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