[AVR] delay_ms in ASF wizard of AVR Studio6.2 is giving inaccurate delay

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Mandar Joshi

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I am using STK600(ATmega2560) and ASF wizard for my project. I have written simple code for LED blinking. But whenever I use delay_ms from ASF it gives inaccurate delay.
As I am using ASF, I haven't defined F_CPU manually and default clock for ATmega2560 is 8MHz as per datasheet. So direct parameter passing to delay_ms() should give appropriate delay but it doesn't.

Fuse values are

Extended 0xFF
High 0x99
Low 0xE2


Is anything wrong with my fuse settings? I have another code for uart but it works fine with same fuse settings and it confirms default clock is 8MHz. So why I am getting inaccurate delays?

My code for LED blink is as follows:
Code: 
#include <asf.h>

int main (void)
{
	// Insert system clock initialization code here (sysclk_init()).

	board_init();

	DDRB=0xff;
	
	while(1)
   {
	 PORTB=0x00;
	 
	 delay_ms(2500);
	 
	 PORTB=0xff;
	 
	 delay_ms(2500);  
   }	
	
	// Insert application code here, after the board has been initialized.
}

- - - Updated - - -

How can I find what F_CPU it is taking by default? I don't have any debugger.
 

it seems your fuses are OK (internal 8Mz without DIV8) and I can guess your led takes about 8 times slower to blink (instead of 2.5s, it takes 20s) if so, you're right, the asf routine isn't getting the right value for sysclk_get_cpu_hz or something related...

sadly i can't help you more, but maybe you can try something...

in the old days we used <util/delay.h> to make quick delays, that avr-libc library needed a F_CPU constant, something similar to:

Code: 
#define F_CPU 8000000UL
#include <avr/io.h>
#include <util/delay.h>

void delay_ms(n){
   while(n--) _delay_ms(1); // _delay_ms can't take big arguments...
}

int main (void)
{

        //	board_init();

	DDRB=0xff;
	
	while(1)
   {
	 PORTB=0x00;
	 
	 delay_ms(2500);
	 
	 PORTB=0xff;
	 
	 delay_ms(2500);  
   }	

}

I'm sure this should work but we are not using that ASF thingie that maybe you are further using...
 

Klaus, inaccurate means as Kurenai_ryu said if I use delay_s() and pass 24 to it LED blinks at 3 seconds. But then I tried by changing CKDIV8 fuse then it works proper for few delays and again fails if I increase delay parameters.

There has to be some kind of limit to parameters we pass to delay functions. But unable to figure out what's the problem is?

And as I said earlier when CKDIV8 is not programmed clock is 8MHz, so after programming CKDIV8 it is 1MHz which I have confirmed from another small uart code.
 

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