PWM accurate 31.25kHz on Pin 9 of Nano for Audio

andrewmcloud · December 3, 2021, 4:16pm

Hi there,
i got stuck with this ...

I´m tied to pin 9 as audio output and this needs some changes in the PWM calculation part ...
If i understood right it does have the same freq. like pin 11, what was the original pin in that code.

Is it generally possible to use that pin the way i need it?

With what i got right now something is going on on the scope, but hard to say what to do now.
I´ve spent some hours with digging through the webs, but now i need to ask someone

#define PWM_PIN       9
#define PWM_VALUE     OCR1A
#define LED_PIN       13
#define LED_PORT      PORTB
#define LED_BIT       5
#define PWM_INTERRUPT TIMER2_OVF_vect

// Wavetables, to be filled in by randomization and some precalculated waveforms
uint16_t wavetable[WT_NO_OF][WT_LENGTH];

uint16_t mapFreq(uint16_t input) {
  return (pgm_read_word_near(freqTable + input));
}

void audioOn() {
  // Set up PWM to 31.25kHz, phase accurate

  TCCR1A = _BV(COM1A1) | _BV(WGM20);
  TCCR1B = _BV(CS20);
  OCR1A = 15;
  TIMSK2 = _BV(TOIE2);
}

Grumpy_Mike · December 3, 2021, 4:49pm

Can you post all your code please?
Something that compiles and we can try ourselves.
Why does the PWM frequency need to be a precise frequency, for an audio output it doesn’t need to.be?

What is “that code” you speak of?

MartinL · December 3, 2021, 5:44pm

Hi @andrewmcloud

If you require PWM output at 31.25kHz (rather than 490Hz) on digital pins 9 and 10 then add this line to the setup() portion of your code:

TCCR1B &= ~_BV(CS11);	// Increase the timer1 PWM frequency to 31.25kHz

Thereafter just use the analogWrite() function as normal and it will output a PWM signal at 31.25kHz:

analogWrite(9, 128);    // Output PWM with 50% duty-cycle on D9

If you'd like to add the same functionality on digital pins 3 and 11 then add these lines to the setup() portion as well:

TCCR2B &= ~_BV(CS22);	// Increase the timer2 PWM frequency to 31.25kHz
TCCR2B |= _BV(CS20);

andrewmcloud · December 3, 2021, 6:57pm

I think analogWrite isn´t the correct function.
Sorry for sharing just a part of the code ...

So what i need is the output on pin 9

// inputs
// Analog in 0: Pitch CV 0-5 V
// Analog in 1: Wavetable select 0-5 V
// Analog in 2: Sweep between two wavetables 0-5 V
// output
// Digital 11: Audio out (PWM)


#include <avr/io.h>
#include <avr/interrupt.h>
#include <avr/pgmspace.h>

uint16_t syncPhaseAcc;
uint16_t syncPhaseInc;
uint8_t wavetableStep;
uint8_t wavetableA;
uint8_t wavetableB;
uint16_t sweepPosition;

// Define wavetable parameters
#define WT_LENGTH 16
#define WT_NO_OF 32
#define WT_POT_SCALING 5

// Map Analogue channels
#define FREQUENCY         (0)
#define WAVETABLE_SELECT   (1)
#define SWEEP  (2)
#define FREQUENCY_OFFSET         (3)

// Changing these will also requires rewriting audioOn()
//    Output is on pin 11
#define PWM_PIN       11
#define PWM_VALUE     OCR2A
#define LED_PIN       13
#define LED_PORT      PORTB
#define LED_BIT       5
#define PWM_INTERRUPT TIMER2_OVF_vect

