[Open-graphics] Re: Jumper count to disconnect DDC from DVI connector

[Jon Smirl]

On Thu, 07 Sep 2006 17:12:07 -0400, Timothy Miller wrote:

> On 9/7/06, Dieter <netbsd at sopwith.solgatos.com> wrote:
>> >    To answer an earlier question: if we want to include a feature on
>> >    OGC1 to disconnect the DDC signals from the DVI conne
>> >ctor so that the external tool will have control of those signals, only
>> >two lines need to be disconnected: SCL and SDA.  So
>> > the cost impact of doing it with jumper plugs would be minimal.
>>
>> If Jack's design is used, (and if I understand it correctly) I think we
>> could look for a signal on the aux DDC, and if one is found then use it,
>> If no signal is found on the aux DDC, then look for a signal on the
>> DVI's DCC pins.  If this scheme works, then we don't need jumpers for
>> DDC.
> 
> That is an interesting thought.  Just provide secondary DDC signals that
> we probe first.  Adding extra pins is always a concern, however, because
> signals from BGAs are hard to route.  Were it not for the density of pins
> on the 3S4000, OGD1 could have fewer layers.  In fact, we have
> inaccessible pins due to the fact that we don't have enough layers to
> route to the inner-most balls.  I would not begrudge 4 extra pins,
> however, so it's something we'll consider.  Indeed, we may find it easy to
> add the pins but find them too costly to route to.  If we add them at all,
> we'll add them to the inner-most set of balls on TRV10.

If the monitor doesn't support DDC then there is nothing hooked up to the
DDC pins on the DVI connector. If these same wires were brought out
somewhere accessible on the PCB the external device could plug in there
and just merge the signals together since they won't be in use at the
same time. There is no need for the ASIC to support two different DDC
signal paths.

Another solution is for the external box to come with a small DVI breakout
cable that plugs into the DVI port and splits off the DDC pins and passes
the rest through. That would be the easiest solution since there is no
need to provide another external connector.

All of this is has been talking about DDC2. I don't know electrically how
DDC1 works but it is something that should be figured out. I think DDC1
may modulate the DDC data onto either HSYNC or VSYNC signals somehow. In
that case you are going to need some hardware to demodulate the DDC data.
DDC1 monitors are a lot more common than fixed frequency ones. I also believe
DDC1 works over the existing VGA connector since it is modulated onto
the SYNC signals. That's how analog monitors do DDC. I'm a programmer, not
a EE so someone else will need to sort this out.

Jon Smirl
jonsmirl at gmail.com