Low freq PWM problem

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Hi All,

I'm kind of stuck at the moment.

I need a pwm signal with a low frequency. Darrel has made the awesome slow pwm module but i can't use because i need timer1 to do a continues pulse count. So i'm trying to code my own, but i can't seem to figure it out.

I'm using timer0 to to make the frequency, with Darrels DT_INTS-14. This works perfect. Preloading timer0 with 190 give about 118Hz where i need 120Hz so thats ok. On every interrupt the PORTC.0 pin is made high.

I'm using timer2 to make the dutycycle. I thought if i have a frequency of 120Hz the period is about 8333uS. If i need 256 dutycycle steps i need a interrupt at 8333uS/256 steps = 32,6uS. Timer2 is setup (presc. 1:1, postsc. 1:1, pr2=64) to generate an interrupt every 32,5uS.

If i need a 50% dutycycle, simply count 128x timer2 interrupts and that make the output low. Could be quite simple I thought, but i was wrong. On my scope i see a nice 117Hz stable signal with a dutycycle of 1 or so. If i alter the dutycycle var i sometimes see a dutycycle that ramps down at every cycle ?

Here's my code :

'==== Set fuses =========================================
'Fuse are set in .inc file

'==== Set includes ======================================
INCLUDE "DT_INTS-14.bas"                                'DTints Interrupt System
include "ReEnterPBP.bas"                                'Enable PBP int reenter

'==== Dec's for OSC =====================================
DEFINE OSC 8                                            'Int OSC @ 8 MHz, for PBP
OSCCON = %01110001                                      'Int OSC @ 8 MHz

'==== Set defines =======================================
OPTION_REG  = %00000111                                 'Set TMR0 precsaler to 1:256, source Fosc/4
WPU         = %00000000                                 'Weak pull-ups disabled

INTCON      = %11100000                                 'Global + Perhip. INT enabled, TMR0 int enabled
PIE1        = %00000010                                 'TMR2 to PR2 match INT enabled
PIE2        = %00000000                                 'Set TMR1 overflow interrupt
PIR1        = %00000000                                 
PIR2        = %00000000                                 
PCON        = %00000000
IOC         = %00000000                                 'Interrupt On Change disabled
PCON        = %00000000                                 'Ultra lowpower + BOR disabled

ANSEL       = %00000000                                 'Select analog inputs: none
ANSELH      = %00000000                                 'Select analog inputs: none
ADCON0      = %00000000                                 'A/D Module is OFF
ADCON1      = %00000000                                 'A/D control register
CM1CON0     = %00000000                                 'Comparator1 Module is OFF
CM2CON0     = %00000000                                 'Comparator2 Module is OFF
VRCON       = %00000000                                 'Voltage ref. disabled

TRISA       = %00000000                                 'Set Input/Output (0 to 5)
PORTA       = %00000000                                 'Ports High/Low (0 to 5)
TRISB       = %00000000                                 'Set Input/Output (4 to 7)
PORTB       = %00000000                                 'Ports High/Low (4 to 7)
TRISC       = %00000000                                 'Set Input/Output (0 to 7)
PORTC       = %00000000                                 'Ports High/Low (0 to 7)

T2CON       = %00000100                                 'Timer2 is On,                                 
T1CON       = %00000000                                 'Timer1 is off
CCP1CON     = %00000000                                 'CCP engine is off

'==== Set Constants =====================================
'none

'==== Setup DTints interrupt handler ====================
ASM
INT_LIST  macro                                         ;Setup IntSource
        INT_Handler    TMR0_INT,  _intTMR0,  PBP,  yes  ;Enable TMR0 interrupts
        INT_Handler    TMR2_INT,  _intTMR2,  PBP,  yes  ;Enable TMR0 interrupts
    endm
    INT_CREATE                                          ;Create interrupt processor
ENDASM

'==== Dec's for pin aliasing INPUT'S ====================
'none  

'==== Dec's for pin aliasing OUTPUT'S ===================
gi1 var PORTC.0                                         'Injector 1 output pin
                                                
'==== Dec's for variables ===============================
dutycycle var byte                                      'Holds the needed dutycycle
dutycount var byte                                      'Counter for dutycycle

'==== Initialise variables ==============================
gistate   = 0
dutycycle = 0
dutycount = 0 

'==== Wait for hardware settle ==========================
pause 500

'==== Enable interrupt processing =======================
TMR0 = 190                                              'Preload Timer0 for interrupt @ 120Hz
PR2 = 64                                                'Preload PR2 for dutycycle interrupt
dutycycle = 128                                         'Set duty cycle to 50%

@   INT_ENABLE   TMR0_INT                               ;Activate TMR0 INT
@   INT_ENABLE   TMR2_INT                               ;Activate TMR2 INT

'==== Main program loop =================================
main:
@   nop                                                 ;Do nothing
goto main
          
'==== The End ===========================================
theend:                                                 ' Endless loop
goto theend                                             ' Endless loop

'Routines and Handlers
'==== TMR0 Interrupt handler ============================
intTMR0:                                                'TMR0 INTERRUPT HANDLER
TMR0 = 190                                          'Reload TMR0
gi1 = 1                                                 'Activate injector 2
@ INT_RETURN                                            ;Jump back to previous process

'==== TMR2 Interrupt handler ============================
intTMR2:                                                'TMR0 INTERRUPT HANDLER
PR2 = 64                                                'Reload PR2

if dutycount = dutycycle then                           'If dutycycle is met,
    gi1 = 0                                             'Deactivate injector 1
    dutycount = 0                                       'Reset the dutycounter
else
    dutycount = dutycount + 1                           'Add 1 to dutycounter
endif
@ INT_RETURN                                            ;Jump back to previous process

Anyone have an idea ?

Best regards, UB