// For converting incoming CV to pitch
const uint16_t freqTable[] PROGMEM = {
549, 550, 552, 554, 556, 557, 559, 561, 563, 565, 566, 568, 570, 572, 574, 575, 577, 579, 581, 583, 585, 586, 588, 590, 592, 594, 596, 598, 600, 601, 603, 605, 607, 609, 611, 613, 615, 617, 619, 621, 623, 625, 627, 629, 631, 633, 635, 637, 639, 641, 643, 645, 647, 649, 651, 653, 655, 657, 659, 661, 663, 666, 668, 670, 672, 674, 676, 678, 680, 683, 685, 687, 689, 691, 693, 696, 698, 700, 702, 705, 707, 709, 711, 714, 716, 718, 720, 723, 725, 727, 729, 732, 734, 736, 739, 741, 743, 746, 748, 751, 753, 755, 758, 760, 763, 765, 767, 770, 772, 775, 777, 780, 782, 785, 787, 790, 792, 795, 797, 800, 802, 805, 807, 810, 812, 815, 818, 820, 823, 825, 828, 831, 833, 836, 838, 841, 844, 846, 849, 852, 855, 857, 860, 863, 865, 868, 871, 874, 876, 879, 882, 885, 888, 890, 893, 896, 899, 902, 905, 908, 910, 913, 916, 919, 922, 925, 928, 931, 934, 937, 940, 943, 946, 949, 952, 955, 958, 961, 964, 967, 970, 973, 976, 979, 982, 985, 989, 992, 995, 998, 1001, 1004, 1007, 1011, 1014, 1017, 1020, 1024, 1027, 1030, 1033, 1037, 1040, 1043, 1046, 1050, 1053, 1056, 1060, 1063, 1067, 1070, 1073, 1077, 1080, 1084, 1087, 1090, 1094, 1097, 1101, 1104, 1108, 1111, 1115, 1118, 1122, 1125, 1129, 1133, 1136, 1140, 1143, 1147, 1151, 1154, 1158, 1162, 1165, 1169, 1173, 1176, 1180, 1184, 1188, 1191, 1195, 1199, 1203, 1207, 1210, 1214, 1218, 1222, 1226, 1230, 1234, 1238, 1242, 1245, 1249, 1253, 1257, 1261, 1265, 1269, 1273, 1277, 1281, 1285, 1290, 1294, 1298, 1302, 1306, 1310, 1314, 1318, 1323, 1327, 1331, 1335, 1339, 1344, 1348, 1352, 1357, 1361, 1365, 1369, 1374, 1378, 1383, 1387, 1391, 1396, 1400, 1405, 1409, 1414, 1418, 1423, 1427, 1432, 1436, 1441, 1445, 1450, 1454, 1459, 1464, 1468, 1473, 1478, 1482, 1487, 1492, 1496, 1501, 1506, 1511, 1515, 1520, 1525, 1530, 1535, 1540, 1545, 1549, 1554, 1559, 1564, 1569, 1574, 1579, 1584, 1589, 1594, 1599, 1604, 1609, 1614, 1620, 1625, 1630, 1635, 1640, 1645, 1651, 1656, 1661, 1666, 1672, 1677, 1682, 1688, 1693, 1698, 1704, 1709, 1715, 1720, 1725, 1731, 1736, 1742, 1747, 1753, 1759, 1764, 1770, 1775, 1781, 1787, 1792, 1798, 1804, 1809, 1815, 1821, 1827, 1832, 1838, 1844, 1850, 1856, 1862, 1868, 1873, 1879, 1885, 1891, 1897, 1903, 1909, 1915, 1921, 1928, 1934, 1940, 1946, 1952, 1958, 1965, 1971, 1977, 1983, 1990, 1996, 2002, 2009, 2015, 2021, 2028, 2034, 2041, 2047, 2054, 2060, 2067, 2073, 2080, 2086, 2093, 2100, 2106, 2113, 2120, 2126, 2133, 2140, 2147, 2153, 2160, 2167, 2174, 2181, 2188, 2195, 2202, 2209, 2216, 2223, 2230, 2237, 2244, 2251, 2258, 2265, 2272, 2280, 2287, 2294, 2301, 2309, 2316, 2323, 2331, 2338, 2346, 2353, 2360, 2368, 2375, 2383, 2390, 2398, 2406, 2413, 2421, 2429, 2436, 2444, 2452, 2460, 2467, 2475, 2483, 2491, 2499, 2507, 2515, 2523, 2531, 2539, 2547, 2555, 2563, 2571, 2579, 2587, 2596, 2604, 2612, 2620, 2629, 2637, 2645, 2654, 2662, 2671, 2679, 2687, 2696, 2705, 2713, 2722, 2730, 2739, 2748, 2756, 2765, 2774, 2783, 2791, 2800, 2809, 2818, 2827, 2836, 2845, 2854, 2863, 2872, 2881, 2890, 2900, 2909, 2918, 2927, 