Delay in C

[savnik]

I have this:

//clock=20000000 //one instruction=0.2us
delay_cycles(59); how much is the delay; is 118;

If i change the xtal from 20 to 4 mhz how much is the delay now ;

 

[Sceadwian]

The delay is 5 times longer, because the clock rate is 5 times slower.

 

[savnik]

The delay is 5 times longer, because the clock rate is 5 times slower.

Yes i know , but what mean : delay_cycles(59)
maybe 59x0.2;

 

[Sceadwian]

Seems pretty self explanatory to me. Judgeing by the delay being .2us and the frequency being 20mhz you're using a pic (the time constant of 20mhz is actually .05us but the PIC instruction cycle is four clocks) So just calculate it that way. 1/Clock frequency * 4 = single cycle delay time. I don't believe such a routine can delay for just a couple cycles as the branching alone takes a certain number of instructions. Delays of only a few instruction cycles would best be achieved by using inline assembly NOP's, there may even be a C function for it, but if you need controlling of timeing that severe you should be programming in assembly anyways.

 

[Papabravo]

Using the instruction cycle time for doing delays, more than a couple of cycles, is a really really poor practice in my opinion. Especially considering how many better ways there are to do it.