8.1 KiB
GPIOs
This document describes some basics for developers. This is useful to understand programming in gobot's digital pin driver.
GPIOs with sysfs
Kernel SYSFS ABI is deprecated since Linux 4.8, see https://www.kernel.org/doc/html/latest/admin-guide/gpio/sysfs.html. For GPIO's we still use the Kernel SYSFS ABI.
GPIOs with character devices
This document provides some test possibilities by using the new character device feature since Kernel 4.8. Please check your Kernel version before using this. Install of "gpiod" is mandatory for the tests.
uname -a
Linux raspi 5.15.61+ #1579 Fri Aug 26 11:08:59 BST 2022 armv6l GNU/Linux
sudo apt install gpiod
For work on character device user space drivers, please refer to our issue #775.
Check available GPIO banks
Example for Tinkerboard (RK3288) with TinkerOS:
ls -la /sys/class/gpio/
total 0
drwxr-xr-x 2 root root 0 Nov 12 06:53 .
drwxr-xr-x 66 root root 0 Feb 14 2019 ..
--w------- 1 root root 4096 Feb 14 2019 export
lrwxrwxrwx 1 root root 0 Feb 14 2019 gpiochip0 -> ../../devices/platform/pinctrl/gpio/gpiochip0
lrwxrwxrwx 1 root root 0 Feb 14 2019 gpiochip120 -> ../../devices/platform/pinctrl/gpio/gpiochip120
lrwxrwxrwx 1 root root 0 Feb 14 2019 gpiochip152 -> ../../devices/platform/pinctrl/gpio/gpiochip152
lrwxrwxrwx 1 root root 0 Feb 14 2019 gpiochip184 -> ../../devices/platform/pinctrl/gpio/gpiochip184
lrwxrwxrwx 1 root root 0 Feb 14 2019 gpiochip216 -> ../../devices/platform/pinctrl/gpio/gpiochip216
lrwxrwxrwx 1 root root 0 Feb 14 2019 gpiochip24 -> ../../devices/platform/pinctrl/gpio/gpiochip24
lrwxrwxrwx 1 root root 0 Feb 14 2019 gpiochip248 -> ../../devices/platform/pinctrl/gpio/gpiochip248
lrwxrwxrwx 1 root root 0 Feb 14 2019 gpiochip56 -> ../../devices/platform/pinctrl/gpio/gpiochip56
lrwxrwxrwx 1 root root 0 Feb 14 2019 gpiochip88 -> ../../devices/platform/pinctrl/gpio/gpiochip88
Example for "Raspberry Pi Model B Rev 2" with "Raspbian GNU/Linux 11 (bullseye)" (26 pin header):
ls -la /sys/class/gpio/
total 0
drwxrwxr-x 2 root gpio 0 Nov 13 09:33 .
drwxr-xr-x 58 root root 0 Sep 13 02:58 ..
--w--w---- 1 root gpio 4096 Nov 13 09:33 export
lrwxrwxrwx 1 root gpio 0 Nov 13 09:33 gpiochip0 -> ../../devices/platform/soc/20200000.gpio/gpio/gpiochip0
--w--w---- 1 root gpio 4096 Nov 13 09:33 unexport
gpiodetect
gpiochip0 [pinctrl-bcm2835] (54 lines)
gpioinfo
gpiochip0 - 54 lines:
gpiochip0 - 54 lines:
...
line 2: "SDA1" unused input active-high
line 3: "SCL1" unused input active-high
line 4: "GPIO_GCLK" unused input active-high
...
line 7: "SPI_CE1_N" unused input active-high
line 8: "SPI_CE0_N" unused input active-high
line 9: "SPI_SDI" unused input active-high
line 10: "SPI_SDO" unused input active-high
line 11: "SPI_SCLK" unused input active-high
...
line 14: "TXD0" unused input active-high
line 15: "RXD0" unused input active-high
...
line 17: "GPIO17" unused input active-high
line 18: "GPIO18" unused input active-high
...
line 22: "GPIO22" unused input active-high
line 23: "GPIO23" unused input active-high
line 24: "GPIO24" unused input active-high
line 25: "GPIO25" unused input active-high
...
General GPIO tests
For Tinkerboard and in general for all other boards:
- the name on system level differ from the header name (normally pin1..pin40)
- the mapping is done in gobot by a file named something like pin_map.go
- for the next tests the system level name is needed
Connect an oscilloscope or at least a meter to the pin (used header pin26 for example). For the output tests a LED with a sufficient resistor to 3.3.V (e.g. 290 Ohm) can be used to detect the output state.
