HP41C with Rp Pico attached

[brouhaha]

(12-02-2023 01:29 PM)blackjetrock Wrote:  f you haven't then it's worth reading the forum post here:

https://forums.raspberrypi.com/viewtopic...2#p2091977

which has the nasty details of the 5V 'tolerance' of the RP2040.

The TL'DR is that in practical terms, the 5V tolerance is like the cake, mostly a lie.

I use 74CBTD3861 voltage clams for all my 3.3V to 5V interfacing, often even for parts that claim 5V tolerance but have significant caveats. The 3861 is an x10 nFET bus switch, with an internal diode from the +5V rail. Due to the diode and the characteristics of the nFETs, it limits voltage through the switch (in either direction) to no more than about 3.5V, which is within tolerance for all 3.3V parts. (Not satisfactory for 3.3V parts running on 3.0V!) Since it does this by the nFET going open, it doesn't present any significant load to either side when it is "clamping" the voltage (unlike e.g. zener or other diode clamps).