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using-ev3-buttons.md

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title Using the EV3 Buttons
group hardware-standard
  • Table of Contents {:toc}

The buttons on the EV3 are mapped as regular keyboard keys

  • BACKSPACE (1)
  • UP (2)
  • LEFT (3)
  • ENTER (4)
  • RIGHT (5)
  • DOWN (6)

{% include /util/screenshot.html source="/images/ev3/labeled-buttons.png" %}

Polling Key States

If you would rather poll the button states instead of using regular keyboard input methods, you can do that via the EVIOCGKEY ioctl on /dev/input/eventX. Key codes and ioctl definitions are in linux/input.h.

Here is an example using python:

#!/usr/bin/env python

import array
import fcntl
import sys

# from linux/input.h

KEY_UP = 103
KEY_DOWN = 108
KEY_LEFT = 105
KEY_RIGHT = 106
KEY_ENTER = 28
KEY_BACKSPACE = 14

KEY_MAX = 0x2ff

def EVIOCGKEY(length):
    return 2 << (14+8+8) | length << (8+8) | ord('E') << 8 | 0x18

# end of stuff from linux/input.h

BUF_LEN = (KEY_MAX + 7) / 8

def test_bit(bit, bytes):
    # bit in bytes is 1 when released and 0 when pressed
    return bool(bytes[bit / 8] & (1 << (bit % 8)))

def main():
    buf = array.array('B', [0] * BUF_LEN)
    with open('/dev/input/by-path/platform-gpio-keys.0-event', 'r') as fd:
        ret = fcntl.ioctl(fd, EVIOCGKEY(len(buf)), buf)

    if ret < 0:
        print "ioctl error", ret
        sys.exit(1)

    for key in ['UP', 'DOWN', 'LEFT', 'RIGHT', 'ENTER', 'BACKSPACE']:
        key_code = globals()['KEY_' + key]
        key_state = test_bit(key_code, buf) and "pressed" or "released"
        print '%9s : %s' % (key, key_state)

if __name__ == "__main__":
    main()

Directly Reading the Event Device

If you want your program to be event driven, you can read the /dev/input/by-path/platform-gpio-keys.0-event file. It will block until a key is pressed. The data is in 16 byte blocks. The first 8 bytes are the time stamp (2 unsigned 64-bit integers, seconds and microseconds), the next 2 bytes are the type (unsigned 16-bit integer), the next 2 bytes are the code (unsigned 16-bit integer) and the last 4 bytes are the value (unsigned 32-bit integer).

Here is an example. It prints out 2 lines each time you press a button on the EV3 and 2 more lines each time you release a button. And of course, press CTRL+C to end.

root@ev3dev:~# hexdump -e \
'"timestamp:%d.%6d""\t""" 1/2 "type:%i""\t"""  1/2 "code:%3i""\t"""  "value:%d\n"' \
< /dev/input/by-path/platform-gpio-keys.0-event 
timestamp:1391366282.119886 type:1  code: 14    value:1
timestamp:1391366282.120100 type:0  code:  0    value:0
timestamp:1391366282.829942 type:1  code: 14    value:0
timestamp:1391366282.830179 type:0  code:  0    value:0
timestamp:1391366284.300099 type:1  code:105    value:1
timestamp:1391366284.300319 type:0  code:  0    value:0
timestamp:1391366284.829989 type:1  code:105    value:0
timestamp:1391366284.830223 type:0  code:  0    value:0
timestamp:1391366286.019866 type:1  code:103    value:1
timestamp:1391366286.020076 type:0  code:  0    value:0
timestamp:1391366286.319967 type:1  code:103    value:0
timestamp:1391366286.320202 type:0  code:  0    value:0
timestamp:1391366287.169925 type:1  code:106    value:1
timestamp:1391366287.169973 type:0  code:  0    value:0
timestamp:1391366287.479994 type:1  code:106    value:0
timestamp:1391366287.480234 type:0  code:  0    value:0
timestamp:1391366288.339882 type:1  code: 28    value:1
timestamp:1391366288.340092 type:0  code:  0    value:0
timestamp:1391366288.699929 type:1  code: 28    value:0
timestamp:1391366288.700291 type:0  code:  0    value:0
timestamp:1391366289.470046 type:1  code:108    value:1
timestamp:1391366289.470263 type:0  code:  0    value:0
timestamp:1391366289.829943 type:1  code:108    value:0
timestamp:1391366289.830176 type:0  code:  0    value:0
^C
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