Sorry for not being clear. Normally, the 575 uses two, 512K, 68 pin, 80ns, VRAM SIMMs. But if you drop in a full 68040 running at 40Mhz, you will not get any video on your screen because the VRAM is too slow. You have to install two 512K, 68 pin, 70ns, VRAM SIMMs to make it work (and do some other stuff)
As I can not find any 70ns VRAM for sale any place, my question is this:
What is the slowest CPU I can run on this 575 logic board? I also have a slower CPU - a XC68LC040RC33B. That is what is running in it now.
Some text below on this mod.
Thanks.
jack the new kid
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2.1.4 — How do I overclock a Mystic?
Full directions for overclocking a 57x motherboard can be found at
the Gabezing Room.
Having done this to a couple 575 boards myself, and having reports from several others who have done it, I highly recommend getting several sets of 80ns VRAM to try in the likely event that your VRAM isn't fast enough to avoid video artifacts. I found a pair of 70ns modules solved all the problems, while some have found that certain 80ns modules work great while some simply give a black screen. Apparently some 80ns modules are faster than others.
40 MHz seems to be a speed that almost everyone can easily achieve, but speeds over about 43 MHz seem to be pretty much luck if they are achieved. I tried a 22.44 MHz oscillator (44.88 MHz CPU/motherboard bus speed) in mine after installing the 70ns VRAM, but got a black screen. Perhaps 60ns VRAM (haven't heard of any ever being made in the proper form factor) would have cured this, but without a display, I couldn't test the serial ports either, which are reported to fail at about 43 MHz.
If you're looking for parts to perform the overclocking, or need a parts list, check out
my parts list page.
Parts List Description Supplier Part Number Cost (USD) Oscillator socket
Digi-Key A462-ND $0.60 20.000MHz crystal oscillator
Digi-Key CTX119-ND $2.78 miniature PC board
Radio Shack 276-148 $1.49 Notes
- If you would like a crystal other than 20.000 MHz, Fox Electronics can custom-build crystals to your desired frequency.
- Digi-Key charges a US$6 handling fee (in addition to normal shipping fees) for all orders under US$25. If you're only ordering these two items, it's not worth it.
- Speeds up to 44MHz have been reported as stable; speeds over 43MHz may cause loss of serial port functionality.
- The CPU and motherboard bus will run at twice the speed of the oscillator, so a 20MHz crystal will give a 40MHz CPU clock.
- I highly recommend using a heatsink on the chip. If you need one, see the information below.
- While you're speeding up the computer, you might as well put a full '040 into it. I have benchmarking results for a Q605 published here comparing the stock motherboard to a clocked one with a full '040. The full '040 will speed up math-related tasks such as JPEG rendering (common on WWW pages) by a factor of about five.
- The prototyping board from Radio Shack is enough for at least four installations if properly cut. Even without cutting, it's good for two because it's made to snap in half.
About that heatsink: there are a number of pitfalls associated with heatsinking the '040 in a 57x motherboard. First of all, there are no heatsink clips on the CPU socket. This makes it terribly hard to attach a heatsink, obviously. A method I've found to be effective (if somewhat tedious) is to put a thin layer of heatsink grease on the bottom of an 040 heatsink and simply set the heatsink on the CPU. The grease will hold the heatsink in place pretty well and will be quite sufficient as long as you don't move the machine around a lot. If you're planning on transporting the machine, just remember to re-align the heatsink on top of the chip when you get it set up in its new location.
If you're planning on going over 40MHz or have a 25MHz '040 installed, I would also recommend getting a heatsink-mounted fan. The
ones I use are from Computers 4 Sure, and are intended for a 486. They piggyback off the HD power connector and draw minimal power. There is a serious problem with these, however, and I suspect it applies to most 486-style heatsink/fan combos. Since the 486 was slightly smaller than the 68040,
there is a small lip on the edge of the bottom of this heatsink. This creates an air space between the heatsink and CPU. Since air is a good insulator, this is a problem. I solved it by machining the lip off of the heatsink with my Dremel; for those of you who don't have this option, I suggest finding a heatsink/fan combo whose heatsink has a perfectly flat bottom. If you find one,
let me know where you got it and, ideally, a part number and cost as well. As with any heatsink, you should apply a small amount of thermally-conductive "heatsink" grease to the surface in contact with the CPU to effect efficient heat transfer.
Last modified on 15 Aug 2001
by Chris Lawson
