[SOLVED] CRO VERIFIED ACCURATE 1usec C-CODE DELAY... Please

[abdul991]

hi, all...

Can anyone.. give me C-program function for a perfect 1usec or 2usec Delay using 12Mhz-PIC18f ,MPLAB.. Which is "verified using CRO.."
any other less or equal to 20Mhz Xtal is also OK....
Please..

 

[jayanth.devarayanadurga]

The 18F4550 is running at 20MHz oscillator frequency, but the internal frequency is 48MHz

4/48 MHz = 0.0833333333333333 us

1 instruction cycle is 0.0833333333333333 us

0.0833333333333333 us * x = 1 us

x = 12

0.0833333333333333 us * y = 2 us

y = 24

1 nop instruction is 0.0833333333333333 us

so call 12 nop instructions for 1 us delay and 24 nop instructions for 2 us delay.

---

Code C - [expand]
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void delay_1us(){
    _asm{
        nop
        nop
        nop
        nop
        nop
        nop
        nop
        nop
        nop
        nop
        nop
        nop
    }
}
 
//call delay_1us once for is and twice for 2 us delay.

---

 

[abdul991]

Thanks jayanth... i didn;t understand the part where you mentioned internal frequency is 48MHz although you are using 20MHz xtal.

But lately after much struggle i was allowed to use a CRO to check my delay program. Here is the 'verified' o/p. I use 12Mhz and 18f452
mplab

void main(void)
{
 int i;
         TRISBbits.TRISB7 = 0;

		 while(1){
         
         PORTBbits.RB7=0;
         //for(i=0;i<3;i++);
         DS1820_DelayUs(3);
         PORTBbits.RB7=1;
         //for(i=0;i<3;i++);
         DS1820_DelayUs(3);
       }
}


void  Delay(unsigned char a)
{
   int i,j;
   for(i = 0; i < a; i++)
   for( j = 0; j < 3; j++);
} 
     
void DS1820_DelayUs(int val)
{
int k;
for(k=0;k<val;k++); 
} 



/*******  

 
         Delay(1);       52usec LOW  AND  52USEC HIGH
         
         Delay(2);       88USEC LOW  88USEC HIGH
         Delay(3);       125USEC LOW  125USEC HIGH
 36 USEC INRCM FOR EACH SINGLE DELAY WITH FUNCTION CALLING TIME 16USEC
 **********************************
  PORTBbits.RB7=0;
         i=7;
         PORTBbits.RB7=1;
         i=5;    //     1.4usec low and 2.1usec high.       

 **********************************

for(i=0;i<1;i++); 13usec LOW  AND  14USEC HIGH
for(i=0;i<2;i++); 19usec LOW  AND  20USEC HIGH
for(i=0;i<3;i++); 26usec LOW  AND  26USEC HIGH

EACH FOR LOOP COUNT IS 7USEC DELAY

**********************************

DS1820_DelayUs(1);  25usec low and 26usec high
DS1820_DelayUs(2);  35usec low and 36usec high
DS1820_DelayUs(3);  44usec low and 45usec high

            ******/

 

[jayanth.devarayanadurga]

Please see datasheet of PIC18F4550. Fosc is external quarts frequency. FCPU is internal clock actually used for executing instructions. Search how to calculate FCPU from Fosc.

 

[abdul991]

Thanks jayanth... I'll look into the datasheet....

 

[betwixt]

That isn't strictly true and wouldn't make sense if it was. The CPU clock depends on the settings in the oscillator control register, the PLL and configuration fuses.

Brian.