PocketBeagle GPIOs and external peripherals

Hi all,

I have one silly question to start off with… I have a PocketBeagle
coming, hopefully next week, and a Lattice IceStick FPGA development
board with which I'll be teaming it up with.

The plan is to produce a logic analysis and stimulus tool. I'll be
using GPIOs on both the PocketBeagle and the FPGA to control and monitor
some arbitrary logic circuit constructed on a bread board.

The project is detailed here: https://hackaday.io/project/28513

One question I have though regards the following warning:

An FTDI 3.3 Volt USB to UART0 connection will not blowup or hurt anything.
You an hook it up and leave it hooked up, and not have to worry about damaging the Pocket Beagle,
A 10K pullup to +3.3 V is also OK, but not actually required for the FTDI cable to work.

Something like the TTL-232R-3V3-WE for wire pigtails.

The damage warning is for things that will try to drive pins with currents high enough to damage semiconductor circuits and/or ESD protection circuits.
Or, in the case of the boot pins, something that will over-ride a 100K resistor.

— Graham

Ahh okay… yeah I used UART here as it's an example of something that
would be generally wired up continuously and has the property of
presenting a voltage when idle using push-pull logic. I'd likely use
the MAX232 for a console port.

By the sounds of things, the problem is more down to inrush current than
voltage alone, and clearly the AM3358 doesn't have its pins in
high-impedance mode at boot-up or else it'd be impossible to achieve
those current levels. 3v3 in series with a megohm or more is never
going to produce more than 3µ3A, and GPIOs in "input" mode often have a
resistance in that ballpark. A GPIO driven as a low output however,
could get nasty.

The plan was to have a series resistor, so this should be sufficient to
prevent catastrophe.

Worst case is prior to the chip being powered up, or while the rails are being sequentially raised.
In that case, internal power can be a low as zero, and ESD diodes on each pin can conduct at 0.7 Volts.
You damage individual pins or can avalanche the whole chip if you can drive enough current.
— Graham