On Tinkerboard the pin26 relates to gpio251. For raspi it relates to gpio7.
Creating gpio251 (sysfs Tinkerboard)
Needs to be "root" for this to work. Switch to root user by "su -".
echo "251" > /sys/class/gpio/export
investigate result:
# cat /sys/class/gpio/gpio251/active_low
0
# cat /sys/class/gpio/gpio251/direction
in
# cat /sys/class/gpio/gpio251/edge
none
# cat /sys/class/gpio/gpio251/value
1
The value can float to "1", if the input is open.
Test input behavior of gpio251 (sysfs Tinkerboard)
Be careful with connecting the input to GND or 3.3V directly, instead use an resistor with minimum 300 Ohm.
Connect the input header pin26 to GND.
# cat /sys/class/gpio/gpio251/direction
in
# cat /sys/class/gpio/gpio251/value
0
Connect the input header pin26 to +3.3V with an resistor (e.g. 1kOhm).
# cat /sys/class/gpio/gpio251/value
1
Test edge detection behavior of gpio251 (sysfs Tinkerboard)
investigate status:
# cat /sys/class/gpio/gpio251/edge
none
The file exists only if the pin can be configured as an interrupt generating input pin. To activate edge detection, "rising", "falling", or "both" needs to be set.
# cat /sys/class/gpio/gpio251/value
1
If edge detection is activated, a poll will return only when the interrupt was triggered. The new value is written to the beginning of the file.
Not tested yet, not supported by gobot yet.
Test output behavior of gpio251 (sysfs Tinkerboard)
Connect the output header pin26 to +3.3V with an resistor (e.g. 1kOhm leads to ~0.3mA, 300Ohm leads to ~10mA).
# echo "out" > /sys/class/gpio/gpio251/direction
# cat /sys/class/gpio/gpio251/direction
out
# cat /sys/class/gpio/gpio251/value
0
# echo "1" > /sys/class/gpio/gpio251/value
# cat /sys/class/gpio/gpio251/value
1
# echo "0" > /sys/class/gpio/gpio251/value
# cat /sys/class/gpio/gpio251/value
0
The meter should show "0V" for values of "0" and "3.3V", when the value was set to "1".
For armbian and Tinkerboard the value remains to "1", although it was set to "0". In this case the pin is not usable as output.
Test inverse output behavior of gpio251 (sysfs Tinkerboard)
# cat /sys/class/gpio/gpio251/value
0
# cat /sys/class/gpio/gpio251/active_low
0
# echo "1" > /sys/class/gpio/gpio251/active_low
# cat /sys/class/gpio/gpio251/value
1
# echo "0" > /sys/class/gpio/gpio251/value
# cat /sys/class/gpio/gpio251/value
0
The meter should show "0V" for values of "1" and "3.3V", when the value was set to "0".
Test input behavior of gpio7 (cdev Raspi)
Use --help to get some information of the command usage and options, e.g. "gpioget --help". Be careful with connecting the input to GND or 3.3V directly, instead use an resistor with minimum 300Ohm. Prefer to use the pull-up or pull-down feature, if working.
sudo gpioget 0 7
1
gpioinfo | grep 'line 7'
line 7: "SPI_CE1_N" unused input active-high
sudo gpioget --bias=pull-down 0 7
0
The value can float to "1", if the input is open. Most likely the raspi device has an internal pull-up resistor. Setting the bias is not possible for sysfs usage. This is one of the advantages of the new character device Kernel feature.
Test output behavior of gpio7 (cdev Raspi)
sudo gpioset 0 7=0
gpioinfo | grep 'line 7'
line 7: "SPI_CE1_N" unused output active-high
sudo gpioset 0 7=1
gpioinfo | grep 'line 7'
line 7: "SPI_CE1_N" unused output active-high
The meter should show "0V" for values of "0" and "3.3V", when the value was set to "1".
A connected LED with pull-up resistor lights up for setting to "0" (inverse).
Test inverse output behavior of gpio7 (cdev Raspi)
sudo gpioset -l 0 7=0
sudo gpioset -l 0 7=1
The meter should show "0V" for values of "1" and "3.3V", when the value was set to "0" (inverse logic).
A connected LED with pull-up resistor lights up for setting to "1" (inverse reversed).
The gpioinfo seems to do not recognize the "active-low" set.