Reading an analog keypad on the ATtiny85

README.md

This is sample code to read the analog keypad I describe in this article. Simply clone the ATtiny85 template library and replace the main.c file with the one contained here.

main.c

/*--------------------------------------------------------------------------*
* Main program
*---------------------------------------------------------------------------*
* 14-Apr-2014 ShaneG
*
* Template program for ATtiny85 C/asm projects.
*--------------------------------------------------------------------------*/
#include <stdint.h>
#include <stdbool.h>
#include <avr/io.h>
#include "softuart.h"
#include "utility.h"
#include "iohelp.h"

// Forward declaration with 'noreturn' attribute
void main() __attribute__ ((noreturn));

//! ADC input for the keyboard
#define ADC_KEYBOARD 1

/* Key values
 */
#define KEYVAL_UP     170
#define KEYVAL_RIGHT  342
#define KEYVAL_DOWN   512
#define KEYVAL_LEFT   682
#define KEYVAL_CENTER 852

/* Error range
 *
 * A key is considered pressed if the ADC reading is withing +/- this amount
 * from the 'ideal' value described above.
 */
#define KEYVAL_RANGE 16

/** Key codes
 *
 * Defined as bit masks so they can be combined easily.
 */
typedef enum _KEYCODE {
  KEYCODE_UP     = 0x01,
  KEYCODE_RIGHT  = 0x02,
  KEYCODE_DOWN   = 0x04,
  KEYCODE_LEFT   = 0x08,
  KEYCODE_CENTER = 0x10,
  } KEYCODE;

/** Determine which (if any) key is pressed
 *
 * @return a bit mask representing the key pressed or 0 if no key.
 */
uint8_t keyRead() {
  uint8_t result = 0xFF;
  for(uint8_t count=0; count<4; count++) {
    uint16_t keyval = adcRead(ADC_KEYBOARD, 1, 1);
    if((keyval>=(KEYVAL_UP - KEYVAL_RANGE))&&(keyval<=(KEYVAL_UP + KEYVAL_RANGE)))
      result &= KEYCODE_UP;
    else if((keyval>=(KEYVAL_RIGHT - KEYVAL_RANGE))&&(keyval<=(KEYVAL_RIGHT + KEYVAL_RANGE)))
      result &= KEYCODE_RIGHT;
    else if((keyval>=(KEYVAL_DOWN - KEYVAL_RANGE))&&(keyval<=(KEYVAL_DOWN + KEYVAL_RANGE)))
      result &= KEYCODE_DOWN;
    else if((keyval>=(KEYVAL_LEFT - KEYVAL_RANGE))&&(keyval<=(KEYVAL_LEFT + KEYVAL_RANGE)))
      result &= KEYCODE_LEFT;
    else if((keyval>=(KEYVAL_CENTER - KEYVAL_RANGE))&&(keyval<=(KEYVAL_CENTER + KEYVAL_RANGE)))
      result &= KEYCODE_CENTER;
    }
  // All done, a consistant keypress will have a valid bit code now
  return result;
  }

/** Program entry point
 */
void main() {
  uartInit();
  adcInit(ADC_KEYBOARD);
  // Keep showing when the key changes
  uint8_t key, last = 0;
  while(true) {
    key = keyRead();
    if(key!=last) {
      if(key==KEYCODE_UP)
        uartPrintP(PSTR("UP\n"));
      else if(key==KEYCODE_RIGHT)
        uartPrintP(PSTR("RIGHT\n"));
      else if(key==KEYCODE_DOWN)
        uartPrintP(PSTR("DOWN\n"));
      else if(key==KEYCODE_LEFT)
        uartPrintP(PSTR("LEFT\n"));
      else if(key==KEYCODE_CENTER)
        uartPrintP(PSTR("CENTER\n"));
      }
    last = key;
    }
  }