Low freq PWM problem

[Mike, K8LH]

You might also consider using the PWM module along with a small interrupt driven "helper" to link together several smaller PWM period "frames" into the much longer 120-Hz PWM period. This method produces a relatively low overhead high resolution jitter free PWM output.

I don't have PBP but perhaps you and other Forum members can muddle through this pseudo C code example.

This example uses a 12-MHz clock (Tcy = 0.3334-usecs) with PWM module settings that produce a PWM period of 333.3334-usecs (250 x 1.3334-usec 'steps'). The interrupt "helper" links together 25 of these short 333.3334-usec PWM period "frames" to produce a precise 120-Hz (8333.3334-usec) period with an output pulse width of 0 to 6250 x 1.3334-usec "steps".

Regards, Mike

;******************************************************************
;
;  // setup PWM for a 333.3 usec period (12-MHz xtal, Timer 2
;  // prescale 4:1, PR2 = 249) and use 25 of these 333.3 usec
;  // PWM "frames" for a precise 120-Hz (8.3334 msec) period.
;
;  char frame = 1;              // initialize "frame" var'
;  int width = 100;             // 0..6250 (1.333 usec steps)
;  int pulse;                   // isr work variable, 0..6250
;
;  void interrupt()             // 120-Hz example
;  { pir1.TMR2IF = 0;           // clear timer 2 interrupt flag
;    frame--;                   // decrement 'frame' number
;    if(frame == 0)             // if end-of-period
;    { frame = 25;              // reset for new period
;      pulse = width;           // refresh work variable
;    }
;    if(pulse > 250)            //
;    { ccpr1l = 250;            // do a 100% duty cycle frame
;      pulse -= 250;            // update 'pulse'
;    }
;    else                       //
;    { ccpr1l = pulse;          // do variable duty cycle
;      pulse = 0;               // force remaining frames to 0%
;    }
;
;
;
;  void Main()                  //
;  { cmcon0 = 7;                // comparators off
;    ansel = 0;                 // a2d convertor off
;    pir1 = 0;                  // clear all interrupt flags
;    pie1.TMR2IE = 1;           // enable TMR2 interrupts
;    ccpr1l = 0;                // set 0% duty cycle (output off)
;    t2con = 0b00000101;        // '0-------' unimplemented bit
;                               // '-0000---' TOUTPS<3:0>, postscale 1
;                               // '-----1--' TMR2ON, turn Timer 2 on
;                               // '------01' T2CKPS<1:0>, prescale 4
;    pr2 = 250-1;               // 250 x 1333-nsec 'ticks' = 333 usecs
;    intcon = 0b11000000;       // '1-------' GIE, enable global ints
;                               // '-1------' PEIE, enable peripheral ints
;                               // '--0-----' T0IE, TMR0 ints disabled
;                               // '----0---' GPIE, IOC disabled
;                               // '-----000' T0IF/INTF/GPIF flags
;    while(1)                   //
;    {                          //
;    }                          //
;  }
;

[ultiblade]

Hi all,

Thanks for the helping hand and code. I used your code samples but it just didn't do the trick. My goal was to be able to have 2 low freqency PWM signals with a 90 degree phase shift.

I finally accomplished this by using a 18F1230 @ 8Mhz , DT-INTS and a few lines of code.

The code below produces a 30Hz PWM that increments it's dutycycle every interrupt until it reaches 100%, it will then decrement to 1% and start over. It uses a 'base' frequency of 60Hz (PTPER 519), and distributes the dutycycle over the 30Hz periods.

'==== Set fuses
@ __CONFIG    _CONFIG1H, _OSC_INTIO2_1H & _FCMEN_OFF_1H & _IESO_OFF_1H
@ __CONFIG    _CONFIG2L, _PWRT_OFF_2L & _BOR_OFF_2L & _BORV_3_2L
@ __CONFIG    _CONFIG2H, _WDT_OFF_2H & _WDTPS_1_2H
@ __CONFIG    _CONFIG3L, _HPOL_HIGH_3L & _LPOL_HIGH_3L & _PWMPIN_ON_3L
@ __CONFIG    _CONFIG3H, _FLTAMX_RA5_3H & _T1OSCMX_HIGH_3H & _MCLRE_OFF_3H
@ __CONFIG    _CONFIG4L, _STVREN_OFF_4L & _BBSIZ_BB256_4L & _XINST_OFF_4L & _DEBUG_OFF_4L
@ __CONFIG    _CONFIG5L, _CP0_OFF_5L & _CP1_OFF_5L
@ __CONFIG    _CONFIG5H, _CPB_OFF_5H & _CPD_OFF_5H
@ __CONFIG    _CONFIG6L, _WRT0_OFF_6L & _WRT1_OFF_6L
@ __CONFIG    _CONFIG6H, _WRTB_OFF_6H & _WRTC_OFF_6H & _WRTD_OFF_6H
@ __CONFIG    _CONFIG7L, _EBTR0_OFF_7L & _EBTR1_OFF_7L

