Electronic – How to modulate PWM frequency in realtime with a Microchip dsPIC

cfrequencymicrochippicpwm

I'm trying to change the PWM output frequency roughly once a millisecond using a dsPIC33FJ256GP710, and I'm having mixed results. I first tried this:

 #include <p33fxxxx.h> 

 _FOSCSEL(FNOSC_PRIPLL); 
 _FOSC(FCKSM_CSDCMD & OSCIOFNC_OFF & POSCMD_XT); 
 _FWDT(FWDTEN_OFF); 

 static unsigned int PWM_TABLE[7][2] = 
 { 
     {132, 66}, {131, 66}, {130, 65}, {129, 65}, {128, 64}, {127, 64}, {126, 63} // Compare, 50% duty 
 }; 

 static int curFreq = 0; 

 int main(void) 
 { 
     int i; 

     PLLFBD = 0x009E;                // Set processor clock to 32 MHz (16 MIPS) 
     CLKDIV = 0x0048;  

     LATCbits.LATC1 = 0;             // Make RC1 an output for a debug pin 
     TRISCbits.TRISC1 = 0;     

     LATDbits.LATD6 = 0;             // Make RD6/OC7 an output (the PWM pin) 
     TRISDbits.TRISD6 = 0; 

     T2CONbits.TON = 0;              // Disable Timer 2                      
     OC7CONbits.OCM = 0b000;         // Turn PWM mode off 
     PR2 = PWM_TABLE[curFreq][0];    // Set PWM period 
     OC7RS = PWM_TABLE[curFreq][1];  // Set PWM duty cycle 
     OC7CONbits.OCM = 0b110;         // Turn PWM mode on 
     T2CONbits.TON = 1;              // Enable Timer 2 

     while (1) 
     {                 
         for (i = 0; i < 3200; i++) {}      // Delay roughly 1 ms         
         curFreq = (curFreq + 1) % 7;       // Bump to next frequency        
         PR2 = PWM_TABLE[curFreq][0];       // Set PWM period 
         OC7RS = PWM_TABLE[curFreq][1];     // Set PWM duty cycle 
         LATCbits.LATC1 = !LATCbits.LATC1;  // Toggle debug pin so we know what's happening         
     } 
 }

The result is that PWM drops out for about 4 ms at what looks to be a repeatable interval, roughly aligned with my debug pin toggle (in other words, when the code is messing with the period and duty cycle registers). I'll attach a photo of my scope trace. Channel 1 is PWM and channel 2 is the debug pin that's toggled when the code attempts to adjust the frequency.

Anyway, I started thinking about timer rollovers, and I did some searching on a few forums. I came up with a few ideas based on a few posts I read. The best idea seemed to be to enable the Timer 2 interrupt, turn PWM mode off inside it, and only change the period and duty cycle registers inside the Timer 2 interrupt. So, I wrote this:

 #include <p33fxxxx.h> 

 _FOSCSEL(FNOSC_PRIPLL); 
 _FOSC(FCKSM_CSDCMD & OSCIOFNC_OFF & POSCMD_XT); 
 _FWDT(FWDTEN_OFF); 

 static int curFreq = 0; 

 static unsigned int PWM_TABLE[7][2] = 
 { 
     {132, 66}, {131, 66}, {130, 65}, {129, 65}, {128, 64}, {127, 64}, {126, 63} // Compare, duty 
 }; 

 int main(void) 
 { 
     int i, ipl; 

     PLLFBD = 0x009E;                // Set processor clock to 32 MHz (16 MIPS) 
     CLKDIV = 0x0048;  

     LATCbits.LATC1 = 0;             // Make RC1 an output for a debug pin 
     TRISCbits.TRISC1 = 0;     

     LATDbits.LATD6 = 0;             // Make RD6/OC7 an output (the PWM pin) 
     TRISDbits.TRISD6 = 0; 

     OC7CONbits.OCM = 0b000;         // Turn PWM mode off     
     OC7RS = PWM_TABLE[curFreq][1];  // Set PWM duty cycle 
     PR2 = PWM_TABLE[curFreq][0];    // Set PWM period 
     OC7CONbits.OCM = 0b110;         // Turn PWM mode on 

