[Arm-netbook] Regarding Improv, EOMA68, Free Software and Open Hardware.

[Luke Kenneth Casson Leighton]

On Fri, Jun 27, 2014 at 12:09 AM, Henrik Nordström
<henrik at henriknordstrom.net> wrote:

>>  the changes i've made since it started open it up to 23or more SoCs
>> *per year*.  i am also considering - reluctantly - reserving the 5.0mm
>> card height for 1280x800 capable SoCs and the 3.3mm card height for
>> 1920x1080 SoCs (over the 24-pin RGB/TTL that is)
>
> Why? I don't get it.
>
> For 1920x1080+ LVDS / DSI / eDP is much better suited interfaces imho.
> Unfortunately all (including RGB/TLL, but except eDP) is plauged by
> configuration issues and vendor extension dependencies which makes then
> somewhat troublesome for EOMA.

 ok, i have a bit more time, so can explain further.  the logic is as follows:

 * SoCs are on the market at 1280x800 maximum resolution over RGB/TTL
 * SoCs are on the market at 2048x2048 @ 30fps max res over RGB/TTL
 * RGB/TTL is the lowest common denominator and is present on all but
   the absolute absolute specialist SoCs such as mobile smartphone ones,
   high-end high-power SoCs (Calxeda, TI etc.)
 * LVDS is fraught with choice between 1 or 2 channels
 * RGB/TTL, precisely because it is lowest-common-denominator,
   may be converted to just about anything with an IC that costs between
   $1 for a single-channel LVDS to $3.50 or $5 for a 1920x1080 DVI/HDMI
   and someone recently found an eDP one for $3 which is awesome.

so the cases we want to cover are:

 * 320x240 RGB/TTL screen at very low cost (these are all RGB/TTL)
 * 1024-1280 x 600-1024 screen at reasonable cost (these are all
   LVDS 1x but a _few_ eDPs are becoming available)
 * 1440x900 to 1920x1080 at not unreasonable cost (these are
   usually Dual or Triple LVDS with some now 4-lane eDP and so on)

 remember that EOMA interfaces are *mandatory*.

 * if you make even LVDS mandatory, you just screwed 2 out of 3 of
   those options (pick any 2: the 320x240 one is my favourite because
   you will need an LVDS converter on the SoC *and* a down-converter
   on the base-board, and you just screwed any chance of having a low
  cost system!)

 * if you make eDP mandatory you just screwed 2 out 3 of those options,
   the 320x240 is screwed on cost, and 1 out of 2 of the others are
   screwed on the number of eDP lanes.

   also you are royally screwed on the pricing of eDP ICs vs the cost
   of RGB/TTL-to-LVDS ICs.

 * if you make MIPI mandatory it's the same thing

 * if you make HDMI mandatory it's *worse* because now you have
    two $5 ICs _and_ you have a $50,000 up-front HDMI manufacturer's
    extortion^Wlicense to pay.

 * if however you make RGB/TTL mandatory you may:

 - use direct RGB/TTL (zero cost) to connect to the 320x240 LCD

 - use a $1 RGB/TTL to LVDS IC or a $3 RGB/TTL to MIPI IC for
   the 1024x600 to 1280x1024 LCDs

 - use a $3 to $5 RGB/TTL to LVDS or eDP IC to connect to the 1440x900
  and the 1920x1080 LCDs.

so overall it is actually very simple: RGB/TTL is the only remaining
logical choice once you have considered all the options from all the
angles covering all the products and all SoC families.  the cost of
the converter ICs is a manageable quantity that is relative to the
cost of the LCD it is connecting to and the sale price of the product.

l.