Question

Using a microcontroller (PIC18F4580), I need to collect data and send it to an SD card for later analysis. The data it collects will have values between 0 and 1023, or 0x0 and 0x3FF.

So what I need to do is convert 1023 into a base 10 string of literal ASCII values (0x31, 0x30, 0x32, 0x33, …).

My problem is that the only way I can think of to split the digits apart requires a lot of division.

char temp[4];
temp[0] = 1023 % 10;
temp[1] = (1023 % 100) / 10;
temp[2] = (1023 % 1000) / 100;
temp[3] = (1023 % 10000) / 1000;

Using this method, finding the ASCII values of an n digit decimal number requires 2n-1 divisions. Is there a method that would be faster?

The end goal of this is to wind up with a .csv file on the SD card that can quickly be plugged into any laptop to see a graph of the data in Excel.

Answer 1

There’s a way of doing it using subtractions, but I am not convinced it’s faster than using subtractions and modulus on a "normal" CPU (may be different in an embedded environment).

Something like this:

char makedigit (int *number, int base)
{
  static char map[] = "0123456789";
  int ix;

  for (ix=0; *number >= base; ix++) { *number -= base; }

  return map[ix];
}


char *makestring (int number)
{
  static char tmp[5];

  tmp[0] = makedigit(&number, 1000);
  tmp[1] = makedigit(&number, 100);
  tmp[2] = makedigit(&number, 10);
  tmp[3] = makedigit(&number, 1);
  tmp[4] = '\0';

  return tmp;
}

Then, a call to makestring() should result in a (static, so copy it before overwriting) string with the converted number (zero-prefixed, at 4 characters width, as the original assumption is a value in the 0-1023 range).