Subj : how to write and read back GPIO pin states with lgpio
To   : All
From : =?UTF-8?Q?Josef_M=C3=B6llers?=
Date : Sun Aug 04 2024 22:13:55

Hi,

I used to use the sysfs interface to the GPIO pins (/sys/class/gpio) but
I understand that is deprecated nowadays. So I tried to switch to lgpio
which looks OK. However, I have problems writing and reading back pin
states from different programs.

My setup is as follows:
I have a couple of relays (solid state and mechanical ones) that control
various external devices.
I use one program to switch devices on and off and want to use another
program to read back the state of the device.

Doing that with sysfs is easy:
1) export the pin:
       echo $pin > /sys/class/gpio/export
       echo $direction > /sys/class/gpio/gpio$pin/direction
    this needs to be done only once.
2) write the state of the pin, thus switching the device on/off:
       echo $newstate > /sys/class/gpio/gpio$pin/value
    this is done every time this is required
3) read back the state of the pin
       value=$(</sys/class/gpio/gpio$pin/value
    this is done every time I want to check the state of the device

Now I switch a device on/off with lgpio as follows:
1) open the GPIO chip:
       h = lgGpiochipOpen(0);
2) claim the pin as an output:
       lgGpioClaimOutput(h, LG_SET_PULL_NONE, pin, value);
    which, to my understanding, already changes the pin's state?!?
3) write the new state
       lgGpioWrite(h, pin, value);
4) close the chip
       lgGpiochipClose(h);

Reading back the state of the pin requires me to
1) open the GPIO chip:
       h = lgGpiochipOpen(0);
2) claim the pin as an input:
       lgGpioClaimInput(h, LG_SET_PULL_NONE, pin);
3) read back the pin's state
       lgGpioRead(h, pin);
4) close the chip
       lgGpiochipClose(h);

However ... When I set the pin's state to "1", I still read back "0"!

What am I doing wrong? Thanks in advance for any pointers.

Josef

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