When you are overclocking a CPU, the voltage is the most often tweaked variable. Higher voltage generally allows for higher clock speeds, but too much and you risk ruining the CPU. Due to a variety of factors, the actual applied CPU voltage (Vcore) can vary significantly from the value set in the EFI/BIOS. Additionally, varying loading conditions on the CPU can have an effect on the applied voltage. To remove this unknown from the overclocking process, Vcore can be measured directly from the motherboard’s VRMs. The output stage of the VRMs represents the Vcore applied to the CPU. The filter capacitors are the largest component of the output stage, making them easier to solder to. Looking at the P8P67 Pro Motherboard, we can see the filter capacitors are the group of tightly packed capacitors placed close to the CPU:
The negative terminal of the capacitor is marked by the dark stripe. Flipping the board over the legs of the filter capacitors are easily identified:
A wire can be easily soldered to any one of the positive capacitor terminals:
This signal voltage can be read by any number of measurement tools. Initially it was a measured using a digital multimeter, but a more permanent solution was desired. A cheap microcontroller filled this requirement, and was installed inside the computer, acting as a USB ADC. The multimeter was used to calibrate the ADC, as it was discovered that the ADC had a +110mV offset. This offset was attributed to the use of the imprecise internal 5V rail as a reference voltage. In the future a more precise voltage reference IC combined with the ARef pin on the microcontroller should allow for more accurate measurement.
To finish the project, a small applet was written to conveniently display the voltage reading to the user in the menu bar.