Request: Better support for MiSTer ao486 core in Direct Video mode

[vess]

Hi marqs,

First, thank you for the amazing work on OSSC Pro and the recent EDID / low-frequency improvements in v0.81.

I’m trying to use the ao486 core on MiSTer FPGA with direct_video=1 (i.e. bypassing the MiSTer internal scaler) so that OSSC Pro receives the raw VGA timing from the core. Unfortunately, when the ao486 core outputs its native DOS text mode 720×400 @ 50 Hz (~26.95 kHz horizontal, ~27 MHz pixel clock) the OSSC Pro input goes black and never locks.

Currently the only reliable workaround is to disable direct_video on the MiSTer side and let the internal scaler upscale everything to 1080p/148.5 MHz, which works perfectly but loses the authentic low-frequency VGA feel and adds the MiSTer scaler into the chain.

Other low-frequency cores (e.g. Minimig, Amiga, some arcade cores) already lock fine in Direct Video mode, so the issue seems specific to this exact 720×400 @ 50 Hz timing (which is a real historical VGA mode used on many European 486/Pentium machines).

Would it be possible in a future firmware to add explicit support for this timing (or widen the HDMI Rx low-frequency acceptance window a bit more)? Even a simple pre-defined sampling profile for 720×400 @ 50 Hz / 70 Hz would solve it for all ao486 users.

Many thanks in advance and happy to provide any logs, modelines or test setups if needed!

[marqs]

The issue should not be related HDMI Rx pixel clock limitations. Have you tried both scaler and LM modes? I’ve used several real 486/Pentium era DOS PCs without problems. They have always outputted 720×400@70, though – I’m not aware of any machines with 720×400@50 although it should work fine too. I can try ao486 core before next release to see if that’s an easily fixable issue.

[Author]

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