IO pull-up vs pull-down

[fangeugene]

It looks like the digital IOs are configured to pull down, as opposed to the pull-ups on the RIO. Is this subject to change in future iterations of the hardware? Keeping interoperability with the RIO would be nice for old robots and teams who have already invested in sensors that expect pull-ups.

Edit: Also the low-value of the 4.7KΩ (as opposed to the RIO's 40KΩ) makes it difficult to override this with a passive circuit.

[bhjelstrom]

The resistor value has been changed for the next revision. Can't give any updates on switching to a pull-up just yet.

[fangeugene]

You're probably already aware of this (and it may or may not be relevant depending on how the circuit is actually implemented on SystemCore) but linking this just in case: https://hackaday.com/2024/09/20/raspberry-pi-rp2350-e9-erratum-redefined-as-input-mode-leakage-current/

[bhjelstrom]

Yes, we were early alpha testers and the strong pulldown was actually in place to address E9 (not saying that this pull configuration is strictly necessary). At this point we are looking at a redesign that gives teams everything they want.

[qwertychouskie]

Yes, we were early alpha testers and the strong pulldown was actually in place to address E9 (not saying that this pull configuration is strictly necessary). At this point we are looking at a redesign that gives teams everything they want.

A strong pull-up would serve the same purpose, right?

[bhjelstrom]

Given the other properties of the circuit, it would not.

[qwertychouskie]

The REV Control/Expansion hubs use a pull-up resistor as well: https://docs.revrobotics.com/duo-control/sensors/5v-sensors#wiring-a-limit-switch-or-micro-switch

I think having the SystemCore use a pull-down setup is just going to cause a lot of pain for a lot of teams. In fact, to be honest, I'm really not sure why pull-down was chosen in the first place, given that both existing control systems (REV in FTC and roboRIO in FRC) use a pull-up setup.

[auscompgeek]

I'm sure you're across it, but the new A4 stepping is out and supposed to address E9 if I've read correctly https://www.theregister.com/2025/07/29/raspberry_pi_rp2350_update/