Sinewaves using interrupts.

[Mike, K8LH]

Someone told me 16 PWM outputs per cycle could generate a decent filtered sine wave. If PBP can handle a much lower 20-KHz (50-usec) interrupt (with 20-MHz clock), maybe we could get Hank to check out the waveforms on his scope? For 16 outputs per cycle he could use an Fout frequency of 20000/16=1250-Hz. For 20 outputs per cycle he could use an Fout frequency of 20000/20=1000-Hz.

So how would you PBP gurus convert this DDS engine for Hank's 20-MHz 16F690 to PBP (Accum and Phase are 24-bit variables)?

//
//  setup CCP in PWM mode for 20-KHz (50-usecs) with prescale 1,
//  postcale 1, and PR2=250-1 (20-MHz clock)
//
//  Fdds = 1 / 0.00005 = 20000 Hz
//  Fres = 20000 / 2^24 = 0.0011920928955078125
//  Phase (1250-Hz) = INT(1250 / 0.0011920928955078125 + 0.5) = 1048576
//  Phase (1000-Hz) = INT(1000 / 0.0011920928955078125 + 0.5) =  838861
//
void interrupt()               //
{ pir1.TMR2IF = 0;             // clear TMR2 interrupt flag
  Accum = Accum + Phase;       // add phase offset to Accum
  ccpr1l = sine[Accum >> 16];  // setup next PWM duty cycle
}

We'd also need a 256 element sine table with PWM values of 0..250. Here's the output from my handy dandy Sine PWM Table Generator;

Number of steps: 256
   Output range: 250
 Table or Array: a

        125,128,131,134,137,140,143,146
        149,152,155,158,161,164,167,169
        172,175,178,181,183,186,189,191
        194,196,199,201,204,206,208,211
        213,215,217,219,221,223,225,227
        228,230,232,233,235,236,237,239
        240,241,242,243,244,245,246,246
        247,248,248,249,249,249,249,249
        250,249,249,249,249,249,248,248
        247,246,246,245,244,243,242,241
        240,239,237,236,235,233,232,230
        228,227,225,223,221,219,217,215
        213,211,208,206,204,201,199,196
        194,191,189,186,183,181,178,175
        172,169,167,164,161,158,155,152
        149,146,143,140,137,134,131,128
        124,121,118,115,112,109,106,103
        100,097,094,091,088,085,082,080
        077,074,071,068,066,063,060,058
        055,053,050,048,045,043,041,038
        036,034,032,030,028,026,024,022
        021,019,017,016,014,013,012,010
        009,008,007,006,005,004,003,003
        002,001,001,000,000,000,000,000
        000,000,000,000,000,000,001,001
        002,003,003,004,005,006,007,008
        009,010,012,013,014,016,017,019
        021,022,024,026,028,030,032,034
        036,038,041,043,045,048,050,053
        055,058,060,063,066,068,071,074
        077,080,082,085,088,091,094,097
        100,103,106,109,112,115,118,121