Guru
Joined: 05/10/2019
Location: United KingdomPosts: 7020
| Posted: 03:40pm 04 Nov 2024 |
The 220R or 270R resistors in the HDMI signal paths at the Pico end don't help the impedance matching in particular. In fact they make it worse. They are current limiters to protect the output drivers in the Pico The terminating impedance is 50R per line, 100R across each pair. This would take about four times the current that the Pico could source while supplying a usable voltage. In order to supply some voltage into the line it's necessary to limit the current and hope that we can get away with the impedance matching!
This doesn't affect the phase of the signals reaching the monitor, although it makes a mess of the wave shape. We still have to supply the data signals in phase with the clock so we can't introduce delay lines made from capacitors to GND and very unmatched trace lengths. We can get so far, as I've proved with Alpha, but just how far out we can get I don't know. This is why I've never attempted to fit both VGA and HDMI to the same board.