Sine Wave Generation without ECCP - Using single CCP Module of PIC16F877A

I had previously shown how to generate sinusoidal pulse width modulation (SPWM) signals using the ECCP module in a PIC for generating a sine wave output for use in DC-AC inverter. I have had requests from people asking how to generate the same SPWM signals with other microcontrollers that don't have the ECCP module, such as the super popular PIC16F877A.

So, here I talk about how to generate the same SPWM signals using just one CCP module as can be commonly found on so many microcontrollers. This allows much greater flexibility in microcontroller selection.

You should go through the other articles related to generating SPWM with the ECCP module (if you haven't already gone through them, that is) to get an idea of what I'm talking about regarding sine wave generation with the ECCP module and about sine wave generation in general, really:

• Generation and Implementation of Sine Wave Table
• Smart Sine - Software to generate sine table
• Generation of sine wave using SPWM in PIC16F684
• 600W 50Hz sine wave inverter test circuit
• Feedback in sine wave inverter (PIC16F series based)
• Demystifying The Use of Table Pointer in SPWM - Application in Sine Wave Inverter

The code I had previously used (utilizing the ECCP module) is:


//----------------------------------------------------------------------------------------
//Programmer: Syed Tahmid Mahbub
//Target Microcontroller: PIC16F684
//Compiler: mikroC PRO for PIC (Can easily port to any other compiler)
//-----------------------------------------------------------------------------------------

unsigned char sin_table[32]={0,25,49,73,96,118,137,
159,177,193,208,220,231,239,245,249,250,249,245,
239,231,220,208,193,177,159,137,118,96,73,49,25};


unsigned int TBL_POINTER_NEW, TBL_POINTER_OLD, TBL_POINTER_SHIFT, SET_FREQ;
unsigned int TBL_temp;
unsigned char DUTY_CYCLE;

void interrupt(){
     if (TMR2IF_bit == 1){
        TBL_POINTER_NEW = TBL_POINTER_OLD + SET_FREQ;
        if (TBL_POINTER_NEW < TBL_POINTER_OLD){
           CCP1CON.P1M1 = ~CCP1CON.P1M1; //Reverse direction of full-bridge
        }
        TBL_POINTER_SHIFT = TBL_POINTER_NEW >> 11;
        DUTY_CYCLE = TBL_POINTER_SHIFT;
        CCPR1L = sin_table[DUTY_CYCLE];
        TBL_POINTER_OLD = TBL_POINTER_NEW;
        TMR2IF_bit = 0;
     }
}

void main() {
     SET_FREQ = 410;
     TBL_POINTER_SHIFT = 0;
     TBL_POINTER_NEW = 0;
     TBL_POINTER_OLD = 0;
     DUTY_CYCLE = 0;
     ANSEL = 0; //Disable ADC
     CMCON0 = 7; //Disable Comparator
     PR2 = 249;
     TRISC = 0x3F;
     CCP1CON = 0x4C;
     TMR2IF_bit = 0;
     T2CON = 4; //TMR2 on, prescaler and postscaler 1:1
     while (TMR2IF_bit == 0);
     TMR2IF_bit = 0;
     TRISC = 0;
     TMR2IE_bit = 1;
     GIE_bit = 1;
     PEIE_bit = 1;
    
     while(1);
}
//-------------------------------------------------------------------------------------

That's the previous code. Now let's look at the code based on a single CCP module and not the ECCP module. For this, I chose the super popular PIC16F877A microcontroller. The chosen frequency, like before, is 16kHz. The code is:

 

//----------------------------------------------------------------------------------------
//Programmer: Syed Tahmid Mahbub
//Target Microcontroller: PIC16F877A
//Compiler: mikroC PRO for PIC (Can easily port to any other compiler)
//-----------------------------------------------------------------------------------------

unsigned char sin_table[32]={0, 25, 50, 75, 99, 121, 143, 163, 181,

198, 212, 224, 234, 242, 247, 250, 250, 247, 242, 234, 224, 212, 198,

181, 163, 143, 121, 99, 75, 50, 25,0};

unsigned int TBL_POINTER_NEW, TBL_POINTER_OLD, TBL_POINTER_SHIFT, SET_FREQ;

unsigned int TBL_temp;

unsigned char DUTY_CYCLE;

sbit MOSA at RD0_bit;

sbit MOSB at RD1_bit;

sbit MOSC at RD2_bit;

sbit MOSD at RD3_bit;

unsigned char FlagReg;

sbit Direction at FlagReg.B0;

//0 -> MOS A + D

//1 -> MOS B + C

void interrupt(){

     if (TMR2IF_bit == 1){

        TBL_POINTER_NEW = TBL_POINTER_OLD + SET_FREQ;

        if (TBL_POINTER_NEW < TBL_POINTER_OLD){

           //CCP1CON.P1M1 = ~CCP1CON.P1M1; //Reverse direction of full-bridge

           if (Direction == 0){

              MOSA = 0;

              MOSD = 0;

              MOSB = 1;

              MOSC = 1;

              Direction = 1;

