Vio Adjustment on the Asus P2B-DS

Testing of the Asus P2B-DS Rev 1.06 D03 mainboard at 150Mhz FSB showed we had a serious problem with memory errors. The memory used for testing was a pair of 256Mb Corsair PC150 CAS2, model # CM654S256-150C2. This memory had tested clean on the P2B-S at 150Mhz FSB, so we knew the problem must lie with the mainboard.

Memory is nominally rated for a supply voltage of 3.3v, which is provided by the ATX power supply, then regulated by the onboard HIP6019BCB regulator. The regulated 3.3v supply is known as Vio. Measuring Vio on the two mainboards revealed a significant difference: P2B-S was supplying 3.47v to the memory, while the P2B-DS was regulating Vio to 3.22v. Could this be the cause of our memory errors?

The HIP6019BCB regulator's VIO output is resistor-programmable from 3.0v to 3.5v. The programming resistor on the P2B-DS is R50, located between the HIP6019BCB regulator and the first CPU slot.

R50 Location


R50 Replaced

R50 Location
R50 Replaced

R50 was originally a 5K ohm surface mount resistor. It was initially replaced with a 4.7K ohm surface mount, which raised Vio from 3.22v to 3.41v.

Testing revealed a large reduction in the number of memory errors at 150Mhz FSB, however memtest86 still reported 2 or 3 errors per pass in test 5.

R50 was then replaced with a 4.42K ohm 1/8 watt standard resistor as shown above as there were no surface mount resistors of the appropriate value to be found on the parts boards. This further raised Vio to 3.49v, almost the maximum available from the HIP6019BCB regulator.

However, testing showed no further improvement in the number of memory errors. The P2B-DS has considerably longer signal paths between the 440BX chipset and the CPU, due to the second CPU slot, than the P2B-S signal paths. We suspect this may be the reason we were unable to run memtest86 at 150Mhz FSB without error on the P2B-DS, after successfully testing the same CPU, memory, video card etc. at 150Mhz FSB on the P2B-S.

Bios Voltages

BIOS Voltages
The screen shot shows voltages as reported by the BIOS after the Vio modification. The +3.3v voltage read 3.2v before the modification, and is now reported as 3.5v. I have no idea why I had the -5v voltage set to ignore when this shot was taken, it should be -5.1v

Since the second modification to increase Vio from 3.41v to 3.49v did not produce any improvement in the memory testing results, we recommend others planning this modification use a 4.7K ohm surface mount resistor to increase Vio to approximately 3.4v

The modified P2B-DS successfully ran memtest86 for 48 hours without error at 140Mhz FSB - this appears to be the upper FSB limit for stability on the P2B-DS.
Last updated May 4th, 2003 by p2b@sympatico.ca