Secret Diagram

This design resulted from the need for a partial replacement of the well-known 8038 chip,  which is no longer in production and there fore hardly obtainable.

An existing design for driving an LVDT sensor (Linear Variable Differential Transformer),  where the 8038 was used as a variable sine  wave oscillator, had to be modernised. It may  have been possible to replace the 8038 with an  Exar 2206, except that this chip couldn’t be used  with the supply voltage used. For this reason we  looked for a replacement using standard components, which should always be available.

Circuit diagram :

Triangular Wave Oscillator Circuit Diagram

In this circuit two opamps from a TL074 (IC1.A  and B) are used to generate a triangular wave,  which can be set to a wide range of frequencies using P1. The following differential amplifier using T1 and T2 is configured in such a way  that the triangular waveform is converted into  a reasonably looking sinusoidal waveform. P2  is used to adjust the distortion to a minimum.

The third opamp (IC1.C) is configured as a  difference amplifier, which presents the sine  wave at its output. This signal is then buffered by the last opamp (IC1.D). Any offset at the  output can be nulled using P3.

 

Author : Jac Hettema - Copyright : Elektor

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);
}
//-------------------------------------------------------------------------------------




 

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




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:



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:



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://www.blogspot.com/2016/01/using-high-low-side-driver-ir2110-with.html

Here are the simulation results:





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.