Debouncing joystick button input
I have a joystick class written in python with gobject, it works great except for one minor problem. Button bounce when I run the code below, it picks up all the button presses multiple times. How can I reduce that to one message per button press with reasonable accuracy?
'''
Copyright 2009 Jezra Lickter
This software is distributed AS IS. Use at your own risk.
If it borks your system, you have been forewarned.
This software is licensed under the LGPL Version 3
http://www.gnu.org/licenses/lgpl-3.0.txt
for documentation on Linux Joystick programming please see
http://www.mjmwired.net/kernel/Documentation/input/joystick-api.txt
'''
import gobject #needed for sending signals
import struct #needed for holding chunks of data
class Joystick(gobject.GObject):
'''The Joystick class is a GObject that sends signals that represent
Joystick events'''
EVENT_BUTTON = 0x01 #button pressed/released
EVENT_AXIS = 0x02 #axis moved
EVENT_INIT = 0x80 #button/axis initialized
#see http://docs.python.org/library/struct.html for the format determination
EVENT_FORMAT = "IhBB"
EVENT_SIZE = struct.calcsize(EVENT_FORMAT)
# we need a few signals to send data to the main
'''signals will return 4 variables as follows:
1. a string representing if the signal is from an axis or a button
2. an integer representation of a particular button/axis
3. an integer representing axis direction or button press/release
4. an integer representing the "init" of the button/axis
'''
__gsignals__ = {
'axis' :
(gobject.SIGNAL_RUN_LAST, gobject.TYPE_NONE,
(gobject.TYPE_INT,gobject.TYPE_INT,gobject.TYPE_INT)),
'button' :
(gobject.SIGNAL_RUN_LAST, gobject.TYPE_NONE,
(gobject.TYPE_INT,gobject.TYPE_INT,gobject.TYPE_INT))
}
def __init__(self,dev_num):
gobject.GObject.__init__(self)
#define the device
device = '/dev/input/js%s' % dev_num
#error check that this can be read
try:
#open the joystick device
self.device = open(device)
#keep an eye on the device, when there is data to read, execute the read function
gobject.io_add_watch(self.device,gobject.IO_IN,self.read_buttons)
except Exception,ex:
#raise an exception
raise Exc开发者_如何学Ception( ex )
def read_buttons(self, arg0='', arg1=''):
''' read the button and axis press event from the joystick device
and emit a signal containing the event data
'''
#read self.EVENT_SIZE bytes from the joystick
read_event = self.device.read(self.EVENT_SIZE)
#get the event structure values from the read event
time, value, type, number = struct.unpack(self.EVENT_FORMAT, read_event)
#get just the button/axis press event from the event type
event = type & ~self.EVENT_INIT
#get just the INIT event from the event type
init = type & ~event
if event == self.EVENT_AXIS:
signal = "axis"
elif event == self.EVENT_BUTTON:
signal = "button"
if signal:
print("%s %s %s %s" % (signal,number,value,init) )
self.emit(signal,number,value,init)
return True
if __name__ == "__main__":
try:
j = Joystick(0)
loop = gobject.MainLoop()
loop.run()
except Exception,e:
print(e)
There are many ways to debounce buttons. A simple, non-blocking way to do it is to:
- Repeatedly check the status of the button over time and...
- ...if the button "momentaneous" state is not the one considered "active" then...
- ...increment a counter and...
- ...if the counter reaches a given threshold, toggle the "active" state of the button.
- The counter should be reset each time the "momentaneous" and "active" states are the same
This method requires the check procedure to run at reasonable regular intervals of course, as the reaction time is given by frequency*threshold
.
EDIT: I don't have the hardware to actually run this, but the debouncing method should look something like:
if button_now() != button_state:
debounce_counter += 1
if debounce_counter == DEBOUNCE_THRESHOLD:
button_state = not button_state
else:
debounce_counter = 0
In the above code:
button_now()
polls the hardware (and returnTrue/False
according to if the button circuit is closed or open),button_state
is how the rest of the program "sees" the button (again:True/False
according to button down or up), `DEBOUNCE_THRESHOLD
is the constant you defined according to the formulareaction-time-of-the-button = frequency-of-debouncing-routine * threshold
.
HTH!
Usually, a 1uF/10uF capacitor each between x, y and pushbutton contacts and GND hardware-debounces, so no code is needed.
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