2937, 2946, 2955, 2965, 2974, 2983, 2993, 3002, 3012, 3021, 3031, 3041, 3050, 3060, 3070, 3079, 3089, 3099, 3109, 3119, 3128, 3138, 3148, 3158, 3168, 3178, 3188, 3198, 3209, 3219, 3229, 3239, 3249, 3260, 3270, 3280, 3291, 3301, 3312, 3322, 3333, 3343, 3354, 3365, 3375, 3386, 3397, 3407, 3418, 3429, 3440, 3451, 3462, 3473, 3484, 3495, 3506, 3517, 3528, 3539, 3551, 3562, 3573, 3584, 3596, 3607, 3619, 3630, 3642, 3653, 3665, 3676, 3688, 3700, 3711, 3723, 3735, 3747, 3759, 3771, 3783, 3795, 3807, 3819, 3831, 3843, 3855, 3867, 3880, 3892, 3904, 3917, 3929, 3942, 3954, 3967, 3979, 3992, 4004, 4017, 4030, 4043, 4055, 4068, 4081, 4094, 4107, 4120, 4133, 4146, 4159, 4173, 4186, 4199, 4212, 4226, 4239, 4253, 4266, 4280, 4293, 4307, 4320, 4334, 4348, 4362, 4375, 4389, 4403, 4417, 4431, 4445, 4459, 4474, 4488, 4502, 4516, 4530, 4545, 4559, 4574, 4588, 4603, 4617, 4632, 4647, 4661, 4676, 4691, 4706, 4721, 4736, 4751, 4766, 4781, 4796, 4811, 4827, 4842, 4857, 4873, 4888, 4904, 4919, 4935, 4950, 4966, 4982, 4998, 5013, 5029, 5045, 5061, 5077, 5093, 5110, 5126, 5142, 5158, 5175, 5191, 5207, 5224, 5241, 5257, 5274, 5291, 5307, 5324, 5341, 5358, 5375, 5392, 5409, 5426, 5443, 5461, 5478, 5495, 5513, 5530, 5548, 5565, 5583, 5601, 5618, 5636, 5654, 5672, 5690, 5708, 5726, 5744, 5763, 5781, 5799, 5818, 5836, 5854, 5873, 5892, 5910, 5929, 5948, 5967, 5986, 6005, 6024, 6043, 6062, 6081, 6100, 6120, 6139, 6159, 6178, 6198, 6217, 6237, 6257, 6277, 6297, 6316, 6337, 6357, 6377, 6397, 6417, 6438, 6458, 6478, 6499, 6520, 6540, 6561, 6582, 6603, 6624, 6645, 6666, 6687, 6708, 6729, 6751, 6772, 6793, 6815, 6837, 6858, 6880, 6902, 6924, 6946, 6968, 6990, 7012, 7034, 7056, 7079, 7101, 7124, 7146, 7169, 7192, 7215, 7237, 7260, 7283, 7306, 7330, 7353, 7376, 7400, 7423, 7447, 7470, 7494, 7518, 7541, 7565, 7589, 7613, 7637, 7662, 7686, 7710, 7735, 7759, 7784, 7809, 7833, 7858, 7883, 7908, 7933, 7958, 7983, 8009, 8034, 8060, 8085, 8111, 8137, 8162, 8188, 8214, 8240, 8266, 8293, 8319, 8345, 8372, 8398, 8425, 8452, 8478, 8505, 8532, 8559, 8586, 8614, 8641, 8668, 8696, 8723, 8751, 8779, 8807, 8834, 8862, 8891, 8919, 8947, 8975, 9004, 9032, 9061, 9090, 9119, 9147, 9176, 9206, 9235, 9264, 9293, 9323, 9352, 9382, 9412, 9442, 9472, 9502, 9532, 9562, 9592, 9623, 9653, 9684, 9714, 9745, 9776, 9807, 9838, 9869, 9901, 9932, 9964, 9995, 10027, 10059, 10090, 10122, 10155, 10187, 10219, 10251, 10284, 10317, 10349, 10382, 10415, 10448, 10481, 10514, 10548, 10581, 10615, 10648, 10682, 10716, 10750, 10784, 10818, 10852, 10887, 10921, 10956, 10991, 11026, 11060, 11096, 11131, 11166, 11201, 11237, 11273, 11308, 11344, 11380, 11416, 11452, 11489, 11525, 11562, 11598, 11635, 11672, 11709, 11746, 11783, 11821, 11858, 11896, 11933, 11971, 12009, 12047, 12085, 12124, 12162, 12201, 12239, 12278, 12317, 12356, 12395, 12435, 12474, 12514, 12553, 12593, 12633, 12673, 12713, 12753, 12794, 12834, 12875, 12916, 12957, 12998, 13039, 13081, 13122, 13164, 13205, 13247, 13289, 13331, 13374, 13416, 13458, 13501, 13544, 13587, 13630, 13673, 13717, 13760, 13804, 13847, 13891, 13935, 13979
};

