80Mhz crystal and 040@40mhz in an A3640...

[AndyLandy]

The A4000 motherboard isn't clocked off the CPU card. If you have an A3630 card, that's clocked off the motherboard, so you can't overclock it without overclocking the whole system. This isn't true of the A3640, that's independently clocked. The problem with overclocking the A3640 is that you are also overclocking the glue logic on the card. You'd have to be very lucky to get a card that will tolerate a 40MHz clock.

[AndyLandy]

The slowest device on the A3640 (except for the 040 Cpu) is a 15 ns Pal or Gal chip (most A3640's have 10 ns or faster) and these are rated by their manufacturers to run at 50 Mhz or above. However, the Amiga custom chips on an A3000 or A4000 motherboard were designed to run at a mere 28 Mhz! So you're lucky if you can overclock your motherboard at 33-35 Mhz. Also, Ramsey is highly dependant on the speed of the installed Fast memory. Also, The A4000 must receive it's Cpu clock and Clock 90 from the Cpu card in order to support the A3640.

I thought the whole point of the A3640 is that you don't overclock the chipset, the card is independently clocked from the motherboard. I understand that's why you can't overclock the A3630, since it's synchronously clocked with the motherboard, but the A3640 isn't. So what am I missing here?

[AndyLandy]

What you are missing is an understanding that the A3640 is a synchronous accelerator which was designed to run at the same clock speed as the motherboard. The clocking difference between the A3640 and A3630 is the motherboard receives the clock from the A3640 and the A3630 receives the clock from the motherboard. If the A3640 had been an asynchronous design then the 40Mhz barrier would have been broken a long time ago.

aah *realisation dawns*

That makes sense now. Quite why the big C= felt the need to do this differently between the '030 and '040 card is beyond me, but so are many of their decisions. Thanks for the info. :-)