Help needed - Newbie Simple delay routine PIC 16F84

[bgs28]

Hi ,
Can anyone help?

Have recently started to look at PIC programming and looking at time delays.

I am struggling to understand some simple concepts it seems so cant make a start.

Can anyone please explain the following lines and how a 10ms delay is achieved.

wait10ms
movlw .256 - .10000/.256 ; set timer to 10ms delay
goto wait
wait50ms
movlw .256 - .50000/.256 ; set timer to 50ms delay
goto wait
wait
movwf TMR0
bcf INTCON, T0IF
clrwdt
btfss INTCON, T0IF
goto $-2
return

im struggling with the .256 - .10000/.256
and the $ -2 in particular

if anyone could explain how the delay is derived from these figures it would be a great help

thanks in advance

B Smyth

 

[millwood]

you need to read timr0 section of the datasheet.

in short, TMR0 is the count-up timer. .256-.10000/.256 load up the timer with an initial offset so that the timer will count up 10000/256 cycles before it overflows.

the prescaler is likely set at 256:1 and the crystal is 4Mhz. so each clock is 1us, and each prescaler is 256us, and since tmr0 takes 10000/256 prescaler output to overflow, each tmr0 overflow takes (roughly) 10000us = 10ms.

but you really need to read the datasheet.

 

[tek_nikkos]

software para generar rutinas PIC16Fxxx

aqui un software que uso mucho para crear rutinas de retardo sin usar timer.
**broken link removed**
solo elijan la frecuencia del cristal, escribir el tiempo en milisegundos, click en calcular ciclos y finalmente click en generar codigo.
solo copiar los archivos desde DEMORA hasta RETURN al mplab, declarar las variables que indica el picdelayer y listo.