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No sine wave mod out

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Analog_guy

Posted: Mar 20, 2012 - 05:31 PM

Joined: Jun 16, 2011
Posts: 31

I'm trying to get a sinewave out by modulating the pwm with a sine table but get no modulation, just expected pulse. I'm using avr studio 4.14, atmega8 chip on a dragonrider. Code is:

Code:

#include <avr\io.h>

#include <avr\interrupt.h>

#include <avr/pgmspace.h>

const uint8_t sinewave[] PROGMEM= //256 values

{

0x80,0x83,0x86,0x89,0x8c,0x8f,0x92,0x95,0x98,0x9c,0x9f,0xa2,0xa5,0xa8,0xab,0xae,

0xb0,0xb3,0xb6,0xb9,0xbc,0xbf,0xc1,0xc4,0xc7,0xc9,0xcc,0xce,0xd1,0xd3,0xd5,0xd8,

0xda,0xdc,0xde,0xe0,0xe2,0xe4,0xe6,0xe8,0xea,0xec,0xed,0xef,0xf0,0xf2,0xf3,0xf5,

0xf6,0xf7,0xf8,0xf9,0xfa,0xfb,0xfc,0xfc,0xfd,0xfe,0xfe,0xff,0xff,0xff,0xff,0xff,

0xff,0xff,0xff,0xff,0xff,0xff,0xfe,0xfe,0xfd,0xfc,0xfc,0xfb,0xfa,0xf9,0xf8,0xf7,

0xf6,0xf5,0xf3,0xf2,0xf0,0xef,0xed,0xec,0xea,0xe8,0xe6,0xe4,0xe2,0xe0,0xde,0xdc,

0xda,0xd8,0xd5,0xd3,0xd1,0xce,0xcc,0xc9,0xc7,0xc4,0xc1,0xbf,0xbc,0xb9,0xb6,0xb3,

0xb0,0xae,0xab,0xa8,0xa5,0xa2,0x9f,0x9c,0x98,0x95,0x92,0x8f,0x8c,0x89,0x86,0x83,

0x80,0x7c,0x79,0x76,0x73,0x70,0x6d,0x6a,0x67,0x63,0x60,0x5d,0x5a,0x57,0x54,0x51,

0x4f,0x4c,0x49,0x46,0x43,0x40,0x3e,0x3b,0x38,0x36,0x33,0x31,0x2e,0x2c,0x2a,0x27,

0x25,0x23,0x21,0x1f,0x1d,0x1b,0x19,0x17,0x15,0x13,0x12,0x10,0x0f,0x0d,0x0c,0x0a,

0x09,0x08,0x07,0x06,0x05,0x04,0x03,0x03,0x02,0x01,0x01,0x00,0x00,0x00,0x00,0x00,

0x00,0x00,0x00,0x00,0x00,0x00,0x01,0x01,0x02,0x03,0x03,0x04,0x05,0x06,0x07,0x08,

0x09,0x0a,0x0c,0x0d,0x0f,0x10,0x12,0x13,0x15,0x17,0x19,0x1b,0x1d,0x1f,0x21,0x23,

0x25,0x27,0x2a,0x2c,0x2e,0x31,0x33,0x36,0x38,0x3b,0x3e,0x40,0x43,0x46,0x49,0x4c,

0x4f,0x51,0x54,0x57,0x5a,0x5d,0x60,0x63,0x67,0x6a,0x6d,0x70,0x73,0x76,0x79,0x7c

};

uint8_t i=0;

ISR(TIMER1_COMPA_vect)

{

OCR1A=pgm_read_byte(&sinewave[i]);

i++;

}

int main(void) {

//Port D pins as input

DDRD=0x00;

//Enable internal pull ups

PORTD=0xFF;

//Set PORTB1 pin as output

DDRB=0xFF;

// initial OCR1A value

OCR1A=80;

//Output compare OC1A 8 bit non inverted PWM

TCCR1A=0x91;

//start timer without prescaler

TCCR1B=0x01;

//enable output compare interrupt for OCR1A (TIMSKI=0x10 for ATmega8)

TIMSK=0x02; // OCIE1A = 1

//enable global interrups

sei();

while (1) {

//loop forever. Interrupts will do the job.