     T2CONbits.TON = 0;              // Disable Timer 2 
     TMR2 = 0;                       // Clear Timer 2 register     
     IPC1bits.T2IP = 1;              // Set the Timer 2 interrupt priority level 
     IFS0bits.T2IF = 0;              // Clear the Timer 2 interrupt flag 
     IEC0bits.T2IE = 1;              // Enable the Timer 2 interrupt 
     T2CONbits.TON = 1;              // Enable Timer 2 

     while (1) 
     {                 
         for (i = 0; i < 1600; i++) {}      // Delay roughly 1 ms 
         SET_AND_SAVE_CPU_IPL(ipl, 2);      // Lock out the Timer 2 interrupt 
         curFreq = (curFreq + 1) % 7;       // Bump to next frequency         
         RESTORE_CPU_IPL(ipl);              // Allow the Timer 2 interrupt 
         LATCbits.LATC1 = !LATCbits.LATC1;  // Toggle debug pin so we know what's happening 
     } 
 } 

 void __attribute__((__interrupt__)) _T2Interrupt(void) 
 {     
     T2CONbits.TON = 0;              // Disable Timer 2 
     TMR2 = 0;                       // Clear Timer 2 register 
     OC7CONbits.OCM = 0b000;         // Turn PWM mode off 
     OC7RS = PWM_TABLE[curFreq][1];  // Set the new PWM duty cycle 
     PR2 = PWM_TABLE[curFreq][0];    // Set the new PWM period     
     OC7CONbits.OCM = 0b110;         // Turn PWM mode on 
     IFS0bits.T2IF = 0;              // Clear the Timer 2 interrupt flag 
     T2CONbits.TON = 1;              // Enable Timer 2 
 }

This looks to be more stable as far as I can tell on my ancient scope, but now the waveform is no longer regularly-shaped (the duty cycle seems to be inexplicably inconsistent) and if I try hard enough I can convince myself that I still see a millisecond of PWM dropout when my scope is set to a 5 or 10 millsecond timebase.

It's certainly better than it was, and I can continue to mess with it in the hopes of fixing the irregular waveform produced by the second bit of code, but my question is:

Is there a "right" way to do this? Or at least a better way than the path I'm on?

Any help would be thoroughly, thoroughly appreciated.

Scope trace http://www.freeimagehosting.net/uploads/c132216a28.jpg

Best Answer

For anyone who's interested, here is the solution I arrived at today:

#include <p33fxxxx.h>

_FOSCSEL(FNOSC_PRIPLL);
_FOSC(FCKSM_CSDCMD & OSCIOFNC_OFF & POSCMD_XT);
_FWDT(FWDTEN_OFF);

static int curFreq = 0;
static int nextFreq = 0;

static unsigned int PWM_TABLE[7][2] =
{
    {132, 66}, {131, 66}, {130, 65}, {129, 65}, {128, 64}, {127, 64}, {126, 63} // Compare, duty
};

int main(void)
{
    int i, ipl;

    PLLFBD = 0x009E;                // Set processor clock to 32 MHz (16 MIPS)
    CLKDIV = 0x0048; 

    LATCbits.LATC1 = 0;             // Make RC1 an output for a debug pin
    TRISCbits.TRISC1 = 0;

    OC7CONbits.OCM = 0b000;         // Turn PWM mode off    
    OC7RS = PWM_TABLE[curFreq][1];  // Set PWM duty cycle
    PR2 = PWM_TABLE[curFreq][0];    // Set PWM period
    OC7CONbits.OCM = 0b110;         // Turn PWM mode on

    T2CONbits.TON = 0;              // Disable Timer 2
    TMR2 = 0;                       // Clear Timer 2 register    
    IPC1bits.T2IP = 1;              // Set the Timer 2 interrupt priority level
    IFS0bits.T2IF = 0;              // Clear the Timer 2 interrupt flag
    IEC0bits.T2IE = 1;              // Enable the Timer 2 interrupt
    T2CONbits.TON = 1;              // Enable Timer 2