// Wavetables, to be filled in by randomization and some precalculated waveforms
uint16_t wavetable[WT_NO_OF][WT_LENGTH];

uint16_t mapFreq(uint16_t input) {
  return (pgm_read_word_near(freqTable + input));
}

void audioOn() {
  // Set up PWM to 31.25kHz, phase accurate

  TCCR2A = _BV(COM2A1) | _BV(WGM20);
  TCCR2B = _BV(CS20);
  OCR2A = 15;
  TIMSK2 = _BV(TOIE2);
}


void setup() {
  pinMode(PWM_PIN,OUTPUT);
  audioOn();
  pinMode(LED_PIN,OUTPUT);
  // Change random seed value to change the randomized wavetables
  randomSeed(123717284);
  int tmpTable[WT_LENGTH];
  uint16_t maxVal;
  
  for (int j = 0; j< WT_NO_OF; j++)
  {
    maxVal = 0;    
    for (int i = 0; i < WT_LENGTH; i++)
    {
      tmpTable[i] = random(-32768, 32767);
      maxVal = abs(tmpTable[i]) > maxVal ? abs(tmpTable[i]) : maxVal;
      
    }
    // Normalize wavetables
    for (int i = 0; i < WT_LENGTH; i++)
    {
      wavetable[j][i] = (32768*tmpTable[i])/maxVal + 32768;
    }

    // Use up some more random values... totally unnecessary
    for (int i = 0; i < random(4); i++)
    {
      i == i;
    }    
  }

  // Hardcoded wavetables sprinkled among the randomized ones

  // Square one octave up
  wavetable[0][0] = 65535;
  wavetable[0][1] = 65535;
  wavetable[0][2] = 65535;
  wavetable[0][3] = 65535;
  wavetable[0][4] = 0;
  wavetable[0][5] = 0;
  wavetable[0][6] = 0;
  wavetable[0][7] = 0;
  wavetable[0][8] = 65535;
  wavetable[0][9] = 65535;
  wavetable[0][10] = 65535;
  wavetable[0][11] = 65535;
  wavetable[0][12] = 0;
  wavetable[0][13] = 0;
  wavetable[0][14] = 0;
  wavetable[0][15] = 0;

  // Lowest square possible
  wavetable[4][0] = 65535;
  wavetable[4][1] = 65535;
  wavetable[4][2] = 65535;
  wavetable[4][3] = 65535;
  wavetable[4][4] = 65535;
  wavetable[4][5] = 65535;
  wavetable[4][6] = 65535;
  wavetable[4][7] = 65535;
  wavetable[4][8] = 0;
  wavetable[4][9] = 0;
  wavetable[4][10] = 0;
  wavetable[4][11] = 0;
  wavetable[4][12] = 0;
  wavetable[4][13] = 0;
  wavetable[4][14] = 0;
  wavetable[4][15] = 0;

  // Square with two extra peaks
  wavetable[8][0] = 65535;
  wavetable[8][1] = 65535;
  wavetable[8][2] = 65535;
  wavetable[8][3] = 65535;
  wavetable[8][4] = 65535;
  wavetable[8][5] = 65535;
  wavetable[8][6] = 65535;
  wavetable[8][7] = 65535;
  wavetable[8][8] = 0;
  wavetable[8][9] = 65535;
  wavetable[8][10] = 0;
  wavetable[8][11] = 0;
  wavetable[8][12] = 65535;
  wavetable[8][13] = 0;
  wavetable[8][14] = 0;
  wavetable[8][15] = 0;

