Quote:
Originally Posted by SpeedGeek
Well, 40 MHz is probably the upper limit for asynchronous FPU clocking but I still don't recommend it. The DSACK1 skew from U7 is already over the 3-5ns spec. Also, the FPU_CS is marginal and at the limit. Running totally reliably with occasional FPU use is not the same as working the FPU hard under the full set of thermal and voltage loss conditions.
You did not mention if it was the Fast RAM or FPU removed which enabled the boot @ 50 MHz? But there is an opportunity to try some of the lesser overclocking frequencies you previously mentioned.
Other than reviewing the PAL checksum and speed ratings, I have no more suggestions at this time.
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Presence or absence of the FPU did not seem to affect my results, it is a 50MHz one anyway. While I was not aware of the problems clocking it asynchronously, I had been clocking it synchronously in my earlier tests for convenience.
Done some more testing now. All tests are with the FPU and expansion memory removed from the G-Force. All Zorro cards except PicassoII removed. All jumpers set for 50MHz. Power is good, 5.05V on the drive connectors, 4.9V on the pins of the clock oscillators in the G-Force.
50MHz - Will boot when cold, but built in memory on G-Force not recognised. PicassoII not recognised.
48MHz - No boot at all, even when cold. Power LED changes brightness. Hard drive light may flash, followed by the bypass relay on PicassoII sometimes giving a brief buzz or power LED may blink.
44.9MHz - Boots from cold OK, machine runs OK, but will fail on reboot or power cycle (behaves just like at 48MHz). Heat related? Nothing on G-Force card gets hot though.
42MHz - Seems to be perfectly reliable.
Here are the details on the PAL chips on my board:
U34 - 74F4, 5nS
U35 - 5F54, 10ns (this chip made by MMI, all others by AMD)
U36 - 90C4, 7nS.
U37 - 4D7B, 5nS.
U38 - 38CC, 7nS.
U39 - F029, 7nS.
U40 - B520, 7nS.
U53 - A2B1, 7nS.
U7 - 4214, 7nS.
U6 - D794, 20nS. (I included this as none of the pictures of this board on the net seem to have this checksum).....I'm starting to think this may be the problem.
EDIT: Since then, I have been testing with the FPU installed, and clocked synchronously with the CPU. Its presence has not altered the results I was previously getting without the FPU.
Edit 2: Seems I have two problems simultaneously (for some reason that ALWAYS happens to me!). With the hard drive removed and J14 closed, I now get fully reliable operation at 44.9MHz, even with all Zorro cards, RAM and FPU fitted. Still fails to boot at 48 or 50 MHz though.
Now investigating my SCSI setup (I have a Guru ROM v 6.14, a SCSI2SD v6 and a CDROM). Correctly terminated at both ends. Just have to find out why it falls over above 42MHz.