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Key de-bouncing in 4x4 matrix keypad

Jason Uy

I'm using 4 x 4 keypad to send the output to 2 7-segment LED.

problem: key de bouncing problem

Summary problem: when I press key, it shows both digit on the left and right. For example, if I first press 1, it should show 1 on the right and blank on the left. If I press 3 next, it should show 1 on the left and 3 on the right. Right now, it I press 1 it shows 1 on the left and 1 on the right. I think this relate to key debouncing. Please check my code.

void read(void) {

char left_val = 'n';

char right_val = 'n';

char next_val = readKeyPad();

while(1){

next_val = readKeyPad();

if (right_val == next_val || next_val == 'n'){

printChar(left_val, Left);

delay(FRAME_PERIOD);

printChar(right_val, Right);

delay(FRAME_PERIOD);

}

else {

left_val = right_val;

right_val = next_val;

}

}
}

Parents
  • 0

    Hey,

    There are plenty of denouncing strategies defined in textbooks, with solutions in hardware, digital logic, and software.

    I suggest you scope the signal, and see a typical period of noise. This would make your solution inherently empirical, so I wouldn't feel the need to make beautiful code.

    Simply assert stability a certain number of iterations. If you have an RTOS, poll it with a task. If you don't want to consume so much resources polling it, your option is applying a period of time where the result is unstable, and effectively, "dont care" 'X' in DLD :).

    Example for a polling code:

    
//Uvision calls compiler with compiler defines. E.g. "C166.exe DEFINE(TDD)"

#include <stdint.h> //XintY_t
#include <stdbool.h> //bool
#include <stdlib.h> //rand

bool eg_mySwitch(void)
{
#ifdef TDD
  //Returns psuedo 1 or 0
  return ((rand() >> 1) >= (32767 >> 1));
#else
  //Your hardware abstraction
#endif
}

//Hardware abstraction layer
#define FILTER_WINDOW_LEN 10 //length of my window for the debouncer, find experimentally
#define FILTER_MAX_ITERATIONS 10000 // Arbitrary exit condition...
#define FILTER_X_GETSTATE eg_mySwitch //callback function that returns 1 or 0

bool poll_switch(bool endState)
{
  static uint8_t s_vals[FILTER_WINDOW_LEN];
  static uint8_t s_index;
  static uint8_t s_total;
  //Start polling
  while(s_index != FILTER_WINDOW_LEN)
  {
    s_vals[s_index] = FILTER_X_GETSTATE();
    if(s_vals[s_index] == endState) s_index++;
    else s_index = 0;
    if(s_total++ == FILTER_MAX_ITERATIONS) return !endState;
  }
  //done
  return endState;
}

Reply
  • 0

    Hey,

    There are plenty of denouncing strategies defined in textbooks, with solutions in hardware, digital logic, and software.

    I suggest you scope the signal, and see a typical period of noise. This would make your solution inherently empirical, so I wouldn't feel the need to make beautiful code.

    Simply assert stability a certain number of iterations. If you have an RTOS, poll it with a task. If you don't want to consume so much resources polling it, your option is applying a period of time where the result is unstable, and effectively, "dont care" 'X' in DLD :).

    Example for a polling code:

    
//Uvision calls compiler with compiler defines. E.g. "C166.exe DEFINE(TDD)"

#include <stdint.h> //XintY_t
#include <stdbool.h> //bool
#include <stdlib.h> //rand

bool eg_mySwitch(void)
{
#ifdef TDD
  //Returns psuedo 1 or 0
  return ((rand() >> 1) >= (32767 >> 1));
#else
  //Your hardware abstraction
#endif
}

//Hardware abstraction layer
#define FILTER_WINDOW_LEN 10 //length of my window for the debouncer, find experimentally
#define FILTER_MAX_ITERATIONS 10000 // Arbitrary exit condition...
#define FILTER_X_GETSTATE eg_mySwitch //callback function that returns 1 or 0

bool poll_switch(bool endState)
{
  static uint8_t s_vals[FILTER_WINDOW_LEN];
  static uint8_t s_index;
  static uint8_t s_total;
  //Start polling
  while(s_index != FILTER_WINDOW_LEN)
  {
    s_vals[s_index] = FILTER_X_GETSTATE();
    if(s_vals[s_index] == endState) s_index++;
    else s_index = 0;
    if(s_total++ == FILTER_MAX_ITERATIONS) return !endState;
  }
  //done
  return endState;
}

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