PWM output on 2 channels of the dsPIC30F4011 with equal duty cycle and 180 degrees out of phase

Reader Mo Hsen wrote in with an interesting query about generating PWM output from the dsPIC30F4011 on two channels with equal duty cycle but 180 degrees out of phase (see the waveforms below for an example). I’ve created a short sample program (for the C30 compiler) which is included in full below. Mo needs the PWM frequency to be 25kHz, so I’m running at the highest clock speed possible with the internal RC oscillator and I’ve set the PWM prescaler to a ratio of 1:1 in order to achieve fine-grained control of the pulse timing.

The basic approach is as follows:

1. Enable up-down counting for centre-aligned PWM (“_PTMOD = 0b10;”).
2. Enable PWM channels 1 and 2 in complementary mode  (“PWMCON1 = 0b00110011;”).
3. Set your PWM period as normal (e.g. “_PTCKPS = 0; PTPER = 600″).
4. Then, whenever you set the pulse width on channel 1, you set the pulse width on channel 2 to be the opposite, i.e. “PDC2 = (2*PTPER )- PDC1;”. In my example below, I’ve phrased this slightly differently, but basically the idea is the same: if you want a duty cycle of 20%, set PDC1 for 20% and PDC2 for 80%.
5. Finally, your 180 degree phase separated outputs are on PWM1H and PWM2L (pins 37 and 36 respectively).

My example program sets up this type of output, then switches back and forth indefinitely between duty cycles of 25% and 75%. Here’s are screenshots of the output waveforms on both channels for each duty cycle:

In the next screenshot, the cursors are enabled so that the period and frequency are displayed above the waveforms (40us and 25 kHz respectively). Note also that due to the high PWM frequency, I have set the sample rate to 1 MHz so that the recorded signal will not be aliased.

I’m using the following software versions:

• MPLAB IDE v8.50
• Microchip C30 Toolsuite v3.23 (free version)
• PICkit 2 Application v2.61.00
• Windows XP (service pack 3, I think)

Here’s the complete code for the program. I’m using the C30 compiler. Also, in MPLAB I have specified Configuration Bits set in code (in MPLAB’s “Configure” menu, click “Configuration Bits…” and then tick the box).

//
// This is a dsPIC30F4011 program to generate PWM on two channels
// simultaneously with equal duty cycle but 180 degrees out of phase.
//
// Written by Ted Burke
// http://batchloaf.wordpress.com
// Last updated 6-4-2012
//

#include <libpic30.h>
#include <p30f4011.h>

// Configuration settings
_FOSC(CSW_FSCM_OFF & FRC_PLL16); // Fosc=16x7.5MHz, Fcy=30MHz
_FWDT(WDT_OFF);                  // Watchdog timer off
_FBORPOR(MCLR_DIS);              // Disable reset pin

// Function prototypes
void configure_pins();
void set_pwm_duty_cycle(double duty_cycle);
unsigned int read_analog_channel(int n);

int main()
{
	// Set up digital i/o, analog input, PWM, UART and interrupt
	configure_pins();

	// Switch back and forth between 25% and 75% duty cycle
	while(1)
	{
		_LATD1 = 1; // LED on
		set_pwm_duty_cycle(0.75); // 75% duty cycle
		__delay32(15000000);

		_LATD1 = 0; // LED off
		set_pwm_duty_cycle(0.25); // 25% duty cycle
		__delay32(15000000);
	}

	return 0;
}

// Sets the duty cycle on PWM channels 1 & 2
void set_pwm_duty_cycle(double duty_cycle)
{
	PDC1 = duty_cycle * (2 * PTPER);
	PDC2 = (1 - duty_cycle) * (2 * PTPER);
}

// This function sets up digital i/o, analog input, PWM,
// UART and timer interrupt.
void configure_pins()
{
	// Configure all four port D pins (RD0, RD1, RD2, RD3)
	// as digital outputs
	LATD = 0;
	TRISD = 0b1111111111110000;

	// Configure all three port C pins (RC13, RC14, RC15)
	// as digital inputs
	TRISC = 0b1111111111111111;

	// Configure AN0-AN8 as analog inputs
	TRISB = 0x01FF;      // All 9 port B pins are inputs
	ADPCFG = 0xFE00;     // Lowest 9 PORTB pins are analog inputs
	ADCON1 = 0;          // Manually clear SAMP to end sampling, start conversion
	ADCON2 = 0;          // Voltage reference from AVDD and AVSS
	ADCON3 = 0x0005;     // Manual Sample, ADCS=5 -> Tad = 3*Tcy
	ADCON1bits.ADON = 1; // Turn ADC ON

	// Configure PWM for free running mode
	//
	//   PWM period = 2 * Tcy * prescale * PTPER = 0.66ns * 1 * PTPER
	//   PWM pulse width = Tcy * prescale * PDC
	//
	PWMCON1 = 0b00110011; // Enable channels 1 & 2 in complementary mode
	PTCON = 0;
	_PTCKPS = 0;      // prescale=1:1 (0=1:1, 1=1:4, 2=1:16, 3=1:64)
	_PTMOD = 0b10;    // Select up-down counting for centre-aligned PWM
	PTPER = 600;      // 25kHz in up/down counting mode
	PDC1 = PTPER;     // 50% duty cycle on PWM channel 1
	PDC2 = PTPER;     // 50% duty cycle on PWM channel 2
	PTMR = 0;         // Clear 15-bit PWM timer counter
	_PTEN = 1;        // Enable PWM time base
}

// This function reads a single sample from the specified
// analog input. It should take less than 2.5us if the chip
// is running at about 30 MIPS.
// Because the dsPIC30F4011 has a 10-bit ADC, the value
// returned will be between 0 and 1023.
unsigned int read_analog_channel(int channel)
{
	ADCHS = channel;          // Select the requested channel
	ADCON1bits.SAMP = 1;      // Start sampling
	__delay32(30);            // 1us delay @ 30 MIPS
	ADCON1bits.SAMP = 0;      // Start Converting
	while (!ADCON1bits.DONE); // Should take 12 * Tad = 1.2us
	return ADCBUF0;
}

Finally, here are two images of the breadboard circuit I used to test the code. I took these using my phone, so unfortunately the image quality is not very good. However, the main details should hopefully be visible. Note that AVdd and AVss (pins 39 and 40) should be connected to the positive and negative supply voltages too, but I ran out of bits of wire!

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