AVR's precise delay program

In C language, it is not easy to use an accurate delay program. In IAR, there is a function called __delay_cycles(), which is defined in the header file intrinsics.h. The function is to delay N instruction cycles. Based on this function, an accurate delay function can be implemented (but it cannot be 100% accurate).

Implementation method:

Create a delay.h header file:

#ifndef __IAR_DELAY_H
#define __IAR_DELAY_H

#include

#define XTAL 8 //Can be defined as the crystal frequency you use (unit: MHz) 

#define delay_us(x) __delay_cycles ( (unsigned long)(x * XTAL) ) 
#define delay_ms(x) __delay_cycles ( (unsigned long)(x * XTAL*1000UL) )
#define delay_s(x) __delay_cycles ( (unsigned long) (x * XTAL*1000000UL) )

#endif 

Note: __delay_cycles(x), x must be a constant or a constant expression. If it is a variable, a compilation error will occur!

Authentication method:

     1. Set up the IAR compiler as follows:

 After entering the software simulation by pressing Ctrl+D, you can call up the running time of the analysis function in the main menu View->Proifing.

Press the Activate button in the red circle in the picture 

, and also turn on "Auto Refresh" on the far right. After running for a while with F5 (Debug->Go), press Debug->Break to interrupt the execution of the program, and the cycles of all functions can be listed.

2. Write a test function

Empty Function

 void delay()
{   
}

It is found that the instruction cycle used by this empty function is 4. I used IAR AVR 5.20 to test it.

Try different test values, test us, ms, s level delays, and then subtract 4 from the instruction cycle of delay() to get the instruction cycle executed by delay_us(), delay_ms(), delay_s()

  void delay()
{   
   delay_us(100);

   //delay_ms(100);

  // delay_s(100);
}

The test found that the accuracy is relatively high and the error is less than 1us.

With this method, you don't need to adjust the delay program when changing the crystal oscillator!