PIC (dsPIC33EP 512MU810) : High Speed PWM module

Question

I'm trying to generate a 10% duty cycle-1kHz PWM with a clock at 80Mhz (F_osc) using the inbuilt (Hardware) high speed PWM.
According to the documentation, Base Period (number for the PTPER) is calculated by

PTPER = F_osc / [(F_pwm) x (PWM_Clk_Prescale)]

Substituting F_osc = 80Mhz, F_pwm = 1khz and Prescale = 8, I'm getting a count of PTPER=10,000.

However, from my output on an oscilloscope, I can see that I have PWM frequency of ~2khz. PWM output .

Am I doing anything wrong? Below is my code --

#include<p33EP512MU810.h>
#include<p33Exxxx.h>

//---------------- compiler directives------------------
_FOSCSEL(FNOSC_FRC);
_FOSC(FCKSM_CSECMD & OSCIOFNC_OFF  & POSCMD_XT);    // Clock Switching Enabled and Fail Safe Clock Monitor is disabled
                                                    // OSC2 Pin Function: OSC2 is Clock Output
                                                    // Primary Oscillator Mode: XT Crystal
_FWDT(FWDTEN_OFF);                                  // Watchdog Timer Enabled/disabled by user software
//------------------------------------------------------


//function to initialise oscillator and set up clock
void Init_Oscillator(void)
{
   // Configure Oscillator to operate the device at 40Mhz
    // Fosc= Fin*M/(N1*N2), Fcy=Fosc/2
    // Fosc= 12M*40/(3*2)=80Mhz for 12M input clock
    // 0.5M < Fin/N1 < 8MHz
    // 100M < Fin*M/N2 < 200M
    // Fcy  = Fosc / 2 = 40 MIPS
    PLLFBD       = 38;  // M=80
    CLKDIVbits.PLLPOST = 0;   // N2=2
    CLKDIVbits.PLLPRE  = 1;   // N1=3
    OSCTUN             = 0;   // Tune FRC oscillator, if FRC is used
    RCONbits.SWDTEN=0;     // Disable Watch Dog Timer

    // Clock switch to incorporate PLL
    __builtin_write_OSCCONH(0x03);              // Initiate Clock Switch to
                                                                                            // Primary Oscillator with PLL (NOSC=0b011)
    __builtin_write_OSCCONL(0x01);              // Start clock switching
    while (OSCCONbits.COSC != 0b011);           // Wait for Clock switch (to XT w/ PLL) to occur
    while(OSCCONbits.LOCK!=1) {}; // Wait for PLL to lock
}

void Init_PWM(void)
{
    /**** PTCON: PWM Time Base Control Register ****/
    PTCONbits.PTEN   = 0;   // Timer Enable bit:    DISABLE MCPWM

        //---------------------------------------------------------------
        //Prescaler 1:8
        PTCON2bits.PCLKDIV = 0b011; // PCLKSEL: 1,2,4,8,16,32,64
        //mode selection
        PWMCON1 = 0x0000;          //PTPER holds period count of PWM
        //Independent mode enable
        IOCON1 = 0xCC00; //fig 14-35 of PWM manual
        // 0xCC00 => PENH high, PENL high, PMOD: 11- True Independent PWM o/p

        //---------------------------------------------------------------
        /**** PTPER: PWM Time Base Period Register ****/
    PTPER = 10000; // Period Value bits
        //Enable PWM timer
        PTCONbits.PTEN   = 1;   // Timer Enable bit:    ENABLE MCPWM
}
//--- extra settings:
// 1) Stop in IDLE MODE :: PTCONbits.PTSIDL = 1 (yes)
// 2) Setting a particular pin as output ==> IOCONxbits.PENH(/L) = 1(enabled)
// 3) MDC,PDC,SDC et al depend on the PWM MODE.
//    Resp, selection of PTPER,PHASE,SPHASE.
//---


int main(void)
{
        Init_Oscillator();
    Init_PWM();// Initialize PWM module
        PDC1 = 1000; // This sets the duty cycle.
    while(1); //loop forever //while

return 0;
}

Doug Currie Doug Currie · Accepted Answer · 2014-06-06T00:01:41

Despite your comments, I believe you have

PLLDIV = 78 // M = 80

So,

FOSC = FIN * PLLDIV / ((PLLPRE + 2) * 2 * (PLLPOST + 1))

FOSC = 8MHz * 80 / ((1 + 2) * 2 * (0 + 1))

FOSC = 8MHz * 80 / 6

FOSC = ~ 107 MHz

PIC (dsPIC33EP 512MU810) : High Speed PWM module

2 Answers