When a patch (pd) is running on the bela, the D6 LED flashes, this one:
When running the bela as a standalone, and having built it into a box, is it possible to assign this LED to a digital output through pd?
In order to give the user a visual ping that the bela is booted and ready?

The signal for that LED is available on P8.38, so if its blinking pattern is fine for you, you could just wire that up to an LED on the outside of your box. Would that work?

Ah, ok! Thanks.
Is there a way to control the blinking? For example, connect it to a red LED and have it as a solid colour? Or is that not possible from pd?

Are you using all 16 GPIOs provided by Bela on Pd? If not, the easiest way is to use one of them.

No, currently only 4, but might be expanding as experimenting is progressing.

Then you need a custom C++ filer. The easiest way is to copy ~/Bela/core/default_libpd_render.cpp into your project and add the following to it:

At the top of the file:

#include <Gpio.h>
Gpio myled;

At the top of setup():

myled.open(8, Gpio::OUTPUT); // P8.35
myled.set();

At the top of cleanup():

myled.clear();

This will make P8.35 (I picked that one as it's not used elsewhere) high all the time as the program is running. Note that if the program crashes unexpectedly, the LED may remain high.