Overvoltage/Undervoltage Protection

General
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One of the biggest concerns I have in working with any foreign part is causing permanent damage to my tools. While it seems there have been a lot of efforts to protect the DUT (I read SPI Flash chip adapters SOP/WSON/DIP - #11 by electronic_eel), I don’t see a lot of protection for the BusPirate itself. Looking at the schematic, there don’t seem to be many diodes clamping inputs to sane ranges; you have overvoltage protection at the inputs to the LMV324’s, but those lie after 100k resistors, and thus can’t protect the 74LVC1T45 buffers, which are (for TI at least) speced to a maximum voltage range at the output of 5.5V before possible damage. If I were to try and use the pirate as an analyzer on a 12V relay, it would probably go up in smoke.

Please consider moving the BAS40t-05 diodes to be at the output of the buffers instead, where they will protect the op-amps as well, and adding additional ones down to ground, in case of undervoltage. I know the V10 is intended to be the final revision, and that the original lacked these features, but if you’re going to have protection, might as well make it through.

2 Likes
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You are correct, it would probably be good to add some additional protection at some point in the future.

One issue I faced is that we are really densely populated around the IO area. Without a drastic redesign of the PCB, I’m not sure how to add 8 more diodes to the IO. I hear rumors that we might get a bigger RP2040 type chip in the next year or so, which would eliminate the need for the 595s and free up a bunch of board space. Another approach is to substitute a bunch of the 74 logic with a cheap RISC V chip running at 5volts, but there’s a ton of opportunity for nasty debugging in that kind of setup.

The BAT40’s on the op-amps do provide some over voltage protection. The primary reason I added them is to protect the op-amp from reverse current/back powering when the Bus Pirate is unplugged but connected to an external power source.