I'm trying to make an interrupt based matrix keypad reader that is loosely based off of this library. However, I'm getting some really weird effects when trying to use GPIO.add_event_detect in that:

  • Despite wanting the falling edge, I get the rising edge as well
  • In some cases, the callback continues to fire until I release the key. Weirdly, this seems to only happen on column 3 (GPIO #22) and not the others, usually.

In my code, I'm removing the callback events as soon as I enter the callback because I have to switch the columns that were inputs with callbacks to outputs so that I can read the rows and determine the key that was pressed. It always figures out the correct key, it's just that it either fires off continuously and/or gets another call back on the rising edge (key released)... I've accommodated for the rising edge issue in code but I don't get why it should happen in the first place.

Code below, please excuse my many debug print statements:


import RPi.GPIO as GPIO

class keypad():
    def __init__(self):

        # CONSTANTS 
        self.KEYPAD = [

        self.ROW         = [18,23,24,25]
        self.COLUMN      = [4,17,22]


    def __colRise(self, channel):
        print ""
        print "Chan: " + str(channel)
        print "State: " + str(GPIO.input(channel))
        if GPIO.input(channel) > 0:

        #remove interrupt temporarily
        for c in range(len(self.COLUMN)):
            print "Remove: " + str(self.COLUMN[c])

        #get column number
        colVal = -1
        for c in range(len(self.COLUMN)):
            if channel == self.COLUMN[c]:
                colVal = c

        print "ColVal: " + str(colVal)

        if colVal >=0 and colVal < len(self.COLUMN):

            #set rows as intputs
            for r in range(len(self.ROW)):
                GPIO.setup(self.ROW[r], GPIO.IN, pull_up_down=GPIO.PUD_UP)

            #set triggered column as output
            GPIO.setup(channel, GPIO.OUT)
            GPIO.output(channel, GPIO.LOW)

            # Scan rows for pushed key/button
            rowVal = -1
            for r in range(len(self.ROW)):
                tmpRead = GPIO.input(self.ROW[r])
                if tmpRead == 0:
                    rowVal = r

            print "RowVal: " + str(rowVal)
            if rowVal >= 0 and rowVal <= 3:
                print str(self.KEYPAD[rowVal][colVal])
                print "Invalid Row!"
            print "Invalid Col!"

        #re-enable interrupts

    def __setInterruptMode(self):
        #set the first row as output low
        #only first one needed as it will ground to all columns
        for r in range(len(self.ROW)):
            GPIO.setup(self.ROW[r], GPIO.OUT)
            GPIO.output(self.ROW[r], GPIO.LOW)

        #set columns as inputs and attach interrupt handlers on rising edge
        for c in range(len(self.COLUMN)):
            GPIO.setup(self.COLUMN[c], GPIO.IN, pull_up_down=GPIO.PUD_UP)
            GPIO.add_event_detect(self.COLUMN[c], GPIO.FALLING, bouncetime=500, callback=self.__colRise)

    def cleanup(self):
        print "Cleanup done!"

import time     
if __name__ == '__main__':
    key = keypad()
        while True:

    except KeyboardInterrupt:
  • I have noticed my hardware sortof works like an antenna also. I dont know what your hardware looks like, but raising the high voltage or lowering the low may be worth trying.
    – ExploWare
    Mar 21, 2014 at 15:52
  • Kinda thought the voltage was fixed at 0V or 3.3V?
    – Adam Haile
    Mar 21, 2014 at 16:04
  • depends on your resistor-usage, can you show us your hardware?
    – ExploWare
    Mar 21, 2014 at 16:06
  • It's a standard 4x3 matrix keypad hooked directly into the GPIO pins as described in the code. The only resistors in use are the built in pull up/down resistors on the chip, also as described in the code. Not really much else to show.
    – Adam Haile
    Mar 21, 2014 at 16:07
  • ok, I don't know much about the internal resistors and how they influence the 3.3v, and if you maybe would be better off make it a pull-down, instead of pullup circuit. But I'm sure someone else has some better knowledge of this part.
    – ExploWare
    Mar 21, 2014 at 16:16

1 Answer 1


I've heard of this problem before (How does python GPIO bouncetime parameter work?). I think the best solution in the end was to program around the issue, adding code to detect the extra bounces (button presses).

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