PIC18f - Dual PWM, sinusoidal PWM

Generating sine wave signal using PIC1xf series is a common project in internet but most of the source code is written in assembly language. Assembly language is good and efficient but it is not portable as C language.

I found a series of tutorials at tahmidmc blog.

The blog explains the sine wave pwm generation from head to toe.

Always start with tutorials for learning.

The key of sine wave is the sine wave duty cycle constants,

tahmidmc has created a useful program to generate sine wave constants. 

It is a handy and easy to use but it requires installation which i do not like.

How TO Use the Software

There are two types of sinusoidal pwm, unipolar pwm and bipolar pwm.

For unipolar pwm, only need two pwm to form sinusoidal wave.

For bipolar pwm, need four pwm to form sinusoidal wave but the DC voltage needed only Vpeak, while unipolar pwm need DC voltage which is double of Vpeak.

At smart sine, for unipolar pwm, you need to add offset to get full sine wave.

A simple example, would be as follow:

Use the 32 samples for better viewing, purpose, but the sample entries can be changed to get desired sine wave frequency.

Example, 

PWM frequency: 20kHz, sine wave frequency = 50Hz.

Sample = (PWM frequency) / (sine wave frequency)

             = (20 000) / (50) = 400

Always tune the PWM frequency to get desired sine wave frequency as the sample entries can be changed.

Now for the PWM settings - ECCP1CON (8 bits)

P1M1:P1M0: Enhanced PWM Output Configuration bits

10 = Half-bridge output: P1A, P1B modulated with dead-band control; P1C, P1D assigned as port pins

DC1B1:DC1B0: PWM Duty Cycle Bit 1 and Bit 0

PWM mode:

These bits are the two LSbs of the 10-bit PWM duty cycle. The eight MSbs of the duty cycle are found in CCPR1L.

CCP1M3:CCP1M0: Enhanced CCP Mode Select bits

1100 = PWM mode: P1A, P1C active-high; P1B, P1D active-high

1101 = PWM mode: P1A, P1C active-high; P1B, P1D active-low

1110 = PWM mode: P1A, P1C active-low; P1B, P1D active-high

1111 = PWM mode: P1A, P1C active-low; P1B, P1D active-low

For dual mode pin assignment,

To skip the datasheet details, it is good to read block diagram,

Half bridge PWM output

The PIC18f enhanced PWM has dead band control which is good for high side and low side switch control. Power transistor has faster switch on time but slower switch off, if both high and low side is switched at the same time, a very high current (shoot-through current) may flow through both power switches, shorting the bridge supply.

Dead band delay allow switch to fully turned off before switch on another power transistor.

ECCP1DEL: PWM DEAD-BAND DELAY REGISTER

PDC6:PDC0: PWM Delay Count bits

Delay time, in number of FOSC/4 (4 * TOSC) cycles, between the scheduled and actual time for a PWM signal to transition to active.

Another important module associated with PWM is timer2. Just for reference, 

Now we have all the details need to write the code:

