[Arm-netbook] EOMA-68 passthrough implementation

[Derek LaHousse]

On Tue, 2012-07-10 at 12:28 +0100, lkcl luke wrote:
> >
> > I'm a little confused by the audio direction.
> 
>  digital signal generation is the *opposite* direction from CPU Cards.
>  however in the case of audio you would still have a mic and
> headphones, one is input, the other is output - you can't reverse
> those :)

> >  A pass-through card
> > should be receiving video signals on the "external" side of the EOMA
> > card, and providing data to the PCMCIA side.  To pass the audio, it
> > would need to break the signal out of the HDMI and then drive a USB
> > sound card, right?
>  the alternative is just to ignore HDMI audio and lay down a USB audio
> IC.  however you can just as well go get a USB audio stick off of ebay
> and it would achieve the same effective result.
>  bottom line: don't complicate things by having audio - at all.
> 
> > It's also important to pass the I2C through to the front of the EOMA
> > passthrough.  A system emulating the EOMA would need to be able to read
> > the EEPROM on the motherboard, as part of the spec.
> 
>  achh, i'd forgotten about that.  hmmm, in a commercial pass-through
> card that might involve dropping an 8-bit micro onto the PCB.  darn.

Luke, Ryan and I were talking on IRC, we're both confused by your last
statement.  I'm going to lay out assumptions, maybe we're not using the
same terms.

EOMA card = cpu-card or passthrough card, the Master side of most busses
motherboard = where the card plugs in, the Slave or device side

Since there is no audio passed across the EOMA header, a passthrough
card would need to go to great lengths to handle audio.  A completely
passive passthrough would have no audio ports at all.  However, a USB
audio device on the motherboard would be passed across the USB bus to
whatever controls the passthrough card.

There shouldn't be an EEPROM or uC on the passthrough card, as the spec
doesn't call for an EEPROM on the EOMA card.  The EEPROM is on the
motherboard, and whatever controls the passthrough would need to read
it.  Or are you saying the passthrough card needs to scale the HDMI
signal to the proper output, according to the EEPROM?

To put a finer point on it:
I have purchased an EOMA68-compliant tablet (1366x768 screen).  It has
onboard storage and a USB audio card driving some pretty crappy
speakers.  I don't yet own an EOMA68 CPUCard, so I will test my
equipment with my laptop and an EOMA68-compliant passthrough card.
This passthrough card looks pretty ugly.  It's shaped like a PCMCIA
card, with a 2mm key which blocks it from going in my laptop (as it
shouldn't).  Then, it expands out past flush to provide me a USB port,
an HDMI, and an eSATA.  I hook all three up to my laptop, where I have
an HDMI-out, a USB, and an eSATA.  I don't connect the I2C to my laptop,
because I don't have a port.
My laptop sees external storage, an audio card, and an external monitor.
I enable the monitor, and my HDMI puts out a 1080p image.  Because the
screen is only 768... What happens?  Who scales the image?

An additional question I hid in there is: According to spec, is it okay
for the EOMA68 card to extend beyond flush?  This seems to be purely
aesthetic.

Derek
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