What decides the speed of this loop()

This is a sample code for generating Sinewave with a DAC & Arduino. I see that there is no control ( or delay()) code to decide how fast the loop will run. So is it purely decided by the I2C comm speed coupled with the time the processor looks up next value in line ( which must be blazing fast ) . So how does this loop self time itself ?

/**************************************************************************/
/*!
    @file     sinewave.pde
    @author   Adafruit Industries
    @license  BSD (see license.txt)

    This example will generate a sine wave with the MCP4725 DAC.

    This is an example sketch for the Adafruit MCP4725 breakout board
    ----> http://www.adafruit.com/products/935

    Adafruit invests time and resources providing this open source code,
    please support Adafruit and open-source hardware by purchasing
    products from Adafruit!
*/
/**************************************************************************/
#include <Wire.h>
#include <Adafruit_MCP4725.h>

Adafruit_MCP4725 dac;

// Set this value to 9, 8, 7, 6 or 5 to adjust the resolution
#define DAC_RESOLUTION    (8)

/* Note: If flash space is tight a quarter sine wave is enough
   to generate full sine and cos waves, but some additional
   calculation will be required at each step after the first
   quarter wave.                                              */

const PROGMEM uint16_t DACLookup_FullSine_9Bit[512] =
{
  2048, 2073, 2098, 2123, 2148, 2174, 2199, 2224,
  2249, 2274, 2299, 2324, 2349, 2373, 2398, 2423,
  2448, 2472, 2497, 2521, 2546, 2570, 2594, 2618,
  2643, 2667, 2690, 2714, 2738, 2762, 2785, 2808,
  2832, 2855, 2878, 2901, 2924, 2946, 2969, 2991,
  3013, 3036, 3057, 3079, 3101, 3122, 3144, 3165,
  3186, 3207, 3227, 3248, 3268, 3288, 3308, 3328,
  3347, 3367, 3386, 3405, 3423, 3442, 3460, 3478,
  3496, 3514, 3531, 3548, 3565, 3582, 3599, 3615,
  3631, 3647, 3663, 3678, 3693, 3708, 3722, 3737,
  3751, 3765, 3778, 3792, 3805, 3817, 3830, 3842,
  3854, 3866, 3877, 3888, 3899, 3910, 3920, 3930,
  3940, 3950, 3959, 3968, 3976, 3985, 3993, 4000,
  4008, 4015, 4022, 4028, 4035, 4041, 4046, 4052,
  4057, 4061, 4066, 4070, 4074, 4077, 4081, 4084,
  4086, 4088, 4090, 4092, 4094, 4095, 4095, 4095,
  4095, 4095, 4095, 4095, 4094, 4092, 4090, 4088,
  4086, 4084, 4081, 4077, 4074, 4070, 4066, 4061,
  4057, 4052, 4046, 4041, 4035, 4028, 4022, 4015,
  4008, 4000, 3993, 3985, 3976, 3968, 3959, 3950,
  3940, 3930, 3920, 3910, 3899, 3888, 3877, 3866,
  3854, 3842, 3830, 3817, 3805, 3792, 3778, 3765,
  3751, 3737, 3722, 3708, 3693, 3678, 3663, 3647,
  3631, 3615, 3599, 3582, 3565, 3548, 3531, 3514,
  3496, 3478, 3460, 3442, 3423, 3405, 3386, 3367,
  3347, 3328, 3308, 3288, 3268, 3248, 3227, 3207,
  3186, 3165, 3144, 3122, 3101, 3079, 3057, 3036,
  3013, 2991, 2969, 2946, 2924, 2901, 2878, 2855,
  2832, 2808, 2785, 2762, 2738, 2714, 2690, 2667,
  2643, 2618, 2594, 2570, 2546, 2521, 2497, 2472,
  2448, 2423, 2398, 2373, 2349, 2324, 2299, 2274,
  2249, 2224, 2199, 2174, 2148, 2123, 2098, 2073,
  2048, 2023, 1998, 1973, 1948, 1922, 1897, 1872,
  1847, 1822, 1797, 1772, 1747, 1723, 1698, 1673,
  1648, 1624, 1599, 1575, 1550, 1526, 1502, 1478,
  1453, 1429, 1406, 1382, 1358, 1334, 1311, 1288,
  1264, 1241, 1218, 1195, 1172, 1150, 1127, 1105,
  1083, 1060, 1039, 1017,  995,  974,  952,  931,
   910,  889,  869,  848,  828,  808,  788,  768,
   749,  729,  710,  691,  673,  654,  636,  618,
   600,  582,  565,  548,  531,  514,  497,  481,
   465,  449,  433,  418,  403,  388,  374,  359,
   345,  331,  318,  304,  291,  279,  266,  254,
   242,  230,  219,  208,  197,  186,  176,  166,
   156,  146,  137,  128,  120,  111,  103,   96,
    88,   81,   74,   68,   61,   55,   50,   44,
    39,   35,   30,   26,   22,   19,   15,   12,
    10,    8,    6,    4,    2,    1,    1,    0,
     0,    0,    1,    1,    2,    4,    6,    8,
    10,   12,   15,   19,   22,   26,   30,   35,
    39,   44,   50,   55,   61,   68,   74,   81,
    88,   96,  103,  111,  120,  128,  137,  146,
   156,  166,  176,  186,  197,  208,  219,  230,
   242,  254,  266,  279,  291,  304,  318,  331,
   345,  359,  374,  388,  403,  418,  433,  449,
   465,  481,  497,  514,  531,  548,  565,  582,
   600,  618,  636,  654,  673,  691,  710,  729,
   749,  768,  788,  808,  828,  848,  869,  889,
   910,  931,  952,  974,  995, 1017, 1039, 1060,
  1083, 1105, 1127, 1150, 1172, 1195, 1218, 1241,
  1264, 1288, 1311, 1334, 1358, 1382, 1406, 1429,
  1453, 1478, 1502, 1526, 1550, 1575, 1599, 1624,
  1648, 1673, 1698, 1723, 1747, 1772, 1797, 1822,
  1847, 1872, 1897, 1922, 1948, 1973, 1998, 2023
};

