After using REV2 as basically my daily driver for over a year, I decided it was time to make another revision to take care of some of the problems that are present in the REV2 design. REV3 has not yet been fabricated yet, but here is an overview of the changes. 

Better Resistance Measurement Circuit

REV2 basically just utilized a voltage divider for resistance measurement. This approach is not ideal because the ADC codes per ohm is not constant. Also there was only two resistance ranges. The combination of these factors meant that the dynamic range of resistance measurement was not very high - only about 30Ω to 500kΩ, and the accuracy could be as bad as 10% at resistances near the limit of a range. To fix these problems I have implemented a constant current source to measure the resistance in REV3.

Additionally there are 4 current ranges: 1mA, 100uA, 10uA, and 1uA. 

Diode Mode
I have realized that diode mode is useful enough to warrant inclusion on REV3. This will be implemented using one of the current ranges reading out the voltage instead of calculating the resistance in firmware.

Analog Continuity Circuit
Previously continuity mode was implemented in software as part of resistance measurement utilizing a threshold. In REV3 I have added an actual amplifier and comparator that will be used with the current source to check for continuity. This way, hopefully the continuity threshold can be very low. I am aiming for 0.5Ω. No More Capacitance Mode
I decided that capacitance mode implemented with an RC step response did not work well enough, and I don't use capacitance mode on a multimeter very often anyway. So REV3 will not have capacitance mode. 

USB B Power Plug
Using the micro USB on the Pi Pico was not ideal. There is now a USB B connector: