Starting with the 80486DX/2 CPUs, the internal clock speed is a multiple of
the CPU clock speed, aka the CPUCLK speed. The PCI slots are usually run at
33MHz, which is the CPUCLK divided by 2 or 3. The ISA slots are usually run
at 8MHz (or there abouts), again determined by the CPUCLK divided by 8 or
12. Most expansion cards are designed with 33MHz and 8MHz in mind.
Only recently did the 100MHz CPUCLK become available for Socket7.
Everything up until now was either 50MHz, 60MHz, or 66MHz CPUCLK. So, a
Pentium-75 was 50MHz CPUCLK x 1.5; the CPU internally ran at 75MHz, and I/O
to and from the CPU ran at 50MHz. P-100 is 66MHz x 1.5; P-133 is 66MHz x 2;
P-233 is 66MHz x 3.5; P-266 is 66MHz x 4, etc.
With the K6-300, the CPUCLK should be set to 100MHz FSB x 3. If you don't
have a M/B that supports the 100MHz CPUCLK, then it should be set to 66MHz x
4.5. You'll loose some speed due to the 66MHz I/O, but it'll work. You
don't want to use 75MHz x 4 because it will throw off the PCI and ISA bus
speeds; remember, PCI is designed to run at 33MHz and ISA at 8MHz.
The dual 1.5/3.5 setting is most likely to save space on the M/B. Only the
P75 and P90 used the 1.5x setting, and they're no longer manufactured. The
other clock speeds of 55, 75, 83.3 are available for "overclocking", running
the M/B at non-standard speeds. This is a cheap way of getting more
performance out of a processor. Of course, you risk compatibility and
thermal related problems. You can get more info about overclocking at
http://www.tomshardware.com or several other sites. Tom's Hardware is a
great site for general info about motherboards, video cards, CPU's, etc.
Good luck. Further discussion should be done via e-mail or on the BitBucket
reflector at QTH.NET.
- Aaron Hsu, KD6DAE
> From: email@example.com[SMTP:firstname.lastname@example.org]
> Sent: Monday, November 23, 1998 9:15 PM
> To: Tentec@contesting.com
> Subject: [TenTec] Computer Motherboard Clock Setting Question???
> OK, OK I know this is for TenTec stuff, but I am an OMNI VI owner and am
> trying to rebuild a computer that I hope will be part of my Modern
> Here goes:
> JUst purchased a new motherboard--it's a TX Pro II Socket 7 Series.
> Along with the board I purchased an AMD K6-2 300 MHZ CPU. Here's the
> question for all you computer whiz types from a computer fizz type:
> I understand that this CPU is capable of running on a 100 MHZ bus board.
> This board is not a 100 Mhz type. On it are jumper settings for 50, 55,
> 60, 66, 75 and 83.3 Mhz CPU External Clock Selectors (BUS SPEEDS???) In
> the manual there's another table titled "CPU INTERNAL CLOCK SPEED
> SELECTORS" in which there several jumper settings shown: 1.5 X /3.5X,
> 2.0X, 2.5X, 3.0X, 4.0X, 4.5X, 5.0X and 5.5X. Below the table the
> following is printed: "NOTE: THE CPU SPEED IS EQUAL TO CPU EXTERNAL CLOCK
> X CPU INTERNAL CLOCK SPEED."
> It would seem to me that the 300 Mhz CPU = 75 MHZ CPU External Clock
> Selector X
> 4.0 CPU INTERNAL CLOCK SPEED Selector.
> I was told to use 66 Mhz and multiply X 4.5 =297, as 75 Mhz might cause
> the computer to hang up.
> What's the story with all this??
> Is there any advantage/disadvantage to running at different external
> clock speeds?
> HOW SHOULD I SET THIS UP??
> Haven't as yet put in case or fired it up.
> Also why the dual setting of 1.5/3.5? Does this relate to this AMD chip??
> Any help would be greatly appreciated.
> Sorry for taking up TenTec B/W-but it is loosely related-isn't it????
> Thanks in advance.
> Art, N2KA..
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