'==== Include DT interrupts
INCLUDE "DT_INTS-18.bas" ;Base Interrupt System
INCLUDE "ReEnterPBP-18.bas" ;Include if using PBP interrupts

'==== Dec's for OSC
OSCCON  = %01110000 'Int OSC @ 8 MHz
'OSCTUNE = %00000000  'PLL disabled, center freqency
DEFINE OSC 8 'Int OSC @ 8 MHz, for PBP

'==== Initialize DT_INTS
ASM
INT_LIST  macro    ; IntSource,	  Label,   Type,	ResetFlag?
        INT_Handler		PT_INT,	_pwmINT,	PBP,	YES
    endm
    INT_CREATE ;Creates the interrupt processor
ENDASM                     

'==== Set port associated registers
ADCON0  = %00000000 'A/D convertor disabled
ADCON1  = %00001111 'All ports configured digital
ADCON2  = %10000000 'Right justify results
CMCON   = %00000000 'Comparator modules disabled
CVRCON  = %00000000

PTCON0  = %00001100 'Fosc/4 Prescale 1:64, Free running mode
PWMCON0 = %00110111 'PWM0,1,2,3 enabled, independent mode
PWMCON1 = %00000001 'Special event on count upwards, Sync PWM timebase 

PORTA   = %00000000
PORTB   = %00000000
TRISA   = %01100000
TRISB   = %00000100

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

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

'==== Reset PWM duty registers
PDC0L = 0: PDC0H = 0 'Reset duty register 0
PDC1L = 0: PDC1H = 0 'Reset duty register 1
PDC2L = 0: PDC2H = 0 'Reset duty register 2
                                       
'==== Dec's for variables
pwmFREQ var word
pwmDUTY_TGT var word
pwmDUTY var word
pwmDIR  var bit
pwmTOG  var bit
pwmFL   var bit


'==== Initialise variables
pwmFREQ = 0
pwmDUTY_TGT = 0
pwmDUTY = 0
pwmDIR  = 0
pwmTOG  = 0
pwmFL   = 0


'==== Wait for hardware settle
Pause 500 'Settle for 0,5 seconds

'==== Enable PWM interrupts
@ INT_ENABLE   PT_INT ;Enable PWM to PTPER match interrupts            

'==== SET PWM frequency and dutycycle
pwmFREQ = 519 'Makes 60,010 Hz, 100% dutycycle = 2076
pwmDUTY = 1 'Initial dutycycle
PTPERL = pwmFREQ.lowbyte 'Set PWM frequency lowbyte
PTPERH = pwmFREQ.highbyte 'Set PWM frequency lowbyte
'pwmFREQ_CO = 2076 '100% dutycycle

'Find out if desired PWM is below 50% or over.
'Double the ammount of dutycycle because 
'the frequency will be half the base frequency .. 
if pwmDUTY <= 1038 then
	pwmDUTY_TGT = pwmDUTY * 2
	pwmDIR = 0
else
	pwmDUTY_TGT = (pwmDUTY - 1038) * 2
	pwmDIR = 1
endif

PTCON1 	= %10000000 'Start PWM output 

'==== Main program loop
main_loop:
@ nop 'Do nothing

goto main_loop
          
'==== The End
theend: 'Endless loop
goto theend 'Endless loop

'==== Routines
pwmINT:
PWMCON1.1 = 1
    if pwmDIR = 0 then	
        if pwmTOG = 0 then
            PDC0L = 0 
            PDC0H = 0
            PDC1L = pwmDUTY_TGT.lowbyte 
            PDC1H = pwmDUTY_TGT.highbyte
            pwmTOG = 1
	   		else     
            PDC0L = pwmDUTY_TGT.lowbyte 
            PDC0H = pwmDUTY_TGT.highbyte
            PDC1L = 0 
            PDC1H = 0
            pwmTOG = 0
	   		endif
    else
        if pwmTOG = 0 then
            PDC0L = 2076 
            PDC0H = 2076
            PDC1L = pwmDUTY_TGT.lowbyte 
            PDC1H = pwmDUTY_TGT.highbyte
            pwmTOG = 1
	   		else
            PDC0L = pwmDUTY_TGT.lowbyte 
            PDC0H = pwmDUTY_TGT.highbyte
            PDC1L = 2076 
            PDC1H = 2076
            pwmTOG = 0
	   		endif
    endif

		if pwmfl = 0 then
    		pwmduty = pwmduty + 1
		else
    		pwmduty = pwmduty - 1
		endif

		if pwmduty >= 2076 then pwmfl = 1
		if pwmduty <= 0 then pwmfl = 0    

		if pwmDUTY < 1038 then
				pwmDUTY_TGT = pwmDUTY * 2
				pwmDIR = 0
		else
				pwmDUTY_TGT = (pwmDUTY - 1038) * 2
				pwmDIR = 1
		endif
PWMCON1.1 = 0
@ INT_RETURN

Problem solved, Thanks guys !
Best regards, UltiBlade