}

I checked and it looks correct. Any ideas?

Posted: Mar 20, 2012 - 10:13 PM

Joined: Mar 28, 2001
Posts: 20637
Location: Sydney, Australia (Gum trees, Koalas and Kangaroos, No Edelweiss)

What does your filter look like? Also I would make i volatile, maybe it's not needed but it won't hurt.

_________________
John Samperi
Ampertronics Pty. Ltd.
www.ampertronics.com.au
* Electronic Design * Custom Products * Contract Assembly

Analog_guy

Posted: Mar 20, 2012 - 11:47 PM

Joined: Jun 16, 2011
Posts: 31

Filter doesn't matter since I don't see any variation in pulsewidths to filter out. I did try a rc filter with fc=1000, but no sinewave.

clawson

Posted: Mar 21, 2012 - 09:30 AM

Joined: Jul 18, 2005
Posts: 62952
Location: (using avr-gcc in) Finchingfield, Essex, England

Why write code like this:

Code:

// initial OCR1A value

OCR1A=80;

//Output compare OC1A 8 bit non inverted PWM

TCCR1A=0x91;

//start timer without prescaler

TCCR1B=0x01;

//enable output compare interrupt for OCR1A (TIMSKI=0x10 for ATmega8)

TIMSK=0x02; // OCIE1A = 1

when it could explain itself like this:

Code:

// initial OCR1A value !!!! Bet this is supposed to be 0x80 !!!

OCR1A = 80;

//Output compare OC1A 8 bit non inverted PWM

TCCR1A = (1 << COM1A1) | (1 << COM1B0) | (1 << WGM10);

//start timer without prescaler

TCCR1B = (1 << CS10);

//enable output compare interrupt for OCR1A (TIMSKI=0x10 for ATmega8)

TIMSK = (1 << undefined);

Having done that, if this is really for mega8, several problems become immediately obvious:

1) trivial but your OCR1A start value is supposed to be 0x80

2) trivial but your comment on setting TCCR1A does not match what's actually being done - why even bother with COM1B0 if you don't use OC1B?

3) most important is the 0x02 you are setting in TIMSK. bit 1 is undefined. The comment even says "(TIMSKI=0x10 for ATmega8)" The bit that enables the COMPA1 interrupt is:

Code:

TIMSK = (1 << OCIE1A);

which I guess you'd write this as:

Code:

TIMSK = 0x10;

So why are you setting the register to 0x02?

Also do you really want to change OCR1A on a compare match anyway? Personally I'd enable the overflow interrupt and change OCR1A in that.

_________________

Analog_guy

Posted: Mar 21, 2012 - 05:55 PM

Joined: Jun 16, 2011
Posts: 31

I'm an older analog engineer who's slowly learning how to program a microcontroller. So, it takes me a while to get something working.
I started with a butterfly and Smileys book and then got a dragon and put it on a dragonrider. Anyway, Smiley likes to use the technique of
TIMSK = (1 << OCIE1A) while others use TIMSK = 0x10. I wish I had looked closer at the code, because I would have noticed that TIMSK = 0x10 is wrong! I used TIMSK = (1 << OCIE1A) and got the sinewave output!!

I copied the code from a pdf file "sine_wave_generator" where the code was copied from :

http://www.scienceprog.com/generate-sin ... ontroller/

so I was sure the code was correct.

Thanks for your help!

Actually 'OCR1A = 80' was correct. 256 would have been 100% while 80 gives ~1/3 duty cycle.

clawson

Posted: Mar 21, 2012 - 06:54 PM

Joined: Jul 18, 2005
Posts: 62952
Location: (using avr-gcc in) Finchingfield, Essex, England

Quote:

Actually 'OCR1A = 80' was correct. 256 would have been 100% while 80 gives ~1/3 duty cycle.

You sure? On the very next interrupt after initialisation the table data has:

Code:

const uint8_t sinewave[] PROGMEM= //256 values

{

0x80,

So the 80 will almost immediately be replaced by 0x80.

Actually, to be honest, I don't suppose it matters and there's not really much point setting an initial value anyway as the interrupting will start so soon after power on that it's moot.

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