Quote  

The I/O pins on the RP2350 series of microcontrollers can withstand 5V (while powered up) so the keyboard
can be directly connected as illustrated with the 5V supplied by the VSYS pin on the Raspberry Pi Pico 2.
The keyboard is enabled with the OPTION KEYBOARD command.

I had it in my head that the RP2350 should be powered before any 5v was applied but this suggests no problem if everything is powered-up simultaneously(?)

Hello,

I had problems with that from the keyboard side: 3v3 was too low on the data / clock lines, so it is suggested (even if the pico could tolerate 5V) that you use a level shifter. And in my opinion the manual should NOT suggest to do it.

I have keyboards which work without this level shifter and I have two keyboards which don't work.

Greetings
Daniel
Edited 2025-06-17 20:44 by Amnesie

There seems to be some discussion on this. There is a big difference between "will withstand 5V" and "5V TTL compatible".

The RP2350 Datasheet says that you can apply 5V to all the GPIO pins apart from those for the ADC providing the IOVDD pin is at 3V3. If you take that literally then you must power up the Pico before applying 5V to any pin. That would make sense as the ESD protection diodes will push 5V onto the internal power rail because the GPIO pin pad isn't running.

If using a PS2 keyboard powered from the same supply as the Pico then it may well be ok as the overvoltage situation only applies for the time taken for the switcher to start up and get 3V3 onto the chip. That's *probably* safe. However, a USB-TTL converter that has the pullups sourced from the USB supply source will have 5V on the pins all the time. Even with current limited by the pullups I'm not too happy with that situation. You really need to use converters with 3V3 data lines.

  Mixtel90 said  

... You really need to use converters with 3V3 data lines.

Exactly, this is my opinion, too. The point is: It MAY perfectly work on the individual situation, but don't count on it, ergo: avoid "half-baked" things, do it right the first time.    Therefore I think it should not be suggested at all (not like the manual suggests)..

I blindly followed this suggestion by the manual, it worked "perfectly" on one keyboard, then I designed a PCB around it (without a lvl shifter) only to discover, that it only works with ONE keyboard, but not with my other two... So I learned this the "hard" way ... and had to redesign the PCB with a lvl shifter.  

Greetings
Daniel
Edited 2025-06-17 21:47 by Amnesie

I prefer to use level shifters for PS2 keyboards. That way it's always safe to plug in a standard Pico, even though you may be perfectly safe with a Pico 2. Raspberry Pi have said that you can get away with 5V on the RP2040 though.

When I first used mmbasic the manual said you can use a 3.3V ps2 keyboard no logic level converters. I found a black one and cut the plug off and stuck pins on the lead and it worked. 3.3V ps2 keyboards must be rare cos it was usb after.