Why do Pimoroni Pico 2 boards have 1k resistors on some/all of the ADC capable GPIO?

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1

I was browsing some schematics, as you do, and noticed there are some 1k resistors on some of Pimoroni’s Pico boards - these do not feature on the Raspberry Pi originals. Why are these there? From my cursory inspection these appear inconsistent.

The appearance of these resistors on the pinout diagram is useful to alert users to the presence of these.

Unrelated to that, the Pimoroni Pico LiPo 2 XL W doesn’t have any pictures which include the underside of the board. If you are used to the flat bottoms of the Raspberry Pi Pico boards these boards with components on both sides could be a surprise. The pinout does include a height diagram which shows this but perhaps this page would benefit from an underside picture too.

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Have a read here: https://electronics.stackexchange.com/questions/694714/what-is-the-role-of-the-resistors-in-this-circuit

Personally, I don’t think they make sense on a generic, multi-purpose dev-board.

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Looks like there’s already a photo showing the underside of Pico LiPo 2 XL on the shop page - are you suggesting we add an end on one to better show the height?

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Sorry, I somehow missed the underside photo. Perhaps I didn’t zoom in on all of them and assumed images were all the same from the small versions.

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These resistors exist on our Pimoroni Pico boards that use the larger RP2350B chip. On this chip, the ADC capabilities are on GP40 to GP47. This means that GP26 to GP29 can no longer be used as ADCs, but to maintain Pico compatibility we need cannot swap to different pins. To get around this we wire both GP26+GP40, GP27+GP41, and GP28+GP42 to the same pads on the board, and include a 1k resistor between them to avoid issues if the two pins are set to conflicting functions, e.g driven high + driven low.

I attempted to convey this dual pin setup on the pinout diagram, though perhaps unsuccessfully.

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As you say: “once you know how a device works, you start to understand the usage instructions”.

Now it is clear, but I must admit I neither picked this up in the pinout diagram, nor in the schematic.

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Thanks for the explanation.

I had looked at the schematic too, briefly, and failed to spot this. When you know to look for it you can see it in the labelling!

I think the current pinout diagram is good for alerting the reader to “something is different here”. I think it could be improved by prefixing 40, 41, 42 with GP and more significantly by adding an asterix (or even an asterisk) and an explanatory footnote on the diagram. A similar addition to Notes section on the shop pages and schematics and then I think most people would pick grasp what’s going on here.

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