I have a design that includes an ATTINY424 doing simple logic functions / I2C monitoring.

From my understanding UPDI is a half-duplex UART. For simple factory programming (no verification or debugging needed), do I need the RX side of the UART or can I just bit bang the programming sequence through a single open-drain I/O of another processor and ignore all responses? (Using an external FET to protect the processor from the possible 12V pulse of an external POD if I decide to use one at some time) Does anyone have experience with that?

Verification is not required since the functionality of the ATTINY will be fully covered by the HW test.

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