Re: [PATCH 24/44] Workaround i8042 chips with broken MUX mode

From: Andries Brouwer
Date: Sat Mar 27 2004 - 20:26:36 EST

On Sat, Mar 27, 2004 at 07:40:39PM -0500, Dmitry Torokhov wrote:

> > The Synaptics multiplexing proposal uses 0xf0, 0x56, 0xa4
> > to activate and 0xf0, 0x56, 0xa5 to deactivate.
> > In both cases the replies must be 0xf0, 0x56, version.
> >
> > Thus, I suppose one might get a more robust detection
> > by checking that both the activation and deactivation
> > sequences yield the same version.
> >
> Unfortunately in this particular case it looks like something flips
> 4th bit on some (but not all, like every 3rd) bytes, so it may very
> well respond with 0xAC to both queries.

If one flips that bit on 0xa5 the result is 0xad, not 0xac.

That the bit is set is not strange.
The standard PS/2 protocol requires bit 3 in the first word of
every 3-byte packet to be 1.

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