Extreme-Overclocking of Sapphire RADEON 9800 XT ULTIMATE Edition: Methods and Technology
Having taken a close look at the card we found that the power circuitry of the memory on RADEON 9800 XT is the same as we saw on RADEON 9800 PRO, save for ratings of some elements. The memory power supply circuit is still based on the Intersil ISL6522 chip and looks like that:
In order to change the voltage of the current that powers the memory, we need to reduce the resistance of the R2 resistor marked with a circle in the scheme. It has a resistance of 562Ohm rather than 1kOhm as in RADEON 9800 PRO. So we soldered up a trimming resistor with the resistance of 10,000 Ohms to pins 5 and 7 of the chip. Having reduced the resistance of the resistor to 2.5kOhm, we made the memory voltage grow from the nominal 2.69V to 3.12V.
We thought it would be not quite safe to continue increasing Vmem, so we stopped at that. We took the voltage measurements from the C746 capacitor (the point where you should take the measurements is marked as VMem in the scheme). The memory worked stable at 400MHz (800MHz DDR) and we couldn’t go up any further. The bonus of extra 10MHz compared to ordinary overclocking is of little value, but we probably deal with a simplified design of the PCB that hasn’t been designed for work with high-frequency memory chips.
As for the VPU voltage, it was not that simple. The circuit we saw in RADEON 9800 XT had nothing in common with the one of RADEON 9800 PRO, so it took some time for us to perform the modification. As it turned out, the VPU power circuitry on the RADEON 9800 XT card is based around the FAN5240 chip from Fairchild Semiconductor.
A curious fact: according to the manufacturer, this chip is specifically designed for the voltage regulator of mobile Athlon XP processors. Its output voltage can vary from 0.925V to 2.0V by sending different 5-bit combinations to its inputs (VID4-VIA0 pins).