Did you forget about the E clock? All 68000 CPU based Amigas use the E clock generated by the CPU. So with a 14 MHz CPU clock you should divide the E clock by 2 and a 28 MHz CPU clock divide it by 4.
Right. That's a good point. So I read earlier that E is 1/10 of the clock input of the 68000. I guess the thing I didn't understand there is the importance of E frequency relative to the chipset versus the processor, and yes, I did not touch it going from 14 to 28MHz.
From the beginning, stock (NTSC) 68000 Amigas have a clock input of 7.15MHz on the processor, the resulting E clock is 715KHz. I measured the E pin on Dennis's unmodified 14MHz accelerator and it is 715KHz, because he divides it in half with a flip flop.
The 28MHz processor in my modification generates 2.8MHz E clock, which is divided in half by the flip flop per Dennis's design. This results in a 1.4MHz E output to the Amiga, which is presumably not good. But like this the machine is mostly working, with KS 1.3 it boots into games and they work, aside from fast music.
So today, after your reminder, I added another flip flop to cut the 1.4MHz E to get 715KHz, as in the original Amiga circuit. I measured the E pin output with my scope and saw 715KHz.
But my Amiga now only displays a bright green screen. It doesn't work, yet anyway. (I re-checked and reverted to prior condition to test, I didn't break anything else.) My bodge wires added about 6-7 inches of length (15cm, give or take), from what I see on the scope the 1.4MHz and 715KHz edges are aligned, there is no more than 1ns delay in there. It's possible, but hard to believe this is a timing problem here.
Looking at Dennis's schematic I can see that the E clock from the processor also drives the clock pin of the flip flip flop synchronizing the /VPA signal. So my 28MHz modification drives that flip flip clock at 2.8MHz. Since the first divider flip flip outputs 1.4MHz, I tried driving that flip flop clock with 1.4MHz like in Dennis's 14MHz design. This doesn't necessarily seem right, but I tried it anyway. Still bright green screen. I don't think this change was right, it makes more sense to leave it 2.8MHz, but was worth a try.
Your post is helpful and you pointed out a thing I hadn't done. What do you think I should change next?
Don't mess with odd asynchronous clock frequencies unless you have the extra logic to generate the proper E clock. This is exactly what the 68020+ CPU based Amigas do.
Yeah, I agree, looking at the division of the E clock it does look inconvenient to do 21MHz, but I can do the division by 3 circuit, it is just more complicated. It's definitely better to get 28MHz working first.
I'm going to take a closer look at timings tomorrow, and I might have some faster flip flops on hand. If all else fails I will connect a logic analyzer and record signals on the two versions of the accelerator and compare to see what isn't right.