[SOLVED] can any understand this code

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Mohamed Slama

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hello everybody

this code is a part of frequency counter code but i cant under stand these

lines (if (--tmr0_cnt==0)) and tmr0_cnt==0 initialized by 100

Code: 
void interrupt()
{

 if(intcon.f2==1) //if timer0 is overlfowed
 {
   if(--tmr0_cnt==100) //decrement and check tmr0_cnt until zer0 which means a second
   {
     intcon.f7=0; t1con.f0=0;  //stop counting and global interrupts
     flag=1;  //set flag
   }
   intcon.f2=0; //reset timer0 flag
   tmr0=98;  //Timer0 starts counting from 98 again
 }

 if(pir1.f0==1) //if timer1 is overlfowed
 {
   OverNumb++;  //increment number of overflow
   pir1.f0=0; //reset timer1 flag
 }

}
 

It means pre decrement the value of tmr0_cnt whenever a timer0 interrupt occurs and check if it is equal to value 100. The initial value of tmr0_cnt should be > 100 so that on first interrupt it gets decremented to 100 if it was 101 previously and then the if() condition becomes true.
 

but when i write :

if (--tmr0_cnt==14) as example

compiler (microC) will equal tmr0 by 14 from the first time ??

- - - Updated - - -

corrected code is
Code: 
void interrupt()
{
 
 if(intcon.f2==1) //if timer0 is overlfowed
 {
   if(--tmr0_cnt==0) //decrement and check tmr0_cnt until zer0 which means a second
   {
     intcon.f7=0; t1con.f0=0;  //stop counting and global interrupts
     flag=1;  //set flag
   }
   intcon.f2=0; //reset timer0 flag
   tmr0=98;  //Timer0 starts counting from 98 again
 }
 
 if(pir1.f0==1) //if timer1 is overlfowed
 {
   OverNumb++;  //increment number of overflow
   pir1.f0=0; //reset timer1 flag
 }
 
}
 

If the initial value of tmr0_cnt is greater than 14 then at each interrupt it is decremented and then compared with 14 and when a match occurs then the if() condition block is executed. == is not the assignment operator but comparison operator.
 
jayanth.devarayanadurga said:
If the initial value of tmr0_cnt is greater than 14 then at each interrupt it is decremented and then compared with 14 and when a match occurs then the if() condition block is executed. == is not the assignment operator but comparison operator.
Click to expand...

firstly : thank u for ur answering

secondly : i know that == is not the assignment operator but comparison operator.
but if wrote this line in MicroC

(if --x==14)
{
code
}

the code will be excuted without comparison , realy i dont know why ?

- - - Updated - - -

jayanth.devarayanadurga can i talk u through facebook account ?
 

jayanth.devarayanadurga

thank u very much
i have found problems in my version of MicroC

and reinstalled it , the program run in correct way , send me ur facebook account
 

you are right , about post #5 i wrote (if --x==14) in wrong way but i have written in correct way in microC compiler , problem is solved thank u very much
 

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