ED#22 : The NVIDIA nForce3's Unlocked PCI Bus

One of our forumers, Don, who is from OverclockerIndo contacted our motherboard specialist, ZuePhOk, and asked him to investigate the NVIDIA's nForce3 PCI bus. He suspected the nForce3's PCI bus was not locked because his friend killed a SCSI card after using a high FSB! If the nForce3 had a PCI lock, the PCI bus would have been locked at 33MHz and the SCSI card would not have been killed even with the system bus running beyond specification.

But how can ZuePhOk test and confirm if the nForce3's PCI bus was really unlocked? Luckily, Team ARP has the IOSS RD2 Pro PC Geiger! If you recall, the RD2 Pro PC Geiger is a multifunction PCI bus analyzer that we reviewed some time ago. As it directly monitors the PCI bus, it is more accurate and trustworthy than any software solution.

Although we do not have our own nForce3 testbed, ZuePhOk had the opportunity to play with an AMD Athlon 64 FX-51 processor and an ASUS SK8N nForce3 motherboard. He was thus able to test the nForce3's PCI bus out.

Let's have a look at his results!

ZuePhOk's Test Results

First, ZuePhOk ran the SK8N's front side bus at 200MHz. As expected, the PC Geiger returned a result of 33.3MHz.

PCI bus speed at 200MHz FSB.

Then, he increased the SK8N's front side bus to 205MHz. If the nForce3's PCI bus was locked, PC Geiger should return a result of 33.3MHz. But did it?

PCI bus speed at 205MHz FSB.

Apparently not! Instead of a 33.3MHz PCI bus, the SK8N ran its PCI bus at 34.1MHz! That is an increase of 2.4% which corresponds to the 5MHz increase in FSB speed.

Now, to confirm that the 34.1MHz was not a fluke, ZuePhOk increased the FSB speed to 215MHz. If the PCI bus was truly locked, PC Geiger should return a value of 33.3MHz. If the PCI bus was unlocked, then we can expect PC Geiger to return a value of 35.8MHz. Let's see what PC Geiger reports.

PCI bus speed at 215MHz FSB.

Will you look at that! 35.8MHz as we would expect if the PCI bus was not locked! So, that pretty much confirms it - the NVIDIA nForce3 chipset does not lock the PCI bus.

Conclusion

While we do not have the chance to test all nForce3 motherboards, this initial finding shows that there are at least the ASUS SK8N motherboard does not come with a locked PCI bus. It would be sheer folly to assume that nForce3 motherboards automatically lock the PCI bus at 33.3MHz. We suspect that a PCI bus lock in the nForce3 motherboards will require at least some form of BIOS support.

For now, it would be safer to assume that the PCI bus is not locked in your nForce3 board and to overclock with caution. PCI devices officially support a maximum bus speed of 33.3MHz but most, if not all, will run quite happily up to 37.5MHz. So, it is advisable to limit the FSB speed of your nForce3 board to 225MHz until you can be sure that your board supports PCI bus speed locking.

Questions & Comments

If you have any questions or comments about this article, please feel free to post them here! We look forward to hearing from you! :)