TMR0 Confusion

[retepsnikrep]

Just after a bit of help with TMR0

I have hacked Darrell's SSPWM routine and am now adapting it to use TRM0 instead of TMR1.

Now I note you cant stop/start TMR0 I dont think that's a major issue in my application which is generating a 2khz pwm.

I'm confused with the reloading of the prescaler i'm going to use 1/4 with a preload of 6 and 8mhz int clock to generate an interrupt at 2000hz.

After the interrupt is dealt with and I reload TMR0 with my PWM duty + preload do I have to reassign the prescaler every time?

My hacked start SSPWM code is below.

Config

'OPTION_REG = %00000001  'Set Option Reg for TMR0 and prescaler 1/4

Start SSPWM

 
StartSPWM: 'Set DutyCycle before calling
'For TMR0 2khz pwm and 8mhz clock Ticks = xxx per cycle 
GIE = 1
PEIE = 1
TMR0_ON_TICKS = DutyCycle '(DutyCycle must be between 25 - 225 (10-90%)
TMR0_OFF_TICKS = 255 - TMR0_ON_TICKS
TMR0_ON_VAL = 255 - TMR0_ON_TICKS + 6
TMR0_OFF_VAL = 255 - TMR0_OFF_TICKS + 6
 
TMR0IE = 1 'Enable TMR0 interrupt enable bit (1 = enabled) 
TMR0IF = 0 'Clear TMR0 interrupt flag
TMR0 = 255 'Load TMR0 with 255 First tick will cause an interrupt 
 
1/4 Prescaler setup here ?????
 
return

My hacked Int routine is below

asm
INT_CODE
      if (CODE_SIZE <= 2)
        movwf   wsave              ; copy W to wsave register
        swapf   STATUS,W           ; swap status reg to be saved into W
        clrf    STATUS             ; change to bank 0 regardless of current bank
        movwf   ssave              ; save status reg to a bank 0 register
        movf    PCLATH,w           ; move PCLATH reg to be saved into W reg
        movwf   psave       ;6     ; save PCLATH reg to a bank 0 register
       endif
 
        btfss   INTCON, TMR0IF     ; is TMR0IF set?   Timer0 Interrupt Flag
        GOTO  NoTimerInt           ; No.  Bypass timer load
        btfss   _SPWMstate         ; Is Output High?
        GOTO  TurnON      ;9/15    ; No.
TurnOFF
        bcf     _CmdPwr             ; Set CmdPwr Low
        bcf     _SPWMstate          ;
        MOVF    _TMR0_OFF_VAL,W     ;  1
        ADDWF   TMR0,F              ;  1    ; reload timer with correct value
        GOTO  TimerDone   ;12/27
TurnON  
        bsf     _CmdPwr             ; Set CmdPwr High
        bsf     _SPWMstate          ;
        MOVF    _TMR0_ON_VAL,W      ;  1
        ADDWF   TMR0,F              ;  1    ; reload timer with correct value
 
TimerDone        
 
        1/4 Prescaler setup here ?????
 
        bcf     INTCON, TMR0IF  ; 1/28 ; Clear Timer0 Interrupt Flag
 
NoTimerInt    
        Movf    psave,w             ; Restore the PCLATH reg
        Movwf   PCLATH
        swapf   ssave,w             ; Restore the STATUS reg   
        movwf   STATUS
        swapf   wsave,f
        swapf   wsave,w    ; 6/34   ; Restore W reg
 
    Retfie                          ; Exit the interrupt routine 
endasm

[cncmachineguy]

Prescaler should be set and forget as I understand it

[mister_e]

When you use the USART, are you setting the Baudrate after each character sent? Nope!

When you use the CCP module in PWM mode, do you need to configure the frequency each time you want to play with the duty cycle? Nope!

Internal hardware are often like so, you set them, you forget them.

[retepsnikrep]

That's why i'm confused the 12F683 data sheet says that writes to the TMR0 reg clear the prescaler :?

The prescaler is not readable or writable. When
assigned to the Timer0 module, all instructions writing to
the TMR0 register will clear the prescaler.

[mister_e]

mm, seems I need to send my crystal ball again for repair... she don't want to tell me the PIC you're using

[cncmachineguy]

That can be a little confusing. the prescaler keeps a count. Say it is set up for 1:8, it will count to 8 then roll over. the roll over is the new tmr count. When you write the tmr0 they are saying the count in pre scale will be reset back to 0. so if it had a count of 5, it will be cleared to zero. But the amount of pre scale will persist, it will still be 1:8

Steve, she just needs glasses, look above the quote

[retepsnikrep]

Thanks for that CNC! makes sense. I'm using a 12F683 FYI. I'll do some bench testing