On Tue, Dec 09, 2025 at 07:31:17PM +1000, James Calligeros wrote:

Imagine for example two codecs sharing a bus. When one codec is
transmitting, the other must ensure that it is holding its side
to 0, or data from the transmitting codec will be corrupted. We
can trust the "idle" codec to simply do this itself, however
this is undefined behaviour. Some devices may leave the line
floating, others still may pull the line high. We need a way to
control this behaviour.

I'm finding this explanation quite confusing - you appear to be
describing the situation where two devices simultaneously drive the same
signal which would be extremely unusual and I can't see how it would
work electrically.

Thus, we define five possible bus-keeping modes that a device can
be in: NONE (UB/as initialised), OFF (explicitly disabled), ZERO
(actively transmit a 0), PULLDOWN, and HIZ (floating).

What is "explicitly disabled" in ths context?  Why aren't pull up or
drive high options?