Whilst it may work, the numbers were coming from the service manual that I'd found that state 5 +/- 1.5V.Actually, while TTL devices use a 5V supply (at least originally) input thresholds and output drive capabilities are asymmetrical.
TTL is 0V and 5V signalling (with tolerances).
Most Pi VGA HATS will be driving the sync signals from the 3.3V GPIOs, so are a bit low for TTL compatibility. 5-1.5 = 3.5V, so 3.3V is technically too low. VGA uses 0.7Vp-p signals, but that is unlikely to be critical.
From memory, input thresholds are <0.8V low, >2.0V high. Output drive is <0.4V low for 16mA current (on original TTL; different for the various TTL families which evolved) and >2.4V at 400uA (yes, uA - again different for the various families).
So driving TTL-compatible inputs from 3.3V CMOS logic should be OK.
It's standard CMOS devices that typically have input thresholds at 1/3 and 2/3 of supply voltage, and symmetrical output drive.
Just to confuse things, there are now devices with TTL-family part numbers, but CMOS characteristics!
3.3V is outside that stated tolerance. Any problems that you experience are therefore your own to solve.
And also referencing the (partial) schematic in that service manual, the vertical sync signal appeared not to be going directly into a TTL logic chip, therefore making assumptions about how many TTL logic chips work is likely invalid for that signal.
Statistics: Posted by 6by9 — Wed Jun 25, 2025 12:31 pm