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Old 14th December, 2001, 03:16 AM
Brad
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stuff dividers, lets fix the pci speed at 33mhz

http://www.overclockers.com/tips745/index.asp

this made me think about making my own little circuit to give a 33mhz signal to the pci circuit.

We could still send in 14.138mhz, and just wire in a 33mhz crystal to the pci output of the pll. Cut the contacts from the pll that control pci speed, and make a little circuit that feeds in 33mhz.


Now, I don't know where to get 33mhz crystals, but in rc aircraft, a popular frequency is the 36mhz range, which goes from 35.450 to 36.550.

These crystals come in Rx and Tx forms (receiver and transmitter). At this stage I don't know which one to use.


Now, what I propose is that we should find the pll of a certain mobo. http://www.icst.com/pdf/94228.pdf this is the one for the Epox 8kha+ Here we can see that pin 10 has two purposes, pciclk out, and 'logic input to select 24mhz or 48mhz for pin7 output' I have no idea what the hell this output thing means, so I doubt we could cut this pin off from the pll and connect a 33mhz input to the mobo.

Now, pins 11, 13, 14, 16, 17, 18, 20, 21 have pciclk on them only. so I think we could solder all these to our new 33mhz input circuit.

The last one is pin 22, which leads the pci clock be 2ns. it mentions in the pdf that it can be stopped by issueing pci_stop# I presume this means it doesn't matter if we just cut it away.



Now, at this point, I think the best board to try this out on would be a cheap kt266a board, as we know the kt266a chipset can handle 190mhz+ in fsb speeds, due to most boards having a pll with a 1/5 and 1/6 divider built in


Is anyone interested in helping me with this project?
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Old 14th December, 2001, 04:43 AM
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Very interesting.

Id love to help although im not sure id be of much use.

As for RC crystals they are slightly different frequencys for TX and RX, im not ecactly sure why this is although i can find out. For our proposed use i dont think it would matter.

Where are you (country) if you want a crystal supplyer for the UK ill probably be able to find somebody.

Ill have to read through the article on overclockers again and do some research before i exactly understand what your proposing. Im sure others here will get it straight away.
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Old 14th December, 2001, 06:28 AM
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pin10 and pin22 are the only ones I am scared about.

The rest I reckon I could do.

also, a crystal has two legs, what do I connect to where?
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Old 14th December, 2001, 01:23 PM
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Re: stuff dividers, lets fix the pci speed at 33mhz

Quote:
Originally posted by Brad
We could still send in 14.138mhz, and just wire in a 33mhz crystal to the pci output of the pll. Cut the contacts from the pll that control pci speed, and make a little circuit that feeds in 33mhz.
It's a nice idea, but might fall foul of a problem or two. Normally the clock generator generates clocks that are syncronised with each other. If the chipset isn't designed to allow it, you may find it won't work with asyncronous clocks. I don't know enough about the VIA chipset to know if this would be a problem or not.

You *might* be able to use the original clock signal (14.138MHz) to generate a reference PCI signal that's got the right phase as the PCI clock should do. I've never tried this, so I'm wary as to how difficult it would be to do.

Quote:
Here we can see that pin 10 has two purposes, pciclk out, and 'logic input to select 24mhz or 48mhz for pin7 output' I have no idea what the hell this output thing means, so I doubt we could cut this pin off from the pll and connect a 33mhz input to the mobo.
What it means, is when you first power the chip up, it reads the voltage level on pin 10. After it's read the pin, and set itself up, it generates a 33MHz output from that pin.

The 24MHz/48MHz bit allows the chip to be told to generate the output for a floppy controller (24MHz) or a USB controller (48MHz) on pin 7. Once you've figured out if it's a 24 or 48MHz clock on pin 7, then you don't have to worry about it, as you can simply pull up/down pin 10 on the PLL itself.

Quote:
The last one is pin 22, which leads the pci clock be 2ns. it mentions in the pdf that it can be stopped by issueing pci_stop# I presume this means it doesn't matter if we just cut it away.
You'll find that if you just cut it anyway, the PCI bus may not work any longer! The 2nS lead is to allow for proper syncronisation for whatever device needs slightly early notification of the clock. The PCI_STOP# signal, I believe is related to power saving modes, where the PCI clock can be halted to reduce the power consumption.

Quote:
Now, at this point, I think the best board to try this out on would be a cheap kt266a board, as we know the kt266a chipset can handle 190mhz+ in fsb speeds, due to most boards having a pll with a 1/5 and 1/6 divider built in
I'm not sure how the fact that the PLL can generate 190MHz plus speeds indicates that the chipset can handle it? That's like having tyres good for over 200MPH on a car and assuming the rest of the car will be able to do 200MPH because of that.

AidanII
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Old 15th December, 2001, 08:37 AM
Brad
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so....what do I do to pin10 and pin 22?

apart from that, I think I know what I want to do.

send 3.3v to a crystal, connect the crystal to all the pins


hey epoxtech, can you provide any input to this?
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