Carrying on from my previous exploits in generating video signals, I decided to try to up the clock frequency and maybe even get a bit of animation going. I used a 16MHz crystal, as the memory is something like 50ns or 60ns, and so 32MHz looked a bad idea. This gives me a 384x480 resolution, with effectively double-width pixels. One you put in all the H- and V-sync areas, that's 256KB, so I can fit two frames in the 512KB flash. By connecting the top-bit of my 24-bit counter to the top bit of the memory address line, I get two frames per seconds.
At least, that's the theory. I change the crystal, program the memory, and the signals look fine on the 'scope, but nothing displays on the monitor. The 16MHz oscillator I have doesn't exactly produce a crisp square wave, so I tried feeding the oscillator output through an inverter to square it up before using it to latch the signals feeding the display. Lo and behold, it works!
I'm still not entirely sure why the inverter helped. Sticking another inverter in the path still works, so it's not about inverting the phase, so perhaps it just needed a squarer wave for the latch, but maybe sticking a bit of propagation delays in helps. I'm going to play about with it a bit more, see what works.
Github repo still pending!