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post #2664 of (permalink) Old 10-26-2018, 03:49 AM
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A note on voltage reporting on the Maximus XI-series, and voltage measurements in general. Like on Crosshair VII Hero, we've improved the SIO voltage readings for better accuracy. The CPU Core Voltage is now reading very close to the voltage the CPU is getting. Applied voltages and load-line levels are comparable to before, but not the software voltage readings.

When measuring voltages on a motherboard and large currents are involved, it's important to use accurate measurement points due to resistance/impedance present on the board. The power plane for example can be modeled as a resistor connected in series between the VRM output and the target device, together with Ohm's law, U = R*I. The larger the current, the larger the voltage drop. If you measure the voltage at the VRM output, for example at the inductor, you'll see a large difference compared to the voltage measured at the CPU socket MLCC. What we're interested in is what voltage the CPU die is actually getting after passing through the output filter, power plane, socket and package.

The Super I/O controller is used by BIOS/AiSuite/CPU-Z/HWInfo/AIDA64 for reading most voltages, temperatures and fan speeds. Its single-ended ADC inputs measure the input voltage referenced to the local SIO ground. With increased VRM output currents, this reading will be more and more inaccurate. We've added a circuit for measuring the differential CPU-die sense and converting it to a single-ended signal to the SIO. The CPU-die sense is accessible through two dedicated pins on the CPU, and are routed to the supply and ground on the CPU die itself. It's typically used by the CPU VRM controller for output voltage control.

Something to look out for is when you're seeing a voltage reported during load which is much higher than what you've set. It would require a negative load-line which is just not supported on any controller as far as I know. At 0 mOhm load-line, you get exactly what you set (Level 8 on M11).

The resulting difference can be seen in the graph below.

Prime95 26.6 12K FFTs

Manual Mode Voltage = Voltage set in BIOS
CPU-die Sense = DMM measurement of the CPU die-sense pins, "true" CPU Core Voltage
Socket MLCC = DMM measurement of the capacitors at the back of the socket
SIO (traditional) = Software reading with standard SIO voltage sense circuit
SIO (Maximus XI Hero) = Software reading with the improved SIO voltage sense circuit

Last edited by elmor; 10-26-2018 at 04:07 AM.
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