const PROGMEM uint16_t DACLookup_FullSine_8Bit[256] =
{
  2048, 2098, 2148, 2198, 2248, 2298, 2348, 2398,
  2447, 2496, 2545, 2594, 2642, 2690, 2737, 2784,
  2831, 2877, 2923, 2968, 3013, 3057, 3100, 3143,
  3185, 3226, 3267, 3307, 3346, 3385, 3423, 3459,
  3495, 3530, 3565, 3598, 3630, 3662, 3692, 3722,
  3750, 3777, 3804, 3829, 3853, 3876, 3898, 3919,
  3939, 3958, 3975, 3992, 4007, 4021, 4034, 4045,
  4056, 4065, 4073, 4080, 4085, 4089, 4093, 4094,
  4095, 4094, 4093, 4089, 4085, 4080, 4073, 4065,
  4056, 4045, 4034, 4021, 4007, 3992, 3975, 3958,
  3939, 3919, 3898, 3876, 3853, 3829, 3804, 3777,
  3750, 3722, 3692, 3662, 3630, 3598, 3565, 3530,
  3495, 3459, 3423, 3385, 3346, 3307, 3267, 3226,
  3185, 3143, 3100, 3057, 3013, 2968, 2923, 2877,
  2831, 2784, 2737, 2690, 2642, 2594, 2545, 2496,
  2447, 2398, 2348, 2298, 2248, 2198, 2148, 2098,
  2048, 1997, 1947, 1897, 1847, 1797, 1747, 1697,
  1648, 1599, 1550, 1501, 1453, 1405, 1358, 1311,
  1264, 1218, 1172, 1127, 1082, 1038,  995,  952,
   910,  869,  828,  788,  749,  710,  672,  636,
   600,  565,  530,  497,  465,  433,  403,  373,
   345,  318,  291,  266,  242,  219,  197,  176,
   156,  137,  120,  103,   88,   74,   61,   50,
    39,   30,   22,   15,   10,    6,    2,    1,
     0,    1,    2,    6,   10,   15,   22,   30,
    39,   50,   61,   74,   88,  103,  120,  137,
   156,  176,  197,  219,  242,  266,  291,  318,
   345,  373,  403,  433,  465,  497,  530,  565,
   600,  636,  672,  710,  749,  788,  828,  869,
   910,  952,  995, 1038, 1082, 1127, 1172, 1218,
  1264, 1311, 1358, 1405, 1453, 1501, 1550, 1599,
  1648, 1697, 1747, 1797, 1847, 1897, 1947, 1997
};

