Odd thing is its a 66mhz input to the shifter
(normally 32) so the double rate would be 64mhz, though I dont have a 64mhz osc to hand at the moment. The MMU clock outputs are going via a /2 chip so the only thing in theory going faster is the MMU, SHIFTER
,CPU. The ST crashes when trying to load from the floppy, but im supprised it got a image at 66mhz. 64mhz would give a proper divide down to 16mhz for a 1.44 floppy, so its probably 17mhz into 16mhz bus, or 66mhz may just be too high, will order a 64mhz osc and try it out.
Also seems the osc chips only outputting 3volts at best, some look like 2 volts, this isn't enough to drive the MMU properly. The onboard (ST) 32mhz clock gen is rubbish, a 64mhz crystal causes the bus to half in speed!! So a clean clock is the way to go. Also I feel its a much sharper image using a better clock input. The shifter
does a great job at buffering the clock. The 32mhz is still only about 3V, put it outputs a nice 5V(ish) clock. Seperating any clocks on the ST bus is just a bad idea. While it does boot, it does not work well at all. With my current setup everything is in sync from a 66mhz master clock.
What I really need is a good 32mhz 5V osc circuit. Then I can feed the shifter
with that, and bypass its clock output and feed that direct with 32mhz also. hopefully it will solve the "double image" on the H scanlines and bring it back to normality in that respect. V sync will still be twice the speed, so only going to get half the picture on the screen, but will have to sort that out another time.
Next up I just need to get things synced with 64mhz, though I have a feeling im going to loose sync again, but wont know until its tried. At the moment the pixel clock is going double speed and the V sync is. Then need a benchmark program, I saw gembench3 someone, so will have to get that onto a floppy, even with the screen a bit messed up, i should still be able to speed test the CPU and MMU/RAM at 200% speed
The 244 and 373 buffers I replaced with ALS / ABT types, both faster than the LS, I think one type maybe the ABT is good for 200mhz! I just mixed them due to them being cheaper options after LS. I don't feel they need changing, they should be good for 25mhz or so, but I upped this to at least 40mhz now, so no problems with speed on the buffers.
The ROM's don't work well at 16mhz bus, So I am using , I presume 150 or 100nS EPROMS. It improved things a fraction so its a good move.
More news towards the end of the week hopefully
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