    while (1)
    {                
        for (i = 0; i < 1600; i++) {}      // Delay roughly 1 ms
        SET_AND_SAVE_CPU_IPL(ipl, 2);      // Lock out the Timer 2 interrupt
        curFreq = (curFreq + 1) % 7;       // Bump to next frequency
        nextFreq = 1;                      // Signal frequency change to ISR
        RESTORE_CPU_IPL(ipl);              // Allow the Timer 2 interrupt        
    }
}

void __attribute__((__interrupt__)) _T2Interrupt(void)
{   
    IFS0bits.T2IF = 0;                  // Clear the Timer 2 interrupt flag     

    if (nextFreq)
    {        
        nextFreq = 0;                   // Clear the frequency hop flag
        OC7RS = PWM_TABLE[curFreq][1];  // Set the new PWM duty cycle
        PR2 = PWM_TABLE[curFreq][0];    // Set the new PWM period         
    }
}

I confirmed with the scope and a debug pin my suspicion: the original code was suffering from a race condition. The main loop did not bother to synchronize changes to PR2 with the actual state of the TMR2 counter, and so would occasionally set PR2 to a value LESS THAN (or maybe equal to) the current TMR2 value. This, in turn, would cause TMR2 to count up until it rolled over, then continue counting until it reached PR2 and generated a rising edge. During the time TMR2 was counting up to 65535 to roll over, no PWM output was being generated. At 16 MIPS, the rollover time for a 16-bit timer like TMR2 is roughly 4 ms, explaining my 4 ms PWM dropout. So, the code was doing exactly what I wrote it to do :)

In the second snippet, the code is correctly synchronizing changes to PR2 and the duty cycle register with the TMR2 rollover event, and so the 4 ms dropout had gone away. I mentioned a "weird" waveform associated with that example: it was due to the RD6/OC7 pin being configured as an output and having a low value set in the LATD register. The second snippet actually turns PWM mode off inside the Timer 2 ISR: this lets the GPIO functionality take over and pulls RD6/OC7 down for a few microseconds before reenabling PWM and generating a rising edge, leading to a "hiccup" waveform.

The second snippet also has a problem in that it reconfigures PR2 and the duty cycle register on every Timer 2 rollover, regardless of whether the main loop has commanded a frequency change or not. It seems to me from observation that the timer rolls over and generates a rising edge on the PWM pin and THEN the Timer 2 ISR gets control a few nanoseconds later (owing I'm sure to vector latency, etcetera). Turning PWM off and rejiggering the registers every time through doesn't get you quite the right frequency and duty cycle in the long run because the hardware has already generated a rising edge and started counting up to the next compare value.

What this means is that in the corrected snippet I posted today, the work done in the Timer 2 ISR needs to be minimized! Because I'm running PWM at such a high frequency, and because there is a small latency between the rising edge generated by the PWM hardware and the invocation of the Timer 2 ISR, by the time I get into the ISR TMR2 has already had time to count up to a fair number. My code needs to set PR2 and the duty cycle register immediately and directly (i.e. no function calls, and even the table lookup is pushing it), otherwise it runs the risk of missing the compare and causing the 4 ms rollover bug that was my original problem.

Anyway, I think this is an accurate description of things, and I'm running the code in my "real" application with encouraging results so far. If anything else changes I'll post here, and of course any corrections to the above would be massively appreciated.

Thanks for your help, pingswept.

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Creating a PWM signal using PIC18F4520

You cannot get a 100ms PWM period with a 20MHz clock. The maximum will be something like 16ms. You can run the micro slower or use a faster period. Generally the period is not too important and you want to maximize the resolution.

Please read the datasheet and reference manual sections on the timers carefully, there are too many issues with your understanding.

Also note that you have confused 0x31249 (201289 decimal) with 31249.

