prescaler calculation atmega328p

prescaler.c

// system clock frequency
#define F_CPU 16000000UL


void timer_freq_prescale(uint32_t a_freq, uint8_t *a_ocr, uint8_t *a_prescaler) {
	// prescaler table for timer 0
	uint8_t prescalers[] = { 0x00, 0x03, 0x06, 0x08, 0x0a, 0x00 };

	uint16_t ocr = 0x00;
	uint8_t prescaler = 0x00;

	do {
		ocr = (uint16_t) (F_CPU / ((a_freq << 1) * (0x01 << prescalers[prescaler])));
		++prescaler;		
	} while ((ocr > 255) && (prescalers[prescaler]));

	--ocr;
	if (ocr > 255) ocr = 255;

	*a_ocr = ocr & 0xff;
	*a_prescaler = prescaler;
}