frequency generation tests

Projects General Guidance
1 
void setup(){
  pinMode (2, OUTPUT);
  cli();
}
void loop(){
    digitalWrite (2, !digitalRead(2));
}

59.042 KHz

void setup(){
  pinMode (2, OUTPUT);
  cli();
}
void loop(){
  while(1){
    digitalWrite (2, !digitalRead(2));
  }
}

63.7466 KHz

void setup(){
  pinMode (2, OUTPUT);
  cli();
}
void loop(){
  while(1){
    PORTD = PORTD - 0b00000100;  // also PORTD = PORTD ^ 0b00000100;
  }
}

1.60004 MHz!

void setup(){
  pinMode (2, OUTPUT);
  cli();
}
void loop(){
  while(1){
    PIND = PIND | 0b00000100; // also PIND |= 0b00000100;
  }
}

2.00005MHz!

void setup(){
  pinMode (2, OUTPUT);
  //cli();
}
void loop(){
  while(1){
    PIND = PIND | 0b00000100;
  }
}

1.988MHz with interrupt jitter

void setup(){
  pinMode (2, OUTPUT);
  cli();
}
void loop(){
  //while(1){
    PIND = PIND | 0b00000100; 
  //}
}

571.443 KHz - loop() really slows things down!
Is PWM the only other controllable high frequency solution? (i.e. w/o setting the system clock out fuse?)

2MHz.jpg
2MHz.jpg960×720 64.3 KB

2

Lower frequencies are no problem :wink:

If your controller should do more than only provide the signal, using timer/counter hardware offers fastest continuous operation (CTC mode).

3

Ok, not familiar with coding for that.

4

loop() really slows things down!

that's because loop() is called from main() in a tight loop that also checks Serial. And you have the overhead of a function call

int main(void)
{
	init();

	initVariant();

#if defined(USBCON)
	USBDevice.attach();
#endif
	
	setup();
    
	for (;;) {
		loop();
		if (serialEventRun) serialEventRun();
	}
        
	return 0;
}
5

BTW have you tried GOTO loop?

void setup(){
  pinMode (2, OUTPUT);
  cli();
}
void loop(){
    label:
    PIND = PIND | 0b00000100; 
    goto label;
}
6

"571.443 KHz - loop() really slows things down!"

I'm not a C expert but looks to me as if while() is speeding things up rather than loop() slowing it down. Very strange behaviour from the compiler!

I guess if you want faster output you need to go to assembly code. Is it possible to embed an assembly code routine with the Arduino IDE?

Russell.

7

I'm not surprised that loop is slowing it down - every iteration requires calling a function, whereas with just the while loop, it doesn't...

I think you can go up to half the system clock using fast pwm mode (or 32mhz, half the fast peripheral clock, if you're using a part with on-chip PLL using the timer that can be clocked off that)

8

russellz:
Is it possible to embed an assembly code routine with the Arduino IDE?

Russell.

see - Programming  Arduino in assembly language - Development - Arduino Forum -

9

After all, what's the aim of this topic?

The closer we reach the clock frequency, the less frequency variations are possible. From a 16 MHz clock you can derive only 8, 4 or 2 MHz, before lower frequencies allow for scale factors other than 2.

10

I do miss the notification of replies feature!
The question comes up frequently as to what frequency can be generated,
for example here looking for 10KHz plus inverted signal,
http://forum.arduino.cc/index.php?topic=324673.msg2243199#msg2243199
I wanted to see for myself, so I tried a few things and posted the results.
I'd only gone to 20KHz previously using blink-without-delay to simulate a camera clock output for a project I was creating.