[Automated-testing] Documentation: Designing for automated testing

[Kevin Hilman]

Chris Fiege <cfi at pengutronix.de> writes:

> now that ELCE and ATS 2019 are coming closer I wanted post a short reminder:
>
> I am collecting facts and hints about how to design an embedded hardware in 
> a way, that embedded software development is easy:
>
> https://designing-for-automated-testing.readthedocs.io/
>
> If you think that your common use-case is missing please send me a patch or 
> open a pull-request:

Sorry, no PR.  I'm not sure where to fit this in your doc, but a couple
topics related to power supply.

1) USB and jack supply

It's pretty common for boards to support 5V wall power (via a jack
connector) or alternatively power over USB.

This can be practical, but also a problem when you want to power cycle
and I've seen many cases where you have to cut power on *both*
jack and USB to power cycle.

Some board designs solve this by disabling
the USB 5V line if there's a jack connector, and I'm not sure if there's
a "right way", but mentioning this would be useful so it can be avoided
if desired.

2) back power from USB serial

I've also seen boards where having a USB/serial adapter has enough
voltage to back-power parts of the board, not allowing it to fully
power-off and reboot.

The hack/solution is to put an in-line resistor on the TX line so there
is enough voltage drop to not back-power ( but not too much voltage drop
to affect the UART signals.)

Again, I'm not a board designer, but this smells like something that
coule be avoided with better board design.

Kevin