Difference between revisions of "GPIOlib"

Revision as of 16:43, 14 May 2012

Introduction

With recent Linux kernel (> 2.6.3x), there is an abstraction layer meant to ease GPIO usage: gpiolib. We will explain here how to use it.

Installation

• Configure Linux kernel

$ make linux26-menuconfig

• Enable the sysfs interface of the GPIOlib If not already done:

Device Drivers  --->
    -*- GPIO Support  --->
        [*]   /sys/class/gpio/... (sysfs interface)

• In order to test the GPIOlib with the user LED and SWITCH of the board you will have to disable the GPIO_keys and GPIO_LEDS drivers that are the regular driver for such device.

Device Drivers  --->
    Input device support  --->
        [*]   Keyboards  --->
            < >   GPIO Buttons (be sure GPIO Buttons is unchecked)
 

Device Drivers  --->
    [ ] LED Support  --->  (be sure "LED Support" or "GPIO connected LEDs" (hereafter) is unchecked)
         < >   LED Support for GPIO connected LEDs
 

• Save your new configuration and rebuild your kernel (if needed)

$ make

• and reflash your kernel (if needed)

Usage

Blink APFxxDev i.MX LED

• Find the GPIO number connected to the LED, eg:
  • for APF27Dev: PF14 -> PortF (n°6) pin 14 -> (6-1)*32 + 14 -> GPIO n°174
  • for APF51Dev: GPIO1_2 -> Port 1 pin 2 -> GPIO n° 2
  • for APF28Dev: Bank 0, pin 21 -> GPIO n° 21 (0*32 + 21)
• Define envt variable (for portability):

# export LED=174    (APF27Dev)
or
# export LED=2    (APF51Dev)
or
# export LED=21    (APF28Dev)

• Ask kernel to export the GPIO:

# echo $LED > /sys/class/gpio/export

• Access functions are created:

# ls /sys/class/gpio/gpio$LED/
active_low  edge        subsystem   value
direction   power       uevent

• Check GPIO direction and put in output:

# cat /sys/class/gpio/gpio$LED/direction
in
# echo out > /sys/class/gpio/gpio$LED/direction
# cat /sys/class/gpio/gpio$LED/direction
out

• Play with GPIO value to blink LED:

# echo 1 > /sys/class/gpio/gpio$LED/value
# echo 0 > /sys/class/gpio/gpio$LED/value

Get APFxxDev i.MX switch status

• Find the GPIO number connected to the switch, eg:
  • for APF27Dev: PF13 -> PortF (n°6) pin 13 -> (6-1)*32 + 13 -> GPIO n°173
  • for APF51Dev: GPIO1_3 -> Port 1 pin 3 -> GPIO n°3 ((1-1)*32 + 3)
  • for APF28Dev: Bank 0, pin 17 -> GPIO n° 17 (0*32 + 17)
• Define envt variable (for portability):

# export SWITCH=173    (APF27Dev)
or
# export SWITCH=3      (APF51Dev)
or
# export SWITCH=17     (APF28Dev)

• Ask kernel to export the GPIO:

# echo $SWITCH > /sys/class/gpio/export

• Access functions are created:

# ls /sys/class/gpio/gpio$SWITCH/
active_low  edge        subsystem   value
direction   power       uevent

• Check GPIO direction and put in output:

# cat /sys/class/gpio/gpio$SWITCH/direction
in

• Get switch status:

# cat /sys/class/gpio/gpio$SWITCH/value
1    (released)
...
# cat /sys/class/gpio/gpio$SWITCH/value
0    (pressed)

• Block until an event occur:

# cat /sys/class/gpio/gpio$SWITCH/edge
none
# echo both > /sys/class/gpio/gpio$SWITCH/edge

Links

• http://www.kernel.org/doc/Documentation/gpio.txt
• how to implement an interrupt driven gpio input in linux