# DDS - 3phase signals 120 degrees out of phase

Hi

I just completed a project of mine that generates 3 Sine waves 120 degrees out of phase. It took my hours to understand how to generate the sine tables and so forth. Thus I commented in as much detail as possible for future programmers to understand and grasp the idea quicker.

Code:

``````/*
*
* DDS Sine Generator mit ATMEGS 328
* Timer2 generates the  31250 KHz Clock Interrupt
* Use Timer2 Interrupt to change duty cycle for the output PWM signals
* D. Tolken
* 120 degress out of phase signals for 3 phase BLDC motor controller
* CPUT, South Africa

a Huge thumbs up and thanks must be given to Martin Nawrath for the developement of the original code to generate a sine wave using PWM and a LPF.
http://interface.khm.de/index.php/lab/experiments/arduino-dds-sinewave-generator/
*/

#include "avr/pgmspace.h" //Store data in flash (program) memory instead of SRAM

// Look Up table of a single sine period divied up into 256 values. Refer to PWM to sine.xls on how the values was calculated
PROGMEM  prog_uchar sine256[]  = {
127,130,133,136,139,143,146,149,152,155,158,161,164,167,170,173,176,178,181,184,187,190,192,195,198,200,203,205,208,210,212,215,217,219,221,223,225,227,229,231,233,234,236,238,239,240,
242,243,244,245,247,248,249,249,250,251,252,252,253,253,253,254,254,254,254,254,254,254,253,253,253,252,252,251,250,249,249,248,247,245,244,243,242,240,239,238,236,234,233,231,229,227,225,223,
221,219,217,215,212,210,208,205,203,200,198,195,192,190,187,184,181,178,176,173,170,167,164,161,158,155,152,149,146,143,139,136,133,130,127,124,121,118,115,111,108,105,102,99,96,93,90,87,84,81,78,
76,73,70,67,64,62,59,56,54,51,49,46,44,42,39,37,35,33,31,29,27,25,23,21,20,18,16,15,14,12,11,10,9,7,6,5,5,4,3,2,2,1,1,1,0,0,0,0,0,0,0,1,1,1,2,2,3,4,5,5,6,7,9,10,11,12,14,15,16,18,20,21,23,25,27,29,31,
33,35,37,39,42,44,46,49,51,54,56,59,62,64,67,70,73,76,78,81,84,87,90,93,96,99,102,105,108,111,115,118,121,124

};
#define cbi(sfr, bit) (_SFR_BYTE(sfr) &= ~_BV(bit)) //define a bit to have the properties of a clear bit operator
#define sbi(sfr, bit) (_SFR_BYTE(sfr) |= _BV(bit))//define a bit to have the properties of a set bit operator

int PWM1= 11;// PWM1 output, phase 1
int PWM2 = 3; //[WM2 ouput, phase 2
int PWM3 = 10; //PWM3 output, phase 3
int offset_1 = 85; //offset 1 is 120 degrees out of phase with previous phase, Refer to PWM to sine.xls
int offset_2 = 170; //offset 2 is 120 degrees out of phase with offset 1. Refer to PWM to sine.xls
int program_exec_time = 6; //monitor how quickly the interrupt trigger
int ISR_exec_time = 7; //monitor how long the interrupt takes

double dfreq;
const double refclk=31376.6;      // measured output frequency

// variables used inside interrupt service declared as voilatile
volatile byte current_count;              // Keep track of where the current count is in sine 256 array
volatile byte ms4_delay;             //variable used to generate a 4ms delay
volatile byte c4ms;              // after every 4ms this variable is incremented, its used to create a delay of 1 second
volatile unsigned long phase_accumulator;   // pahse accumulator
volatile unsigned long tword_m;  // dds tuning word m, refer to DDS_calculator (from Martin Nawrath) for explination.

void setup()
{
pinMode(PWM1, OUTPUT);      //sets the digital pin as output
pinMode(PWM2, OUTPUT);      //sets the digital pin as output
pinMode(PWM3, OUTPUT);      //sets the digital pin as output
pinMode(program_exec_time, OUTPUT);      //sets the digital pin as output
pinMode(9, OUTPUT);         //sets the digital pin as output
sbi(PORTD,program_exec_time); //Sets the pin
Setup_timer1();
Setup_timer2();

//Disable Timer 1 interrupt to avoid any timing delays
cbi (TIMSK0,TOIE0);              //disable Timer0 !!! delay() is now not available
sbi (TIMSK2,TOIE2);              //enable Timer2 Interrupt

dfreq=1000.0;                    //initial output frequency = 1000.o Hz
tword_m=pow(2,32)*dfreq/refclk;  //calulate DDS new tuning word

}
void loop()
{
while(1)
{
sbi(PORTD,program_exec_time); //Sets the pin
if (c4ms > 250) // c4ms = 4ms, thus 4ms *250 = 1 second delay
{
c4ms=0;                          //Reset c4ms
dfreq=analogRead(0);             //Read voltage on analog 1 to see desired output frequency, 0V = 0Hz, 5V = 1.023kHz
cbi (TIMSK2,TOIE2);              //Disable Timer2 Interrupt
tword_m=pow(2,32)*dfreq/refclk;  //Calulate DDS new tuning word
sbi (TIMSK2,TOIE2);              //Enable Timer2 Interrupt
}
}
}

//Timer 1 setup
//Set prscaler to 1, PWM mode to phase correct PWM,  16000000/510 = 31372.55 Hz clock
void Setup_timer1(void)
{
// Timer1 Clock Prescaler to : 1
sbi (TCCR1B, CS10);
cbi (TCCR1B, CS11);
cbi (TCCR1B, CS12);

// Timer1 PWM Mode set to Phase Correct PWM
cbi (TCCR1A, COM1A0);
sbi (TCCR1A, COM1A1);
cbi (TCCR1A, COM1B0);
sbi (TCCR1A, COM1B1);

// Mode 1 / Phase Correct PWM
sbi (TCCR1A, WGM10);
cbi (TCCR1A, WGM11);
cbi (TCCR1B, WGM12);
cbi (TCCR1B, WGM13);
}

//Timer 1 setup
//Set prscaler to 1, PWM mode to phase correct PWM,  16000000/510 = 31372.55 Hz clock
void Setup_timer2()
{
// Timer2 Clock Prescaler to : 1
sbi (TCCR2B, CS20);
cbi (TCCR2B, CS21);
cbi (TCCR2B, CS22);

// Timer2 PWM Mode set to Phase Correct PWM
cbi (TCCR2A, COM2A0);  // clear Compare Match
sbi (TCCR2A, COM2A1);
cbi (TCCR2A, COM2B0);
sbi (TCCR2A, COM2B1);

// Mode 1  / Phase Correct PWM
sbi (TCCR2A, WGM20);
cbi (TCCR2A, WGM21);
cbi (TCCR2B, WGM22);
}

//Timer2 Interrupt Service at 31372,550 KHz = 32uSec
//This is the timebase REFCLOCK for the DDS generator
//FOUT = (M (REFCLK)) / (2 exp 32)
//Runtime : 8 microseconds
ISR(TIMER2_OVF_vect)
{
cbi(PORTD,program_exec_time); //Clear the pin
sbi(PORTD,ISR_exec_time);          // Sets the pin

phase_accumulator=phase_accumulator+tword_m; //Adds tuning M word to previoud phase accumulator. refer to DDS_calculator (from Martin Nawrath) for explination.
current_count=phase_accumulator >> 24;     // use upper 8 bits of phase_accumulator as frequency information

OCR2A=pgm_read_byte_near(sine256 + current_count); // read value fron ROM sine table and send to PWM
OCR2B=pgm_read_byte_near(sine256 + (uint8_t)(current_count + offset_1)); // read value fron ROM sine table and send to PWM, 120 Degree out of phase of PWM1

OCR1A = pgm_read_byte_near(sine256 + (uint8_t)(current_count + offset_2));// read value fron ROM sine table and send to PWM, 120 Degree out of phase of PWM2
OCR1B = pgm_read_byte_near(sine256 + (uint8_t)(current_count + offset_2));// read value fron ROM sine table and send to PWM, 120 Degree out of phase of PWM2

//increment variable ms4_delay every 4mS/125 =  milliseconds 32uS
if(ms4_delay++ == 125)
{
c4ms++;
ms4_delay=0; //reset count
}

cbi(PORTD,ISR_exec_time);            //Clear the pin
}
``````

