When I connect the MCU pin directly to the 3.3V power rail, the pin reads 3.3V and an ammeter shows 6.8 uA of current going into the pin.
I assert the RS-485 signal and measure the voltage at the SP3485 receiver output (RO) pin with the pin floating (disconnected from the MCU). I see 3.3V.
When I connect the RO pin to the MCU the MCU pin is only driven to 1V and the ammeter shows 3.7 uA of current going into the pin.
What is going on? Surely the RO output pin isn't current-limited to 3.7 uA?