calculation error

[kichapi]

i used interrupt add my adc program and some doubt the calculation.
my operating frequancy 20mhz. i want20us delay .how to calculate this?
please help me? iam waiting your valid replay

fout=fclk/prescaler*(256-TMR0)*count=?

 

[Diver300]

I guess that you are using a 16F series PIC. For those, the PIC takes 4 clock cycles to execute one instruction.

A 20 MHz clock has a cycle time of 50 ns so 4 cycles is 200 nS. A 20 µs delay is 100 instruction cycles.

A delay loop of 100 cycles would look something like this:-

      movlw    d'33'
      movwf   delay_reg ; You need to define a register for this
delay_loop
      decfsz    delay_reg, f
      goto       delay_loop

That just goes round the loop 33 times. The GOTO instruction takes 2 instruction times, so the loop takes 3 instruction times. Running it 33 times takes 100 cycles (might not be exact, but it is close) so just about 20 µs at 20 MHz