  // Somewhat random?
  wavetable[9][0] = 8345;
  wavetable[9][1] = 50000;
  wavetable[9][2] = 7643;
  wavetable[9][3] = 65535;
  wavetable[9][4] = 52000;
  wavetable[9][5] = 10000;
  wavetable[9][6] = 20000;
  wavetable[9][7] = 40000;
  wavetable[9][8] = 300;
  wavetable[9][9] = 5120;
  wavetable[9][10] = 60240;
  wavetable[9][11] = 2048;
  wavetable[9][12] = 65535;
  wavetable[9][13] = 0;
  wavetable[9][14] = 58755;
  wavetable[9][15] = 36289;

  // Sawtooth
  wavetable[16][0] = 0x0000;
  wavetable[16][1] = 0x1000;
  wavetable[16][2] = 0x2000;
  wavetable[16][3] = 0x3000;
  wavetable[16][4] = 0x4000;
  wavetable[16][5] = 0x5000;
  wavetable[16][6] = 0x6000;
  wavetable[16][7] = 0x7000;
  wavetable[16][8] = 0x8000;
  wavetable[16][9] = 0x9000;
  wavetable[16][10] = 0xa000;
  wavetable[16][11] = 0xb000;
  wavetable[16][12] = 0xc000;
  wavetable[16][13] = 0xd000;
  wavetable[16][14] = 0xe000;
  wavetable[16][15] = 0xf000;

  // Sine
  wavetable[24][0] = 128 << 8;
  wavetable[24][1] = 176 << 8;
  wavetable[24][2] = 218 << 8;
  wavetable[24][3] = 246 << 8;
  wavetable[24][4] = 255 << 8;
  wavetable[24][5] = 246 << 8;
  wavetable[24][6] = 218 << 8;
  wavetable[24][7] = 176 << 8;
  wavetable[24][8] = 128 << 8;
  wavetable[24][9] = 79 << 8;
  wavetable[24][10] = 37 << 8;
  wavetable[24][11] = 9 << 8;
  wavetable[24][12] = 0 << 8;
  wavetable[24][13] = 9 << 8;
  wavetable[24][14] = 37 << 8;
  wavetable[24][15] = 79 << 8;

  wavetableStep=0;
}

void loop() {
  // Get CV in and convert it to the phase accumulator
    int pwmv = analogRead(FREQUENCY)+analogRead(FREQUENCY_OFFSET);
  if (pwmv > 1023) pwmv = 1023;
      if (pwmv < 0) pwmv = 0;
  syncPhaseInc = mapFreq(pwmv);

  // Read position of sweep
  sweepPosition = analogRead(SWEEP);

  // Read current wavetable and set up wavetables to morph between
  uint8_t readWavetable = analogRead(WAVETABLE_SELECT) >> WT_POT_SCALING;
  wavetableA = readWavetable;
  wavetableB = (readWavetable+1) & 0x1f;
}

SIGNAL(PWM_INTERRUPT)
{
  uint16_t output;
  uint16_t waveA;
  uint16_t waveB;
  uint32_t delta;  
  bool aLarger;

  // Phase accumulator
  syncPhaseAcc += syncPhaseInc;
  if (syncPhaseAcc < syncPhaseInc) 
  {
    // Time to increase the wavetable step
    wavetableStep++;
    wavetableStep &= WT_LENGTH - 1;
    LED_PORT ^= 1 << LED_BIT; // Faster than using digitalWrite
  }

  // Calculate morphing between wavetables, keep delta value positive
  waveA = (wavetable[wavetableA][wavetableStep]);
  waveB = (wavetable[wavetableB][wavetableStep]);
  if (waveA >= waveB)
  {
    aLarger = true;
    delta = waveA - waveB;
  }
  else
  {
    aLarger = false;
    delta = waveB - waveA;
  }
    
  delta = (delta * sweepPosition) >> 10;
  output = aLarger ? waveA - delta : waveA + delta;

  // Output to PWM (this is faster than using analogWrite)  
  // Scale down to 8 bit value
  PWM_VALUE = output  >> 8;
}```

johnwasser · December 3, 2021, 7:47pm

Technically, you should be using the Timer1 constants WGM10 and CS10 with Timer1 registers instead of the Timer2 constants WGM20 and CS20.

