General Purpose Input Output
General Purpose Input Output (GPIO)
==
General-purpose input/output (GPIO) is a generic pin on an integrated circuit whose behavior—including whether it is an input or output pin—is controllable by the user at run time. Wikipedia
Kernel Integration
Kernel Display Message
root@edison:~# dmesg | grep -i gpio
[ 0.000000] SFI: GPIO E3A27, 0964 (v1 INTEL INTELFDK)
[ 0.189472] wifi_platform_data: GPIO == 64
[ 0.189618] IPC bus, name = msic_gpio, irq = 0x31
[ 0.209216] virtio_rpmsg_bus virtio0: creating channel rpmsg_msic_gpio addr 0x5
[ 1.584512] vwlan gpio 96
[ 1.971456] info[ 0]: name = power_btn, gpio = -1
[ 1.971473] info[ 1]: name = SW1UI4, gpio = 61
[ 2.004409] input: gpio-keys as /devices/platform/gpio-keys/input/input0Userspace Interfaces
root@edison:~# ls /sys/class/gpio/
export gpio125 gpio127 gpio129 gpio131 gpio133 gpiochip0
gpio124 gpio126 gpio128 gpio130 gpio132 gpio134 unexportYou are seeing 3 kind of entries:
Control interfaces used to get userspace control over GPIOs;
GPIOs themselves; and
GPIO controllers ("gpio_chip" instances).
Watching it change (a primer on GPIO and GPIO configuration this content is moving to GPIO Subsystem)
To see the status of our exported pins in the Edison, type this your Edison's terminal:
root@edison:~# watch -n 1 cat /sys/kernel/debug/gpioBasically, it will output the configured GPIO's to console every second:
Every 1s: cat /sys/kernel/debug/gpio 2015-10-13 21:02:27
PIOs 0-191, pci/0000:00:0c.0, 0000:00:0c.0:
gpio-61 (SW1UI4 ) in hi
gpio-64 (bcm43xx_irq ) in hi
gpio-71 (bcm_bt_lpm ) out lo
gpio-77 (sd_cd ) in hi
gpio-96 (vwlan ) out hi
gpio-111 (Arduino Shield SS ) out hi
gpio-124 (hsu ) in hi
gpio-125 (hsu ) in hi
gpio-126 (hsu ) in hi
gpio-127 (hsu ) in hi
gpio-128 (hsu ) in hi
gpio-129 (hsu ) in hi
gpio-130 (hsu ) in hi
gpio-131 (hsu ) in hi
gpio-132 (hsu ) in lo
gpio-133 (hsu ) out lo
gpio-134 (hsu ) in hi
gpio-184 (bcm_bt_lpm ) out lo
gpio-185 (bcm_bt_lpm ) in lo
GPIOs 200-215, i2c/1-0020, pcal9555a, can sleep:
gpio-207 (sysfs ) in hi
gpio-215 (sysfs ) out lo
GPIOs 216-231, i2c/1-0021, pcal9555a, can sleep:
GPIOs 232-247, i2c/1-0022, pcal9555a, can sleep:
GPIOs 248-263, i2c/1-0023, pcal9555a, can sleep:The gpio's displayed above, are the ones reserved (AKA exported) by default in a newly flashed yocto image Poky (Yocto Project Reference Distro) 1.7.2 edison, kernel 3.10.17-poky-edison+
To reserve and use a GPIO, Before:
root@edison:~# ls /sys/class/gpio
export gpio127 gpio131 gpio207 gpiochip216
gpio124 gpio128 gpio132 gpio215 gpiochip232
gpio125 gpio129 gpio133 gpiochip0 gpiochip248
gpio126 gpio130 gpio134 gpiochip200 unexportlet's say 48 lets type the following:
root@edison:/# echo 48 > /sys/class/gpio/exportby this mechanism, a new directory is created in /sys/class/gpio, which should be gpio48: After:
root@edison:/# ls sys/class/gpio/
export gpio127 gpio131 gpio207 gpiochip200 unexport
gpio124 gpio128 gpio132 gpio215 gpiochip216
gpio125 gpio129 gpio133 gpio48 gpiochip232
gpio126 gpio130 gpio134 gpiochip0 gpiochip248This directory, is a control interface used to get userspace control over GPIO48, therefore can have the following read/write attributes:
"direction" ... reads as either "in" or "out". This value may
normally be written. Writing as "out" defaults to
initializing the value as low. To ensure glitch free
operation, values "low" and "high" may be written to
configure the GPIO as an output with that initial value.
Note that this attribute *will not exist* if the kernel
doesn't support changing the direction of a GPIO, or
it was exported by kernel code that didn't explicitly
allow userspace to reconfigure this GPIO's direction.
"value" ... reads as either 0 (low) or 1 (high). If the GPIO
is configured as an output, this value may be written;
any nonzero value is treated as high.
If the pin can be configured as interrupt-generating interrupt
and if it has been configured to generate interrupts (see the
description of "edge"), you can poll(2) on that file and
poll(2) will return whenever the interrupt was triggered. If
you use poll(2), set the events POLLPRI and POLLERR. If you
use select(2), set the file descriptor in exceptfds. After
poll(2) returns, either lseek(2) to the beginning of the sysfs
file and read the new value or close the file and re-open it
to read the value.
"edge" ... reads as either "none", "rising", "falling", or
"both". Write these strings to select the signal edge(s)
that will make poll(2) on the "value" file return.
This file exists only if the pin can be configured as an
interrupt generating input pin.
"active_low" ... reads as either 0 (false) or 1 (true). Write
any nonzero value to invert the value attribute both
for reading and writing. Existing and subsequent
poll(2) support configuration via the edge attribute
for "rising" and "falling" edges will follow this
setting.Exercise Change GPIO Direction
As explained before the GPIO control interface has some read/write attributes, so lets go ahead and change the default direction of the GPIO48.
Review the current GPIO direction using the command to debug GPIO statuses; If the direction is "out" let's change it to "in", if it is "in", change it to "out"
To change it, from "in" to "out " we only have to write "out" to attribute GPIO48/direction like this:
echo out > /sys/class/gpio/gpio48/directionThen checking again the GPIO status we can see, that direction has changed. Here is a table that shows the direction before and after the change:
| GPIO48 Previous Direction | GPIO48 New Direction |
| -- | -- |
| gpio-48 (sysfs ) **in** lo| gpio-48 (sysfs ) **out** lo |Exercise Use Python Bindings
mraa
apt-get install python-smbusLab
Blinks the Edison LED Here
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