juanm

Problem with PWM DAC

Discussion created by juanm on Jun 2, 2012
Latest reply on Jun 5, 2012 by juanm

Hi! I'm trying to implement a simply DAC using PWM, but I have the following problem. I'm using a table to produce a sinewave on the output, but the resulting signal isn't perfect. You can see the problem in this video:

 

http://www.youtube.com/watch?feature=player_embedded&v=jdVHTpKcCuM

 

This is the code:

 

unsigned char i = 0;unsigned char sine[256]={  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};void interrupt VectorNumber_Vtpm1ovf tpm1ovf_isr(void){ TPM1SC_TOF = 0; TPM1C0V=sine[i]; i++;}void main(void) {  TPM1SC = 0b01101000;  TPM1C0SC = 0b00101000;  TPM1MOD = 0x00FF;  TPM1C0V = sine[i];  EnableInterrupts;    // enable interrupts  while (1)  {  }}

 

 I'm using an SH8 with a bus frequency of 8Mhz using the internal oscillator.

Thanks! 

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