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#include <p18f4553.h> //****************************************************************************** // CONFIG //****************************************************************************** #pragma config PLLDIV = 5 // (20 MHz crystal on PICDEM FS USB board) #pragma config CPUDIV = OSC1_PLL2 #pragma config USBDIV = 2 // Clock source from 96MHz PLL/2 #pragma config FOSC = HSPLL_HS #pragma config FCMEN = OFF #pragma config IESO = OFF #pragma config PWRT = OFF #pragma config BOR = ON #pragma config BORV = 3 #pragma config VREGEN = ON //USB Voltage Regulator #pragma config WDT = OFF #pragma config WDTPS = 32768 #pragma config MCLRE = ON #pragma config LPT1OSC = OFF #pragma config PBADEN = OFF // #pragma config CCP2MX = ON #pragma config STVREN = ON #pragma config LVP = OFF // #pragma config ICPRT = OFF // Dedicated In-Circuit Debug/Programming #pragma config XINST = OFF // Extended Instruction Set #pragma config CP0 = OFF #pragma config CP1 = OFF // #pragma config CP2 = OFF // #pragma config CP3 = OFF #pragma config CPB = OFF // #pragma config CPD = OFF #pragma config WRT0 = OFF #pragma config WRT1 = OFF // #pragma config WRT2 = OFF // #pragma config WRT3 = OFF #pragma config WRTB = OFF // Boot Block Write Protection #pragma config WRTC = OFF // #pragma config WRTD = OFF #pragma config EBTR0 = OFF #pragma config EBTR1 = OFF // #pragma config EBTR2 = OFF // #pragma config EBTR3 = OFF #pragma config EBTRB = OFF const rom unsigned int unipolar_32_sine_wave_constant [32] = { 511, 611, 707, 795, 872, 936, 983, 1012, 1022, 1012, 983, 936, 872, 795, 707, 611, 511, 411, 315, 227, 150, 86, 39, 10, 0, 10, 39, 86, 150, 227, 315, 411 }; struct pwm_settings { unsigned char mode; unsigned int duty_cycle; unsigned int frequency; }; struct pwm_settings pwm; unsigned char counter=0; void main() { RCONbits.IPEN=1; // set the dual mode ECCP1CONbits.P1M = 0b10; ECCP1CONbits.CCP1M = 0b1101; // set the pin output TRISCbits.TRISC2 = 0; TRISDbits.TRISD5 = 0; //--------------------------------------------------- // set timer 2 T2CONbits.T2CKPS = 0b00; T2CONbits.TOUTPS = 0b0000; T2CONbits.TMR2ON = 0; // STOP TIMER2 registers to POR state PIE1bits.TMR2IE = 0; // Disable Timer2 overflow interrupts PR2 = 0xFF; // Set period //--------------------------------------------------- // set dead-bank delay ECCP1DELbits.PDC = 0b000000; CCPR1L = 0xFF; ECCP1CONbits.DC1B = 0b00; INTCONbits.GIEH=1; T2CONbits.TMR2ON = 1; // Enable TIMER2 registers to POR state PIE1bits.TMR2IE = 1; // Enable Timer2 overflow interrupts } //---------------------------------------------------------------------------- // High priority interrupt vector void InterruptHandlerHigh() ; #pragma code InterruptVectorHigh = 0x08 void InterruptVectorHigh(void) { _asm goto InterruptHandlerHigh //jump to interrupt routine _endasm } //---------------------------------------------------------------------------- // High priority interrupt routine #pragma code #pragma interrupt InterruptHandlerHigh // // Timer0 overflow void InterruptHandlerHigh() { if (PIR1bits.TMR2IF) PIR1bits.TMR2IF = 0; counter++; if(counter >= 32) counter=0; pwm.duty_cycle = unipolar_32_sine_wave_constant[counter]; /* Duty cycle PWM port1 */ CCPR1L = (unsigned char) (pwm.duty_cycle >> 2); ECCP1CONbits.DC1B = (unsigned char) (pwm.duty_cycle); }

Result:

PIC18f - 10 bits PWM

After I have my hand on kingst logic analyser, I suddenly realise there a lot of things that I can do. I can write a simple coding and check the output signal using logic analyser. It makes the development way easier.

"Previously, I was lack of this kind of equipment for verification purpose." - no more excuse for myself and start coding.

I decide to try on PIC18f PWM, PWM is widely used in power converter control to boost / buck voltage or convert DC voltage to AC voltage.

PWM also generally adapted to emulate DAC in order to generate SINE wave, sawtooth wave.


Before started, always good to look in datasheet and familiar with register first.

1. PWM FREQUENCY CALCULATION

   The equation is pretty much self explainable.

