[Arm-netbook] Testing: GPIO

Luke Kenneth Casson Leighton lkcl at lkcl.net
Fri Mar 2 03:40:01 GMT 2018

On Thu, Mar 1, 2018 at 10:42 PM, Richard Wilbur
<richard.wilbur at gmail.com> wrote:

> Luke, have you tested the D/A circuit on the micro-desktop board?

 yep it works great up to 1024x768.  i haven't yet been able to get it
to sync at anything greater than that, because you have to manually
convert the signals into A20 timings... and of course if you can't
read the EDID data you don't know *exactly* what the settings are in
the first place for any given monitor.

 1024x768, being a common VESA standard, has worked consistently on
every monitor i've tried.

> Only thing I would worry about is the hold time on the data lines.  If
> the A20 sets up the data quickly (relative to the pixel time) and
> holds it until the next setup, we should be in good shape.

 sigh yeah i thought about that... using buffer ICs with a "hold", and
linking up the clock line to it.... never got round to it.  i'd prefer
to just skip the entire circuit and use a TFP410 (or maybe it's a
TFP401a), or a Chrontel RGB/TTL to VGA converter IC.  CH7036 i think
it is.

>> Much easier suggestion: get a small LCD. *ANY* small LCD. Like a five or
>> seven inch display at the largest. Raw panel, no driver board. Get the
>> datasheet and a compatible connector. (If you source from eBay this is very
>> easy; those are almost all commodity displays with available datasheets.)
>> If it's a SMALL DISPLAY it *will* be RGBTTL, 90%+ of the time (I've seen
>> one exception to this ever and it was in an off-brand portable DVD player).
>> Wire it up. Wire it to the card connector. Add power. If you get a screen
>> that works, you've done it right.
> I think this is why Luke put the display signals on the EOMA68
> standard in the RGBTTL format--to simplify the job of connecting to
> LCD's.  (I'm thinking of the laptop, tablet, gaming console, phone,
> etc.)

 yyup.  exactly.  remember, you can't do more than one interface on
any given set of pins, so i had to pick one (RGB/TTL or LVDS or MIPI
or eDP), that then means you have to have a conversion IC in-place on
the Card if a particular SoC doesn't *have* that interface... and many
of the lower-cost SoCs don't because they're not part of the MIPI or
DisplayPort cartel(s)....

 ... and even if you had LVDS, the cost on the other side (Housing
side) of having an LVDS-to-RGB/TTL converter is so high relative to
the cost of the LCD itself that companies would rebel and not bother
with the standard at all.

 so, bizarrely, RGB/TTL, by being both "free" and also unencumbered by
patents *and* by being lowest-common-denominator, wins out on all
fronts.  except for the fact that you need a 125mhz clock-rate for
1920x1080 at 60fps, which is a bit... high.  but hey.


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