On Mar 1, 2018, at 20:40, Luke Kenneth Casson Leighton lkcl@lkcl.net wrote:
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.
So if we could read the EDID the driver would figure out the A20 timings? Does the A20 already have a graphics driver capable of that? (In which case the bit-banging VESA DDC driver becomes very important.) How much of this infrastructure already exists? I'm bringing my tools, where do we start building?
I have a collection of VGA monitors with different aspect ratios and sizes (3 CRT and 3 LCD). I'd be happy to test resolutions above and below 1024x768.
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.
Are you thinking of octal D flip-flops? I'll have to look up those datasheets. What do those chips offer over the flip-flops? How do the prices compare?
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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)....
Yes, that's the awful thing about so many industry standards: you can't get the text without signing documents and paying a handsome price, you can't use them without paying royalties to the patent owners.
... 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.
Will the A20 clock the RGBTTL interface that high?