Subj : Re: how to write and read back GPIO pin states with lgpio
To   : josef@invalid.invalid
From : Computer Nerd Kev
Date : Mon Aug 05 2024 09:04:21

Josef M?llers <josef@invalid.invalid> wrote:
> 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.

Personally I switched to using the "gpio" command that's one of the
example programs included with the bcm2835 library. If you're just
trying to build C programs to replace the /sys/class/gpio devices
in a shell script, it's an existing option. It does have some bugs
though.

https://www.airspayce.com/mikem/bcm2835/

See examples/gpio/gpio.c in the source code.

> However, I have problems writing and reading back pin
> states from different programs.
> [snip]
> 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.

At a GUESS, you're reading the input buffer instead of the output
buffer. In output mode the input is disabled and always reads zero
or is meaningless.

Unfortunately I've forgotten whether this is the case with the Pi,
or at least where to look to confirm I'm not mis-remembering, so
check for yourself. But this is a common way for IO hardware to
work.

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