PCI Speed and Overclocking: A Closer Look at A64 and P4 Chipsets
by Wesley Fink on February 16, 2004 9:42 AM EST- Posted in
- CPUs
Sometimes things are not as they appear. Nowhere is this more true than the Computer Industry. Technology should be very straightforward, but all too often, marketing concerns get thrown into the mix and we end up with specifications with a marketing slant. It has been observed in reviews of the nForce3-150 boards that they often overclock better than VIA K8T800 boards. The nF3 sports a PCI/AGP adjustment in BIOS and it is usually stated that the nF3 should overclock better because of the fixed AGP/PCI bus.
Generally, we have also seen that Intel chipsets with Northwood processors reach incredible overclock levels. Intel has a widely published PCI/AGP lock in 865/875 chipsets, and it is clear that speeds like DDR550 (275x4 FSB) could never be reached without the fixed or locked AGP/PCI speeds. Most peripherals simply can't handle a PCI speed of 45.8 (275/6) or AGP of 91.7 (275/3) when specification is 33/66. Our Radeon 9800 PRO cards can rarely handle anything above about 70-72 on the AGP bus; this is typical of most current AGP cards.
Many tests have demonstrated the ability of Intel chipsets to lock the PCI/AGP bus, but questions are being raised again with the introduction of VIA's new PT880 chipset for the Pentium 4. VIA has been historically the only chipset maker not to utilize a PCI/AGP lock, but in the new PT880, VIA claims that they have implemented an asynchronous PCI/AGP bus. This is another way of saying that the processor and memory bus run at one speed and the PCI/AGP bus is not run at fixed divisors of that speed. This keeps the sensitive PCI and AGP cards in specification and allows a potentially higher overclock of the CPU and memory. Since this is VIA's first AGP/PCI, many are asking whether VIA really pulled this off - it has been such an issue with their chipsets in the recent past.
The most effective way to measure PCI/AGP bus speed is probably oscilloscope testing of bus input frequencies. To see an example of how this is done, you may want to check out tests on the nForce3-150 PCI bus at Lost Circuits. There is also a simpler tool available with the feature of measuring and displaying PCI bus speed, and that is PC Geiger. This device is a card that is mounted in a PCI slot and a digital display that provides all kinds of information - post codes, bus utilization, and PCI speed. The Taiwanese company, IOSS, markets PC Geiger in a joint venture with VICS in Japan. While PC Geiger provides many types of information, we will only be looking at PCI speed for these tests.
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PrinceGaz - Monday, February 16, 2004 - link
I'm still awaiting any sort of reponse to the first post in this thread about why no tests seem to have been done at around 233 or 234FSB to see if a 1/7 divider kicks in. Thats the first thing I'd have tried after establishing no PCI lock at say 210 or 220. And yes, you can lower the multiplier on A64 chips so they aren't an issue like I said.Pumpkinierre - Monday, February 16, 2004 - link
Yes I second ionpro2's #13 suggestion. If you can get higher clockspeed by lowering the multiplier. try your geiger at these higher speeds and see what's going on.Icewind - Monday, February 16, 2004 - link
Now im curious as well as how you got it that highionpro2 - Monday, February 16, 2004 - link
Wesley Fink: No way that the motherboard would've posted with no PCI lock. Think about it: the devices that are dangling off of the PCI bus on the board itself would be running *way* the heck out of specification at the overclocks achieved on the Aopen. Perhaps these have higher tolerences, but I seriously doubt the ALC655 would run at 43Mhz, or the Realtek Gigabit LAN chip, either. Most likely, there are merely windowed dividers for these chips, with a 1/6 and 1/7 ratio. It would make sense for this to kick in at 220Mhz -- otherwise, you are running the PCI out of spec almost 10% down; and I'd be willing to bet whatever optimization is done for aftermarket cards is done for higher PCI/AGP speeds, not lower. *Please* test the AOpen board at 9x240 and see what PCI speed is reported.Wesley Fink - Monday, February 16, 2004 - link
Pumpkinierre -Your question is exactly the reason I stated that I don't know what is happening with AGP on A64 boards. With no PCI cards in my test setup, a working AGP lock could provide quite a bit of OC room. Some sites have reported AGP IS fixed on VIA, but I have no way of testing that. I can tell you for a fact that the PCI slots are not frequency locked on the A64 boards I tested, but the way frequencies are derived in HT it IS possible AGP is locked on some A64 boards.
Pumpkinierre - Monday, February 16, 2004 - link
I dont understand. You got 253MHzx8 and 240x9 on the Aopen A64 mobo when you tested it. That's a lot more than 220MHz. Even if the AGP was asynchronous, which I very much doubt, surely other components on the M'board would have seized?!impar - Monday, February 16, 2004 - link
I would like to see the same test in nForce2 boards...Icewind - Monday, February 16, 2004 - link
Sorry, guess I missed the defination of "go pound salt" so i'll take it with a grain of salt.idgaf13 - Monday, February 16, 2004 - link
If all The motherboard chipset suppliersdo the same thing ,such as the PCI lock,
musi be a "deeper"reasoning.
Certainly if possible ,contractually or technically, they would do it.
They all need that special selling point ,A point of differintation.
Basically "go pound salt" ,Icewind.
Icewind - Monday, February 16, 2004 - link
Cause VIA knows better and they still won't comply with the wishes of system builders. For that, I dispute them.