params.c

#include <stdio.h>
#include <stdint.h>
#include <math.h>
#include <limits.h>
#include <assert.h>
 
// Figure out a good approximation for (x * numerator / denominator), where
// x is an int16_t to uint16_t.  Approximation will be of the form x * p >> s.
void figure_out_approximation(uint64_t numerator, uint64_t denominator)
{
  printf("    ideal:  x * %llu / %llu\n", numerator, denominator);
  assert(numerator < INT_MAX);
  assert(denominator < INT_MAX);
 
  int input_min = -0x8000;
  int input_max = 0xFFFF;
  uint32_t p_max = 0x7FFF;
 
  // Figure out the approximation
  double k = (double)numerator / denominator;
  uint16_t p = 0;
  uint8_t shift = 0;
  for (uint8_t shift_candidate = 0; shift_candidate < 32; shift_candidate++)
  {
    uint32_t p_candidate = round(k * (1 << shift_candidate));
    if (p_candidate > p_max) { break; }
    p = p_candidate;
    shift = shift_candidate;
  }
  while ((p & 1) == 0)
  {
    p >>= 1;
    shift--;
  }
 
  // Figure out its worst error.
  int worst_error = 0;
  int worst_i = -99999;
  double worst_error_percent = 0;
  for (int i = input_min; i <= input_max; i++)
  {
    int ideal = i * (int64_t)numerator / (int)denominator;
    int approximate = ((int32_t)i * p) >> shift;
    int this_error = ideal - approximate;
    if (abs(this_error) > abs(worst_error))
    {
      worst_error = this_error;
      worst_error_percent = 100 * this_error / ideal;
      worst_i = i;
    }
  }
  printf("    approx: x * %d >> %u", p, shift);
  if (worst_error == 0)
  {
    printf(" (no error)\n");
  }
  else
  {
    printf(" (worst error is %d (%.2lf%%) at x=%d)\n", worst_error, worst_error_percent, worst_i);
  }
}
 
void calculate_params(uint8_t isense_range, uint32_t riso, uint32_t rsense)
{
  printf("Board: riso = %u ohm, isense_range = %u A, rsense = %u ohm\n",
    riso, isense_range, rsense);
 
  printf("  vcodes to mA:\n");
  figure_out_approximation((riso + rsense) * 250, 27500 * rsense);
 
  printf("  icodes to mA:\n");
  figure_out_approximation(1000 * isense_range, 27500);
 
  printf("  pinstant to mW:\n");
  figure_out_approximation(isense_range * (riso + rsense) * 5, (rsense * 462));
 
  printf("\n");
}
 
int main()
{
  // Settings for Pololu boards:
  calculate_params(30, 4000000, 1000);
  calculate_params(30, 4000000, 2000);
  calculate_params(30, 4000000, 4000);
}