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MCP_INT is push-pull on boot - expected open-drain, needs a PNP transistor in series #63
MCP23017, unexpectedly, has the INT pins in the push-pull mode on boot (at least it's still active low). This interferes with HAT EEPROM detection by Pi Zero, since it expects the GPIO0 to not be driven and instead pulled up to VCC. So, we need to add this solution between MCP23017 and Pi Zero/EEPROM:
This also means we need to add a jellybean PNP transistor to the BOM. This is a suitable solution by now, but I'll look into whether we have a way around it.