           }

           else{

                MOSB = 0;

                MOSC = 0;

                MOSA = 1;

                MOSD = 1;

                Direction = 0;

           }

        }

        TBL_POINTER_SHIFT = TBL_POINTER_NEW >> 11;

        DUTY_CYCLE = TBL_POINTER_SHIFT;

        CCPR1L = sin_table[DUTY_CYCLE];

        TBL_POINTER_OLD = TBL_POINTER_NEW;

        TMR2IF_bit = 0;

     }

}

void main() {

     SET_FREQ = 410;

     PORTD = 0;

     TRISD = 0;

     PR2 = 249; // 16kHz

     CCPR1L = 0;

     CCP1CON = 12; //PWM mode

     TRISC = 0xFF;

     TMR2IF_bit = 0;

     T2CON = 0x04; //TMR2 on

     while (TMR2IF_bit == 0);

     TMR2IF_bit = 0; //Clear TMR2IF

     PORTC = 0;

     TRISC = 0;

     TMR2IE_bit = 1;

     GIE_bit = 1;

     PEIE_bit = 1;

    

     while (1);

    

}

Now let's talk about the changes I've made in order to be able to use a single CCP module instead of the ECCP module.

When the ECCP module is used, it generates the SPWM signals and sends the modulation signals to the required "MOSFETs" (of course there's a drive circuit in between) depending on the "direction" as dictated by CCP1CON.P1M1 (bit 7 of CCP1CON register). Since this bit does not exist in the CCP module (obviously, since it's "uni-directional"), this functionality must be achieved in software. Since we don't have the ECCP module and have chosen to use a single CCP module only, the 4 drive signals come from other pins not associated to the PWM module. I've chosen PORTD bits 0 to 3. Of course, you can select any other 4 pins.

This is the circuit diagram of the SPWM signal generation portion:

Fig. 1 - Circuit diagram of SPWM generation section - microcontroller + AND gates (Click image to enlarge)

Below (Fig. 2) is the circuit diagram for the configuration of the MOSFETs and the drivers - and the synchronization with the signals generated from Fig. 1 above.

Fig. 2 - MOSFET Configuration Section (Click image to enlarge)

The SPWM generation is done by the single CCP module and which MOSFETs to send the signals to is set by the "Direction" bit and the hardware trick employing the AND gate. When "Direction" is equal to 0, the high side MOSFET A is kept on for 10ms during which time the SPWM signals on CCP1 output (RC2) are sent to low side MOSFET D by sending a "1" to RD3, which, with the help of the AND gate "diverts" the CCP1 signal to the low side MOSFET D (see Fig. 1 above). The same thing is achieved when "Direction" is equal to 1, just with high side MOSFET C and low side MOSFET B. When MOSFETs A and D are operated, MOSFETs B and C are kept off and vice versa. The MOSFETs are first turned off before the other two are turned on, as can be seen in the code block:

           if (Direction == 0){

              MOSA = 0;

              MOSD = 0;

              MOSB = 1;

              MOSC = 1;

              Direction = 1;

           }

           else{

                MOSB = 0;

                MOSC = 0;

                MOSA = 1;

                MOSD = 1;

                Direction = 0;

           }

To understand how the timing and the table pointer operation work, go through this:

Demystifying The Use of Table Pointer in SPWM - Application in Sine Wave Inverter

I've modified the sine table to increase the deadtime. Notice how there's a 0 at both the start and the end. This achieves the additional deadtime. See Fig. 4 below. I did this by using my software "Smart Sine" to generate a sine table with 31 values and then adding a 0 at the end.
 
Besides that, the other functionality are the same - the PWM initialization and setting, the table and table pointer are used the same way as before. So make sure you go through this tutorial if you aren't completely clear regarding it:

Demystifying The Use of Table Pointer in SPWM - Application in Sine Wave Inverter

For the MOSFET drivers, you require high/low side MOSFET drivers. One of the most popular such driver is the IR2110. For a thorough tutorial on using the IR2110, go through this tutorial:
http://tahmidmc.blogspot.com/2013/01/using-high-low-side-driver-ir2110-with.html

Here are the simulation results:

 Fig. 3 - Generated SPWM  Drive Signals (Click image to enlarge)

Fig. 4 - Clear demonstration of the "deadtime" (Click image to enlarge)

Fig. 5 - Simulation results showing signal frequencies (Click image to enlarge)

Fig. 6 - Generated Sine Wave Signal (Click image to enlarge)

The operation is quite simple to understand. The trick lies in a simple software modification and the use of the external AND gates. It's quite simple really! All we've needed are 5 IO pins from the PIC16F877A leaving all the other IO pins unused - for use for so many other tasks you can carry out. Observe how the main function in the code is not doing anything and all is done in the interrupt. Notice the empty endless while(1) loop where you can carry out any other required task.

I hope you've understood how to generate SPWM signals using just the single CCP module of a microcontroller and can now use it for all your applications! Keep in mind that this isn't restricted to only PICs but can be used for any microcontroller containing one PWM module. Let me know your feedback and comments.