pack_temp.c

#include <stdio.h>
#include <stdint.h>

uint8_t pack_temp(uint8_t *scratchpad) {

  uint8_t v;

  if ( *(scratchpad + 1) & 0x80 ) {
    *(scratchpad + 1) ^= 0xff;
    *scratchpad ^= 0xff;
    if ( *scratchpad == 0xff )
      (*(scratchpad + 1))++;
    (*scratchpad)++;
    v = (*(scratchpad + 1) << 5) | (*scratchpad >> 3);
    if ( v > 56 )
      v = 56;
    v = 56 - v;
  }
  else {
    v = (*(scratchpad + 1) << 5) | (*scratchpad >> 3);
    if ( v > 199 )
      v = 199;
    v += 56;
  }

  return v;

}

int main(int argc, char **argv) {

  uint8_t x[2] = {0x90, 0xfc};

  printf("%d\n", pack_temp(x));
  return 0;
}