Another suggestion: Try using DPI. You might not even need a HAT: you could drive it more or less directly from the GPIO header. But for compatibility and to avoid any risk of damage, it would be wise to use a buffer (maybe 74HCT125). That will drive the H/V sync signals at proper TTL levels.
For the intensity signal, if only black and white is required, you could maybe drive it from Green[7] (which by default is GPIO 21), through the buffer, then divide the output voltage with a pair of 150R resistors (that should give 2.5V peak unloaded, ~1.25V loaded, which is too high for VGA but I'm going to guess this monitor will be fine with it. Increase the top resistor if cautious.)
Getting rid of the HDMI-to-VGA converter gives you more flexibility and control over voltages and timings. And as there's no EDID, the emulator might be less tempted to switch to a different video mode.
With your scope, check all the voltages, frequencies, widths and polarities are as expected before connecting the Atari monitor! Compare it with the real ST's mono output if you can.
For the intensity signal, if only black and white is required, you could maybe drive it from Green[7] (which by default is GPIO 21), through the buffer, then divide the output voltage with a pair of 150R resistors (that should give 2.5V peak unloaded, ~1.25V loaded, which is too high for VGA but I'm going to guess this monitor will be fine with it. Increase the top resistor if cautious.)
Getting rid of the HDMI-to-VGA converter gives you more flexibility and control over voltages and timings. And as there's no EDID, the emulator might be less tempted to switch to a different video mode.
With your scope, check all the voltages, frequencies, widths and polarities are as expected before connecting the Atari monitor! Compare it with the real ST's mono output if you can.
Statistics: Posted by njh — Fri Jun 27, 2025 12:25 pm