Hallo Michael,

your picture looks nice, especially the arrows of bus lines.

I wonder if there is a certain bus contention problem at power-up, when at first moment port lines are undefined. You know, mcu needs a certain time for start-up oscillator, and as long as reset mode isn't invoked port lines are undefined. As consequence two or more devices could enable their outputs at the same time! Did you ever see bus contention at this moment?

I try to avoid this situation by additionally gating decoding circuitry with reset signal of supervisory chip. Then, only when Vcc is smaller than about 1V...2V bus contention is possible, but then output impedances are so high that no damaging currents can flow.

Bye,
Kai