On Thu, May 31, 2012 at 8:51 PM,  <alan.b.pearce@stfc.ac.uk> wrote:
>> Slightly confused; I need to drive a transformer in a push pull mode wit=
h
>> 2 FET's from the PIC. I wonder whether the ECCP mode is much suitable to=
 do that
>> (since I will get CCP2 free in this case), or whether it is better to us=
e
>> CCP1 and CCP2 to drive the FET's.
>
> I think you may find this is one of the reasons why they introduced the E=
CCP module. I think you will find it has methods for setting dead time and =
other such requirements. I haven't used the ECCP module in this way, so am =
not qualified to talk 'with knowledge' but seem to remember these were area=
s it improved over the CCP.



I found the dead time a good feature, so I decided to give it a try.

I did some quick testing with a sine table taken from Roman Black's
1kHz sinewave generator


http://www.romanblack.com/onesec/Sine1kHz.htm

using the following code.


#include <p18f4550.h>
#include <delays.h>
#include <timers.h>
#include <pwm.h>

#include "stdint.h"

#pragma config WDT	=3D OFF
#pragma config PLLDIV 	=3D 5 /* 20MHz / 5 =3D 4 MHz, 96MHz PLL i/p */
#pragma config CPUDIV 	=3D OSC1_PLL2 /* CPU CLK =3D 96 MHz/2 =3D 48MHz */
#pragma config USBDIV 	=3D 2 /* 96MHz PLL/2 =3D 48MHz USB clock */
#pragma config FOSC 	=3D HSPLL_HS /* Osc type: HS Crystal, PLL enabled */
#pragma config VREGEN 	=3D ON /* USB Internal Voltage Regulator enabled */
#pragma config PWRT 	=3D ON
#pragma config MCLRE 	=3D OFF
#pragma config LVP 	=3D OFF
#pragma config BOR 	=3D OFF

/*
 * this is the 50 entry "harmonic compensated"
 * sine table (updated 20th Jun 2011)
 */
const rom char sine[50] =3D {
	52, 57, 62, 66, 70, 74, 77, 80, 82, 84,
	85, 86, 86, 86, 85, 83, 81, 78, 75, 72,
	69, 65, 61, 56, 52, 48, 44, 39, 35, 31,
	28, 25, 22, 19, 17, 15, 14, 14, 14, 15,
	16, 18, 20, 23, 26, 30, 34, 38, 43, 48
};

void main (void)
{
	char dc =3D 0;
	char step =3D 0;

	TRISCbits.TRISC2 =3D 0; /* P1A output */
	TRISDbits.TRISD5 =3D 0; /* P1B output */

	/*
	 * Half bridge mode
	 * P1A & P1B used, Active HIGH
	 */
	SetOutputPWM1(HALF_OUT, PWM_MODE_1);

	/*
	 * start PWM
	 * 1PWM cycle/period =3D 100ticks
	 */
	OpenPWM1(100 - 1);

	/*
	 * CPU CLK =3D 48MHz
	 * Tcy =3D 4/48MHz =3D 1/12uS =3D 83.3nS
 	 * ie, Nop() =3D 84nS
	 * 1tick =3D 16*84nS =3D 1344nS =3D 1.344uS;
	 * 100ticks =3D 134.4uS
	 * PWM freq =3D 1/134.4uS =3D 7.4404kHz
	 *
	 * PWM freq =3D 50Hz
	 */
	T2CON =3D 0b00000100; /* TMR2 ON, 1:1 post, 1:16 pre */

	while (1) {
		while (!PIR1bits.TMR2IF);

		dc =3D sine[step];
		SetDCPWM1(dc); /* setup new duty cycle */
		PIR1bits.TMR2IF =3D 0;

		step++;
		if (step >=3D 50)
			step =3D 0;
	}
}

However the output waveform doesn't seem to be as expected ?
I find that the P1B is high most of the time, while P1A low.
At least the duty cycle should vary a bit more consistently ?

Feeling even more confused by now, whether it is the implementation
what I did, or is it due to the ECCP, or in fact whether this what's to
be expected...


P1B
http://www.flickr.com/photos/79743192@N07/7308947670/in/photostream

P1A
http://www.flickr.com/photos/79743192@N07/7308947960/in/photostream


Any ideas ?

Best Regards,
Manu

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