Hi All,
The Gigatron expends a considerable percentage of it's clock cycles generating the 1/4 resolution 64 colour VGA display.
However it should be possible to use a shift register to clock out each pixel (monochrome) using a 25MHz VGA clock.
This would provide a considerable boost in the processing throughput.
Had anyone explored this option?
Ken
An 80 column monochrome display?
Re: An 80 column monochrome display?
I haven't explored it. Making a 25 MHz VGA circuit on a PCB is pushing things for most people. You'd need tricks such as using 74ACT logic and likely pipelining.
If the Gigatron is driving it, it would be simpler in that you wouldn't need a sync circuit, and the CPU has no problem with producing the syncs for that on time, even at 6.25 MHz. So I guess put the top 2 bits in a flip-flop and the other 6 in a shift register (or flip-flop wired as one). I guess you'd be sending 6 pixels at a time, 4 would fit into the 6.25 MHz clock, and there'd be the other 2. So 2 slow cycles would send 12 pixels, taking the time of 3 Gigatron cycles.
If the Gigatron is driving it, it would be simpler in that you wouldn't need a sync circuit, and the CPU has no problem with producing the syncs for that on time, even at 6.25 MHz. So I guess put the top 2 bits in a flip-flop and the other 6 in a shift register (or flip-flop wired as one). I guess you'd be sending 6 pixels at a time, 4 would fit into the 6.25 MHz clock, and there'd be the other 2. So 2 slow cycles would send 12 pixels, taking the time of 3 Gigatron cycles.