Overclocking A3640

[SpeedGeek]

A3640 rev3.1 should be no problem but you did not say if you used the A3640 on an A3000 or A4000. The A3000 motherboard won't run reliably past 30 MHz but the A4000 motherboard should run @ 33-37 MHz depending on the speed of the installed fast memory. Double check your delay line hack. The 10 ns tap must be disconnected and replaced with the 5 ns tap. Also, check the +5V supply on the motherboard and make sure the 200 pin CPU slot connector is clean. I assume you installed the new caps with corrected polarity as well.

P.S. Once you get the problem solved you should get a cooling fan since the XC68040 tends to run very hot!

[SpeedGeek]

It's not signal integrity limiting the A3000 mobo it's SuperDMAC. Remember, the A4000 does not have one, it has IDE PIO logic instead.

80 ns memory should be fast enough to run RAMSEY @ 30 MHz (the A3000/4000 custom chips were actually designed to run @ 28 MHz and under-clocked to 25 MHz for production reasons and because Motorola would not guarantee performance on over-clocked 25 MHz CPU's. With 70 ns memory you could run RAMSEY @ 33 MHz on an A4000.

You still need to check the mobo +5V and if you installed the new caps with correct polarity. Also, what versions of RAMSEY and SuperDMAC are installed?

[SpeedGeek]

Erm.... no. The custom chips always need to run on 28 MHz or you wouldn't get anywhere near NTSC/PAL frequencies. The Amiga has two distinct busses that may be run asynchronously.

No. It's the only the graphics chips (FAT AGNUS and ALICE)    which use a master clock of 28 MHz. This master clock gets divided down to 7 and 3.5 MHz for ECS systems (DENISE) and 14/7 and 3.5 MHz for AGA systems. (LISA).

RAMSEY, SUPER BUSTER, FAT GARY and SUPER DMAC are 030 bus protocol chips which run synchronous to the CPU and I quoted Dave Haynie on them being tested @ 28 MHz.

>Perhaps it doesn't like to run on anything other than 25MHz. "Well, overclocking the A3000 motherboard isn't recommended, though I did most of my Buster simulations at 28MHz, just because our testers didn't support multiple asynchronous clocks, so I had to run at 28MHz to get the 7MHz timing correct."

Dave Haynie

[SpeedGeek]

@Sandman

Thanks for the updated info. There were some early versions of Ramsey and SuperDMAC which had problems with the A3640 even at 25 MHz. But I have the same versions as you in my A3000 and managed to get a boot at 30 MHz. The heat sinks are needed for reliable operation when the chips start to warm up but are not needed for a cold boot.

Check the ROM speed jumpers to make sure both are set to the 25 MHz position. (It's still possible that some masked ROM's could be to slow to run @ 30 MHz). You could also try connecting the CLK90 jumper back to the 10 ns tap just to see what happens. Also, please describe what happens when the power is first switched on. (i.e. power LED cycles dim/bright or HD led stuck on or none of the above).

[SpeedGeek]

If U209 was the problem here than the A3640 would not even work @ 25 MHz on an A3000. But just in case this is one those A3640's which does not like to run above 28 MHz maybe it would be good idea to try it with an A4000 @ 30-33 MHz.

[SpeedGeek]

If you can find a rev. 4 SuperDMAC it would certainly be worth a try. You could also try another rev. 2 SuperDMAC as some work better than others. Another thing you might want try is replacing the 74F08 @ U103 with a 74HC08.

[SpeedGeek]

I never could get my A3000 mobo to run reliably past 30 MHz until I hacked SuperDMAC to run @ 1/2 CPU clock. The reason I didn't suggest this before was because DMA performance will get much slower. However, Bebek on Amibay says his A3000 is working @ 33 MHz with the SMD 030 so I'm waiting for more info on this.