Sine wave table calculater: PWM to sine.xlsx
DDS Frequency Calc (Not My Document, helps to explain): dds_calc.xls
Measured Results: Output.bmp

Cheers

dds_calc.xls (8 KB)

PWM to sine.xlsx (44 KB)

Output.BMP (219 KB)

I suspect your phase accumulator handling in the ISR could be improved, if the compiler hasn't already done it for you.
That 24 place shift will probably be slow, so extracting the eight bit integer part of the fixed-point accumulator might be quicker.
Might be worth a peek at the generated code.

That 24 place shift will probably be slow, so extracting the eight bit integer part of the fixed-point accumulator might be quicker.

Good idea, I have a few spots that might benefit from a union in a similar application.

Duane B

rcarduino.blogspot.com

If we're looking for ways to save time, I'll suggest declaring ISR_exec_time and program_exec_time as const. It's surprising how many cycles it takes to do a cbi() or sbi() with a variable bit number. With a const bit number, those collapse to single-byte cbi and sbi instructions.

I think there could be additional improvement - but maybe not much - from declaring current_count to be local to the ISR. It's not used outside the ISR. It's volatile, so it has to be read or stored each time it's used; if it was a local variable, it might just stay in regsiters, and never make it to memory at all.

//Disable Timer 1 interrupt to avoid any timing delays
cbi (TIMSK0,TOIE0); //disable Timer0 !!! delay() is now not available
sbi (TIMSK2,TOIE2); //enable Timer2 Interrupt

what did you mean here?

//Timer 1 setup
//Set prscaler to 1, PWM mode to phase correct PWM, 16000000/510 = 31372.55 Hz clock
void Setup_timer2()
{
// Timer2 Clock Prescaler to : 1

It seems like there's the need of some code-cleaning..

of course, other than that, that's very interesting!!

i want to generate 6 sine wave because i want o fire six pluse igbt so how can i do that??

Hi , i would like to ask , where did you get this equation Lookuptable value = (2^(PWM_bit-1)) + (2^(PWM_bit-1)) sin(2pi/number of samples) ? how the value of ampilitude (3phase) is got ? i didn t understand ( 127 + 3 , 130 + 3 , 133+3 .... ? ) , and this code generate 3 signals like PWM . l didn t understand why dfreq=1000 hz ? . PLease help i am confused
THANKS #gnusmas

Hello, first of all, thanks you for sharing your work, till now helped me a lot to understand how is the way to go.

There is any option to change the frequency of the pwm (output) , keeping the 50Hz of the sine wave?