MartinL · December 3, 2021, 9:37pm

Hi @andrewmcloud

I see in your code you're using timer 2, an 8-bit timer capable of counting between 0 and 255. In phase correct mode with th prescaler set to 1, this produces an output PWM frequency of 31.25kHz:

PWM frequency = 16MHz / (2 * (255 + 1)) = 31250Hz = 31.25kHz

Resolution = log(255 + 1) / log(2) = 8-bit

However, switching over to the 16-bit timer 1 doesn't solve the resolution problem, since it won't give you 16-bit resolution at 31.25kHz.

If you use phase correct mode with the prescaler set to 1, you'll get 16-bit resolution, but a different PWM frequency entirely:

PWM frequency = 16MHz / (2 * (65535 + 1)) = 122Hz

Resolution = log(65535 + 1) / log(2) = 16-bit

If you attempt to get around this by using timer 1's phase and frequency correct mode, you'll find that you need to load the ICR1 register with 255 (0xFF), in order to generate a 31.25kHz PWM output. However, now your resolution is reduced to 8-bits.

andrewmcloud · December 4, 2021, 7:50am

this sounds a bit too complex for me ...
8 bits is ok, that´s what i want at the end.
i thought it´s a bit simpler as they have the same frequency.
just like swapping the connections, or sources.

i´m asking a bit more specific,
how should the code look like to run on pin 9?
i´m pretty fine with how the code works in its original state, but not with output on pin 11

solving this would probably save my christmas

MartinL · December 4, 2021, 8:50am

Hi @andrewmcloud

If you're just directly substituting timer 2 on pin D11 for timer 1 on D9, then you should just need to make the following changes:

#define PWM_PIN       9

#define PWM_VALUE     OCR1A

#define PWM_INTERRUPT TIMER1_OVF_vect

void audioOn() {
  // Set up PWM to 31.25kHz, phase accurate
  TCCR1B &= ~_BV(CS11);	// Increase the timer1 PWM frequency to 31.25kHz
  OCR1A = 15;
  TIMSK1 = _BV(TOIE1);
}

andrewmcloud · December 4, 2021, 9:41am

You saved my day

it´s running!

Now the world is becoming another chiptuny synthesizer voice.
If you´re into synths you can check ecolabaudio.de

My Banana-Farm says THANK YOU!

andrewmcloud · December 14, 2021, 5:58pm

I´m very sorry to ask,
but would this work with pin10 aswell?

i´ve tried this here, but without results ... if i understood right 9&10 have the same pwm frequency and share the same prescaler, but i´m sure i´ve messed this up.

#define PWM_PIN       10
#define PWM_VALUE     OCR1B
#define LED_PIN       13
#define LED_PORT      PORTB
#define LED_BIT       5
#define PWM_INTERRUPT TIMER1_OVF_vect

void audioOn() {
  // Set up PWM to 31.25kHz, phase accurate

   TCCR1A = _BV(COM1B0) | _BV(WGM10);
TCCR1B &= ~_BV(CS11);
OCR1B = 15;
 TIMSK1 = _BV(TOIE1);
}

MartinL · December 14, 2021, 9:30pm

Hi @andrewmcloud

Thanks for the link by the way. A nice collection of Eurorack modules. I really like the artistic designs as well, a fusion of art and engineering.

Regarding the code, I think you just need to change the COM1B0 to COM1B1 in this line:

TCCR1A = _BV(COM1B1) | _BV(WGM10);

andrewmcloud · December 15, 2021, 7:41am

Thanks MartinL,

i highly welcome your help and it worked again
And thanks for digging into my page.

If you like contact over one of the given channels, i´m looking for someone who´s better in coding than me to realize some further stuff

system · June 13, 2022, 7:41am

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PWM accurate 31.25kHz on Pin 9 of Nano for Audio

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