2. REGISTER 15-1: CCPxCON: STANDARD CCPx CONTROL REGISTER

bit 5-4:	PWM mode: These bits are the two LSbs (bit 1 and bit 0) of the 10-bit PWM duty cycle. The eight MSbs of the duty cycle are found in CCPR1L.
bit 3-0:	CCPxM3:CCPxM0: CCPx Module Mode Select bits: 11xx = PWM mode

3.If you want to prescale your FOSC, please take note on this
   PWM Period = [(PR2) + 1] • 4 • TOSC • (TMR2 Prescale Value)

4. For prescaling the at Timer 2.
    T2CON: TIMER2 CONTROL REGISTER

bit 1-0

T2CKPS1:T2CKPS0: Timer2 Clock Prescale Select bits 00 = Prescaler is 1 01 = Prescaler is 4 1x = Prescaler is 16

I always clock my PIC18f at 48Mhz because I want the maximum performance and I want the clock frequency can be applicable to all projects so that I dont have to change the frequency all the times.

Using equation for note 1:
FPWM = FOSC / (4(PR2 + 1))
my maximum frequency is 46875 where PR2 = 255, FOSC = 48,000,000.

PR2 = 255, mean that I can have duty cycle of 10 bits resolution.

For the duty cycle, I have 8 MSB at CCPR1L, 2 LSB at CCP1CON,
I do like following:

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pwm_h = (unsigned char) (duty_cycle >> 2); CCPR1L = pwm_h; pwm_l = (unsigned char) (duty_cycle << 4); CCP1CON = (CCP1CON & 0xCF) | (pwm_l & 0x30);

Basically, we have everything we need, now for the whole code:

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#pragma config PLLDIV = 5 // (20 MHz crystal on PICDEM FS USB board) #pragma config CPUDIV = OSC1_PLL2 #pragma config USBDIV = 2 // Clock source from 96MHz PLL/2 #pragma config FOSC = HSPLL_HS #pragma config FCMEN = OFF #pragma config IESO = OFF #pragma config PWRT = OFF #pragma config BOR = ON #pragma config BORV = 3 #pragma config VREGEN = ON //USB Voltage Regulator #pragma config WDT = OFF #pragma config WDTPS = 32768 #pragma config MCLRE = ON #pragma config LPT1OSC = OFF #pragma config PBADEN = OFF // #pragma config CCP2MX = ON #pragma config STVREN = ON #pragma config LVP = OFF // #pragma config ICPRT = OFF // Dedicated In-Circuit Debug/Programming #pragma config XINST = OFF // Extended Instruction Set #pragma config CP0 = OFF #pragma config CP1 = OFF // #pragma config CP2 = OFF // #pragma config CP3 = OFF #pragma config CPB = OFF // #pragma config CPD = OFF #pragma config WRT0 = OFF #pragma config WRT1 = OFF // #pragma config WRT2 = OFF // #pragma config WRT3 = OFF #pragma config WRTB = OFF // Boot Block Write Protection #pragma config WRTC = OFF // #pragma config WRTD = OFF #pragma config EBTR0 = OFF #pragma config EBTR1 = OFF // #pragma config EBTR2 = OFF // #pragma config EBTR3 = OFF #pragma config EBTRB = OFF #include <p18f4553.h> void main() { unsigned char pwm_l, pwm_h; unsigned int period; unsigned int frequency = 46875; // maximum available freq for 10 bits unsigned int duty_cycle = 512; // half duty for testing purpose period = 0xFF; // for 10 bits operation CCP1CON |= 0b00001100; //ccpxm3:ccpxm0 11xx=pwm mode TRISCbits.TRISC2 = 0; //--------------------------------------------------- T2CONbits.TMR2ON = 0; // STOP TIMER2 registers to POR state PR2 = period; // Set period T2CONbits.TMR2ON = 1; // Turn on PWM1 pwm_h = (unsigned char) (pwm.duty_cycle >> 2); CCPR1L = pwm_h; pwm_l = (unsigned char) (pwm.duty_cycle << 4); CCP1CON = (CCP1CON & 0xCF) | (pwm_l & 0x30); }

Result:
Time to show off my logic analyser:

The readings are located at right side, where the frequency is shown as 45.45kHz, 50% duty cycle. The result is good for me.
45kHz and 10 bits duty cycle is suffice for most application.