Low freq PWM problem

[ultiblade]

You're so right ! I don't know how i missed this. Thanks !

A couple of day's ago i found a timer calculator, i just entered the frequency and the calculations were made. However, it seems that the scrollbars that are used to set the prescalers and preload values in this calculator are refreshed after clicking calculate. I was using bad combinations every calculation.

I tried your values, and it worked like a charm, thanks for replying !

Thanks !

[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