 1: TCCR1A, TCCR1B and, you guessed it, TCCR1C.  In TCCR1A, two bits control the behavior of OC1A when the timer matches OCR1A. Those two bits are COM1A1 and COM1A0. When set to 01, OC1A is toggled when there's a compare match.

TCCR1A |= (1<<COM1A0);

To tell Timer1 to operate in CTC mode, we set bits WGM13, WGM12, WGM11, WGM10 across two control registers. Actually for CTC mode, WGM12=1 and the rest are 0 (initial value on powerup)

TCCR1B |= (1<<WGM12);

To configure Timer1 prescaling, set bits CS12, CS11 and CS10 in TCCR1B. For no prescaling, use 001, respectively. That is, set CS10=1

TCCR1B |= (1<<CS10);

The only thing left to do is to enable the OC1A (PB1) pin for output

DDRB |= _BV(1);

Putting it all together, here's the code that generates the clock signal in the picture above.

TCNT1=0;
OCR1A = 15;
TCCR1A |= (1<<COM1A0);
TCCR1B |= (1<<CS10) | (1<<WGM12);
DDRB |= _BV(1);


Finally, if you want to sync your code to the rising and falling clock edge, check the TIFR1 register for the OCF1A flag:

if ((TIFR1 & _BV(1)) == _BV(1)) {
// do something here
}

Or, you can sync your code to a high or low clock value by reading PB1 from the PINB register.  The MCU can also be set up to call an interrupt service routine whenever there's a match.