TimerA: How to get PWM in upMode configuration?(TI MSP432 Launchpad)

Problem solved. I forgot to enable INT_TA0_N interrupt which includes CCR1 to CCR7 vector. In this case CCR1 determines the duty Cycle so I just added one more Interrupt to NVIC and enabled the interrupt. New code is like this:

#include "msp432.h"
#include "driverlib.h"

void SetConfiguation(void);

int main(void) 
{
    WDT_A_holdTimer();
    SetConfiguation();
    while(1);
}

void SetConfiguation(void)
{

    Interrupt_disableMaster();

    CS_setDCOCenteredFrequency(CS_DCO_FREQUENCY_24);
    CS_initClockSignal(CS_SMCLK, CS_DCOCLK_SELECT, CS_CLOCK_DIVIDER_128);

    /*
    Timer_A_UpModeConfig upConfig =
    {
            TIMER_A_CLOCKSOURCE_SMCLK,              // SMCLK Clock SOurce
            TIMER_A_CLOCKSOURCE_DIVIDER_64,          // SCLK/64 = 24MHz/128/64 = 2929 Hz
            24,                           // 50000 tick period
            TIMER_A_TAIE_INTERRUPT_ENABLE,         // Enable Timer interrupt
            TIMER_A_CCIE_CCR0_INTERRUPT_ENABLE ,    // Enable CCR0 interrupt
            //TIMER_A_CCIE_CCR0_INTERRUPT_DISABLE,     // Disable CCR0 interrupt
            TIMER_A_DO_CLEAR                      // Clear value
    };
    */
    Timer_A_PWMConfig pwmConf =
    {
        //uint_fast16_t clockSource;
        TIMER_A_CLOCKSOURCE_SMCLK,
        //uint_fast16_t clockSourceDivider;
        TIMER_A_CLOCKSOURCE_DIVIDER_64,
        //uint_fast16_t timerPeriod;
        1600,
        //uint_fast16_t compareRegister;
        TIMER_A_CAPTURECOMPARE_REGISTER_1,
        //uint_fast16_t compareOutputMode;
        TIMER_A_OUTPUTMODE_SET_RESET,
        //uint_fast16_t dutyCycle;
        200
    };

    Timer_A_generatePWM(TIMER_A0_MODULE, &pwmConf);

    P1DIR |= BIT0;
    P1OUT &= ~BIT0;

    //Timer_A_configureUpMode(TIMER_A0_MODULE, &upConfig);
    /*
    Timer_A_CompareModeConfig compConfig =
    {
        //uint_fast16_t compareRegister;
        TIMER_A_CAPTURECOMPARE_REGISTER_1,
        //uint_fast16_t compareInterruptEnable;
        TIMER_A_CAPTURECOMPARE_INTERRUPT_ENABLE,
        //uint_fast16_t compareOutputMode;
        TIMER_A_OUTPUTMODE_SET_RESET,
        //TIMER_A_OUTPUTMODE_TOGGLE_RESET,
        //uint_fast16_t compareValue;
        6
    };
    */
    //Timer_A_initCompare(TIMER_A0_MODULE, &compConfig);

    Timer_A_enableCaptureCompareInterrupt(TIMER_A0_MODULE, TIMER_A_CAPTURECOMPARE_REGISTER_0);
    Timer_A_enableCaptureCompareInterrupt(TIMER_A0_MODULE, TIMER_A_CAPTURECOMPARE_REGISTER_1);

    Interrupt_enableInterrupt(INT_TA0_0);
    Interrupt_enableInterrupt(INT_TA0_N);

    PCM_setCoreVoltageLevel(PCM_VCORE0);
    PCM_setPowerState(PCM_AM_LDO_VCORE0);

    //Timer_A_startCounter(TIMER_A0_MODULE, TIMER_A_UP_MODE);
    Interrupt_enableMaster();
}


void Port1IsrHandler(void)
{

}

void Timer_AIsrHandler(void)
{
    Timer_A_clearCaptureCompareInterrupt(TIMER_A0_MODULE, TIMER_A_CAPTURECOMPARE_REGISTER_0);
    //Timer_A_clearCaptureCompareInterrupt(TIMER_A0_MODULE, TIMER_A_CAPTURECOMPARE_REGISTER_1);
    //Timer_A_clearInterruptFlag(TIMER_A0_MODULE);
    P1OUT ^= 1;

}

void Timer_A_N_IsrHandler(void)
{
    Timer_A_clearCaptureCompareInterrupt(TIMER_A0_MODULE, TIMER_A_CAPTURECOMPARE_REGISTER_1);
    P1OUT ^= 1;
}

Best Answer

Related Solutions

Creating a PWM signal using PIC18F4520

TimerA: How to get PWM in upMode configuration?(TI MSP432 Launchpad)

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