Timer with TMR0

[piombazzo]

Help me,
i want create a counter in 1 sec with the TMR0 and interrupt, the pic is 16f877a osc 20Mhz.
I have utilized the formule Time = Mhz * periode * (Tmr0-N) * prescaler
Time = (20 * 50 *(256)*64)/1M= 16384*61 =999.424
but it not functional.
Help Me
SOS.
Aiutoooooo

[robert0]

Look here, even if on a different pic than yours:

http://www.microengineeringlabs.com/...BP/CLOCKX4.BAS

bye ( se sei italiano ciaooo)

roberto

[piombazzo]

Hy Robert (ciao si sono Italiano )
thank you for answer
but the osc in the example is 4 Mhz,
i use a osc to 20 Mhz.
Help Me.

[Bruce]

 @ DEVICE LVP_OFF,HS_OSC
    DEFINE OSC 20
    DEFINE INTHAND TMRint
    
    'Variables for saving state in interrupt handler
    wsave	VAR	BYTE $70 system		' Saves W
    ssave	VAR	BYTE bank0 system	' Saves STATUS
    psave	VAR	BYTE bank0 system	' Saves PCLATH
    fsave	VAR	BYTE bank0 system	' Saves FSR

    TMR0_Ticks VAR BYTE system 
    
    TMR0 = 96
    
    GoTo Init

Asm
TMRint
    ; Uncomment the following if the device has less than 2k of code space
	; movwf	wsave			; Save W
	; swapf	STATUS, W		; Swap STATUS to W (swap avoids changing STATUS)
	; clrf	STATUS			; Clear STATUS
	; movwf	ssave			; Save swapped STATUS
	; movf	PCLATH, W		; Move PCLATH to W
	; movwf	psave			; Save PCLATH

    ; Increment ticks, reload, clr int flags
    ;------------------------------------------------------------------------------
    incf    TMR0_Ticks,f    ; increment overflow ticks
    movlw   96              ; 256-96=160. 160x200nSx256=8.192mS. 8.192mSx122=0.999mS= ~1S
    movwf   TMR0            ; reload
    bcf     INTCON,2        ; clear TMR0 int flag
    ;---------------------------------------------------------------------------- 
    
    movf	fsave, W		; retrieve FSR value
	movwf	FSR				; Restore FSR
	movf	psave, W		; Retrieve PCLATH value
	movwf	PCLATH			; Restore PCLATH
	swapf	ssave, W		; (swap to avoid changing STATUS)
	movwf	STATUS			; Restore STATUS
	swapf	wsave, F		; Swap the stored W value
	swapf	wsave, W		; Restore it to W (swap to avoid changing STATUS)
	retfie					; Return from the interrupt
EndAsm

Init:
    PORTD = 0
    TRISD = 0
    OPTION_REG = %00000111 ' 1:256 prescaler to TMR0
    INTCON = %11100000     ' Enable TIMR0 interrupts

Main:
    IF TMR0_Ticks >= 122 THEN
     TMR0_Ticks = 0
     PORTD.0 = PORTD.0 ^ 1 ' ~1 second pulse on RD0
    ENDIF
    GOTO Main
    
    End

Timer1 would be better, but this should get you started.

[piombazzo]

Thank You
Bruce.

[robert0]

I used Timer2 on a PIC16F628 for 1.5 seconds:

T2CON=%01111010 'prescaler 1:16 POSTSCALER 1:16 ,STOP TIMER2

Then start timer and in interrupt handler:

if PIR1.1=1 then
Count= Count +1
if Count =100 then
high led
endif
T2CON.2=0
PIR1.1=0 'reset flag interrupt
resume
endif

Every interrupt of timer Count is incremented....when is 100 =1,5 sec
It's easy to change to TMR0 or Timer1....and also you can change the number of count

Ciao
Roberto