Question regarding HDMI Audio support on i.MX6 with vanilla kernel

[Luís Mendes]

Hi,

On Tue, Nov 7, 2017 at 10:43 PM, Russell King - ARM Linux <
linux at armlinux.org.uk> wrote:

> On Tue, Nov 07, 2017 at 10:24:37PM +0000, Luís Mendes wrote:
> > Hi,
> >
> > On Tue, Nov 7, 2017 at 8:07 PM, Russell King - ARM Linux <
> > linux at armlinux.org.uk> wrote:
> > > 2. there is an (optional?) TXS0102 level converter on the DDC signals
> > >    which, although marketed for I2C buses, doesn't actually conform to
> > >    the I2C bus spec.  The I2C bus spec calls for 30% and 70% of the
> > >    bus voltage for input thresholds, which would be 1.5V and 3.5V.
> > >    The TXS0102 datasheet gives more stringent levels, which are 0.15V
> > >    and 4.6V assuming a 5V "B" supply.
> > >
> > >    (Note: HDMI calls for the DDC bus to conform to the I2C bus spec.)
> > >
> > > So, I think this Wandboard design has some question marks over the
> > > hardware design, and so I wouldn't expect HDMI to be reliable.
> > >
> >
> > This chip must be the culprit, it really does a bad job at translating
> I2C
> > signals. The logic levels are completing wrong then.
> > If the chip is not populated because iMX6 may have some 5V tolerant pins,
> > then it is barely able to drive the logic 1 level for the monitor,
> although
> > it can receive properly
>
> I just looked at what was done on the Hummingboard, and it uses a
> N-channel mosfet with the gate connected to the 3.3V supply, source to
> the imx6 and drain to the HDMI connector, with 4.7K pull-ups on both
> sides - obviously to 3.3V on the imx6 side and 5V on the HDMI side.
>
> IOW, a standard single-mosfet level shifter for open-drain signalling.
>
> I haven't heard of that causing any problems, and as it's on the uSOM,
> and remains the same on the more recent rev1.5 uSOM, I suspect that
> no one has had a problem with it.
>
> It's probably cheaper than a TXS0102!
>

Ok, thanks! I'll probably employ such a solution for the base board that I
am going to design.


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