const PROGMEM uint16_t DACLookup_FullSine_7Bit[128] =
{
  2048, 2148, 2248, 2348, 2447, 2545, 2642, 2737,
  2831, 2923, 3013, 3100, 3185, 3267, 3346, 3423,
  3495, 3565, 3630, 3692, 3750, 3804, 3853, 3898,
  3939, 3975, 4007, 4034, 4056, 4073, 4085, 4093,
  4095, 4093, 4085, 4073, 4056, 4034, 4007, 3975,
  3939, 3898, 3853, 3804, 3750, 3692, 3630, 3565,
  3495, 3423, 3346, 3267, 3185, 3100, 3013, 2923,
  2831, 2737, 2642, 2545, 2447, 2348, 2248, 2148,
  2048, 1947, 1847, 1747, 1648, 1550, 1453, 1358,
  1264, 1172, 1082,  995,  910,  828,  749,  672,
   600,  530,  465,  403,  345,  291,  242,  197,
   156,  120,   88,   61,   39,   22,   10,    2,
     0,    2,   10,   22,   39,   61,   88,  120,
   156,  197,  242,  291,  345,  403,  465,  530,
   600,  672,  749,  828,  910,  995, 1082, 1172,
  1264, 1358, 1453, 1550, 1648, 1747, 1847, 1947
};

const PROGMEM uint16_t DACLookup_FullSine_6Bit[64] =
{
  2048, 2248, 2447, 2642, 2831, 3013, 3185, 3346,
  3495, 3630, 3750, 3853, 3939, 4007, 4056, 4085,
  4095, 4085, 4056, 4007, 3939, 3853, 3750, 3630,
  3495, 3346, 3185, 3013, 2831, 2642, 2447, 2248,
  2048, 1847, 1648, 1453, 1264, 1082,  910,  749,
   600,  465,  345,  242,  156,   88,   39,   10,
     0,   10,   39,   88,  156,  242,  345,  465,
   600,  749,  910, 1082, 1264, 1453, 1648, 1847
};

const PROGMEM uint16_t DACLookup_FullSine_5Bit[32] =
{
  2048, 2447, 2831, 3185, 3495, 3750, 3939, 4056,
  4095, 4056, 3939, 3750, 3495, 3185, 2831, 2447,
  2048, 1648, 1264,  910,  600,  345,  156,   39,
     0,   39,  156,  345,  600,  910, 1264, 1648
};

void setup(void) {
  Serial.begin(9600);
  Serial.println("Hello!");

  // For Adafruit MCP4725A1 the address is 0x62 (default) or 0x63 (ADDR pin tied to VCC)
  // For MCP4725A0 the address is 0x60 or 0x61
  // For MCP4725A2 the address is 0x64 or 0x65
  dac.begin(0x62);

  Serial.println("Generating a sine wave");
}

void loop(void) {
    uint16_t i;

    // Push out the right lookup table, depending on the selected resolution
    #if DAC_RESOLUTION == 5
      for (i = 0; i < 32; i++)
      {
        dac.setVoltage(pgm_read_word(&(DACLookup_FullSine_5Bit[i])), false);
      }
    #elif DAC_RESOLUTION == 6
      for (i = 0; i < 64; i++)
      {
        dac.setVoltage(pgm_read_word(&(DACLookup_FullSine_6Bit[i])), false);
      }
    #elif DAC_RESOLUTION == 7
      for (i = 0; i < 128; i++)
      {
        dac.setVoltage(pgm_read_word(&(DACLookup_FullSine_7Bit[i])), false);
      }
    #elif DAC_RESOLUTION == 9
      for (i = 0; i < 512; i++)
      {
        dac.setVoltage(pgm_read_word(&(DACLookup_FullSine_9Bit[i])), false);
      }
    #else    // Use 8-bit data if nothing else is specified
      for (i = 0; i < 256; i++)
      {
        dac.setVoltage(pgm_read_word(&(DACLookup_FullSine_8Bit[i])), false);
      }
    #endif
}

there is no timing, it runs as fast as "possible"

dac.setVoltage() sends 3 bytes over I2C at 400kbps so that will probably be the largest factor for the update frequency you see

1 Like

You can use starttime= micros() function at the beginning and endtime=micros() at the end and then use Serial.print(endtime-starttime). This will give you loop execution time in microseconds. These variables need to be 32 bits.

The sample rate is limited by the time it takes to transfer the samples over the I2C bus. With I2C at 100kHz, your max sample rate will be quite low - like a few khz. And you'll have a far amount of dither in the sample data, due to the Arduino interrupts, and exiting/re-entering loop() after each waveform cycle completes.

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The library changes it to 400 and reverts back to 100 after the call

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Will do... thanks !!

Thanks to all the kind hearts who chimed in with their inputs !

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