[Arm-netbook] EOMA-68 passthrough implementation

[Henrik Nordström]

tis 2012-07-10 klockan 16:22 -0400 skrev Derek LaHousse:

> 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. 

Correct.

>  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?

There is this "little" requirement on EOMA68 cards:

http://elinux.org/Embedded_Open_Modular_Architecture/EOMA-68#Start-up_procedure

The reason is to protect both the EOMA68 card and the I/O it connects to
from incompatibilities in future upgrades of the EOMA68 standard, and
for configuring the LCD output to the right format for the connected
screen. LCD in itself do not have any DDC like control channel for
discovering hte LCD panel capability and the needed information need to
be passwd out of band (i.e. the I2C EEPROM information in EOMA68)

> 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?

The HDMI receiver chip that converts HDMI to LCD TTL signalling.

> 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.

Afaik yes, but undesired as it would fit badly with tablet like use
cases.

I did raise this a little bit before quesioning if we should have
Ethernet in EOMA68 1.0. Imho it's better if Ethernet is directly
connected to the CPU card. But doing so requires the card to extend to
provide an RJ45 jack. Use of strange adapter cable for connectors is
highly undesired.

Having Ethernet in an EOMA68 passthrough card is kind of pointless other
than for testing purposes. That ethernet only goes to the Ethernet port
on the I/O board/unit if anywhere.

I would suggest revising EOMA68 to 

a) remove Ethernet and instead define physical parameters to allow class
II height at the last 2(?) CM of the card, allowing for a wider choice
of external connectors on the CPU card.

b) Add analog stereo audio output. It's a very common requirement and
expect most I/O boards to want audio. Higher quality audio is possible
via USB, but at significant overhead cost which impairs both software
complexity and battery lifetime.

Regards
Henrik