adc_random.c

   1 /*
   2  * File:   adc_random.c
   3  *
   4  * Generate a random 8-bit value by reading an unused (floating) IO pin.
   5  * Pin 1, RA2 (AN2) is free in the current design.
   6  */
   7
   8 #include <htc.h>
   9 #include "tmr.h"
  10
  11 unsigned char adc_random()
  12 {
  13     static bit save_ansa2;
  14     static bit save_trisa2;
  15     unsigned char accumulate = 0;
  16
  17     /* Turn on the FVR, configured for 1.024V to the ADC */
  18     FVRCON = 0b10000001;
  19     while (!FVRRDY); /* wait for ready signal */
  20
  21     /* Configure RA2 (AN2) for ADC input */
  22     save_ansa2 = ANSA2;
  23     ANSA2 = 1;
  24     save_trisa2 = TRISA2;
  25     TRISA2 = 1;
  26     ADCON1 = 0b11110011; /* Right justified result, Frc clk, FVR/Vss refs */
  27     ADCON0 = 0b00001001; /* Enable channel AN2, enable ADC */
  28
  29     /* Sample the ADC several times, accumulating the LSB of the result */
  30     for (unsigned i = 0; i < 128; i++) {
  31         tmr_uwait(10); /* Sampling time */
  32         ADGO = 1; /* Start the conversion */
  33         while (ADGO); /* wait for completion */
  34         accumulate += ADRESL;
  35     }
  36
  37     /* Turn off ADC, FVR, and revert PORTA changes */
  38     ADON = 0;
  39     FVREN = 0;
  40     TRISA2 = save_trisa2;
  41     ANSA2 = save_ansa2;
  42
  43     /* Return the result */
  44     return accumulate;
  45 }