On Thu, Mar 1, 2018 at 10:42 PM, Richard Wilbur richard.wilbur@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@60fps, which is a bit... high. but hey.
l.