Crazy high frequency PWM with ATMega32U4

Timer 4 on the ATMega32U4 is a high speed one, but I have never tried it out before. Now that I have a scope, I decided to toy with it. It can work from the internal PLL, meaning some really high frequencies can be achieved. I managed to get up to 8Mhz, but probably more can be achieved with setting the PLL to even higher frequency. I tried out frquencies from 10Hz to 8Mhz. Anything above 700Khz is less accurate (or at least I couldn’t make it better), below that the frequency generated is pretty well matched. Here are couple tests with 1.712Mhz 33%Duty:

123456hz, 50% Duty:

So overall interesting experiment, I am not sure if it has any practical value other than testing my o-scope. Here is my code, if someone wants to try it out, I tested it on a Funky v2:

#define TIMER4_RESOLUTION 1023UL
#define PLL_FREQ 48000000UL

unsigned long pwmPeriod = 0;
ISR(TIMER4_OVF_vect) {}

void enable_intr(){
    TIMSK4 = _BV(TOIE4);
}

void disable_intr(){
    TIMSK4 = 0;
}

void initialize(unsigned long freq) {

    /* Init the internal PLL */
   PLLFRQ = _BV(PDIV2);
   PLLCSR = _BV(PLLE);
   while(!(PLLCSR & _BV(PLOCK)));
   PLLFRQ |= _BV(PLLTM0); /* PCK 48MHz */

   TCCR4A = (1<<PWM4A);  
   TCCR4E = (1<<ENHC4); 
   TCCR4D = (1<<WGM40); //set it to phase and frequency correct mode    
   TCCR4C = 0;     
   setPeriod(freq);    
} 
void setPwmDuty(unsigned int duty) { 	
   unsigned long dutyCycle = pwmPeriod; 	
   dutyCycle *= duty; 	
   dutyCycle >>= 9;
   TC4H = (dutyCycle) >> 8; 
   OCR4A = (dutyCycle) & 255; 
}

void start() {
  TCCR4A |= _BV(COM4A1);
}
void stop()  {
  TCCR4A &= ~(_BV(COM4A1));
}

void setPeriod(unsigned long freq)  {
unsigned long cycles = PLL_FREQ / 2 / freq;
unsigned char clockSelectBits = 0;

        if (cycles < TIMER4_RESOLUTION) {
		clockSelectBits = _BV(CS40);
		pwmPeriod = cycles;
	} else
	if (cycles < TIMER4_RESOLUTION * 2) {
		clockSelectBits = _BV(CS41);
		pwmPeriod = cycles / 2;
	} else
	if (cycles < TIMER4_RESOLUTION * 4) {
		clockSelectBits = _BV(CS41) | _BV(CS40);
		pwmPeriod = cycles / 4;
	} else
	if (cycles < TIMER4_RESOLUTION * 8) {
		clockSelectBits = _BV(CS42);
		pwmPeriod = cycles / 8;
	} else
	if (cycles < TIMER4_RESOLUTION * 16) {
		clockSelectBits = _BV(CS42) | _BV(CS40);
		pwmPeriod = cycles / 16;
	} else 
        if (cycles < TIMER4_RESOLUTION * 32) {
		clockSelectBits = _BV(CS42) | _BV(CS41);
		pwmPeriod = cycles / 32;
	} else 	
        if (cycles < TIMER4_RESOLUTION * 64) {
		clockSelectBits = _BV(CS42) | _BV(CS41) | _BV(CS40);
		pwmPeriod = cycles / 64;
	} else 	
        if (cycles < TIMER4_RESOLUTION * 128) {
		clockSelectBits = _BV(CS43);
		pwmPeriod = cycles / 128;
	} else 	
        if (cycles < TIMER4_RESOLUTION * 256) {
		clockSelectBits = _BV(CS43) | _BV(CS40);
		pwmPeriod = cycles / 256;
	} else 	
        if (cycles < TIMER4_RESOLUTION * 512) {
		clockSelectBits = _BV(CS43) | _BV(CS41);
		pwmPeriod = cycles / 512;
	} else 	
        if (cycles < TIMER4_RESOLUTION * 1024) {
		clockSelectBits = _BV(CS43) | _BV(CS41) | _BV(CS40);
		pwmPeriod = cycles / 1024;
	} else 	
        if (cycles < TIMER4_RESOLUTION * 2048) {
		clockSelectBits = _BV(CS43) | _BV(CS42);
		pwmPeriod = cycles / 2048;
	} else 	
        if (cycles < TIMER4_RESOLUTION * 4096) {
		clockSelectBits = _BV(CS43) | _BV(CS42) | _BV(CS40);
		pwmPeriod = cycles / 4096;
	} else 	
        if (cycles < TIMER4_RESOLUTION * 8192) {
		clockSelectBits = _BV(CS43) | _BV(CS42) | _BV(CS41);
		pwmPeriod = cycles / 8192;
	} else 	
        if (cycles < TIMER4_RESOLUTION * 16384) { 		clockSelectBits = _BV(CS43) | _BV(CS42) | _BV(CS41) | _BV(CS40); 		pwmPeriod = cycles / 16384; 	}  /*else    		clockSelectBits = _BV(CS43) | _BV(CS42) | _BV(CS41) | _BV(CS40); 		pwmPeriod = TIMER4_RESOLUTION - 1; */	         TCCR4B = clockSelectBits;             TC4H = pwmPeriod >> 8; 
        OCR4C = pwmPeriod; 

    }

void setup() {

pinMode(13, OUTPUT);
digitalWrite(13,LOW);
initialize(123456);  // Frequency to generate in Hz
setPwmDuty(512);     // Duty cycle 0-1024, 1024 is 100%, 512 is 50%
start();

}
void loop(){
}