Setting precise voltage offsets with over_voltage

Question

On the BCM2711, the base VCore was greatly lowered from previous processors, though the over_voltage setting still uses 0.025V increments. On my Pi 4, vcgencmd measure_volts returns 0.8563V when not overvolted and 1.0063V at over_voltage=6, the highest allowed without tripping the warranty bit.

However, it seems the Raspberry Pi 400, which uses a newer stepping of the BCM2711, ships with over_voltage=0x11170 by default. I have not seen hexadecimal values for this setting documented anywhere, but as far as I can tell, this corresponds to an offset of 70,000 microvolts (0.07V), just shy of over_voltage=3 (0.075V).

Given that over_voltage=6 corresponds to a 0.15V offset, can I safely use an arbitrary value between 0x0 and 0x249F0 for precise voltage offsets?

Answer 1

Well, I just tried over_voltage=0xEA60 (+0.060V offset) with a 1.7 GHz overclock and it works as expected:

pi@azazel:~ $ vcgencmd measure_volts
volt=0.9163V
pi@azazel:~ $ vcgencmd measure_clock arm
frequency(48)=1700419968

So the answer is yes, you can enter the voltage offset in microvolts by specifying a hexadecimal value. The warranty bit was not tripped (the revision number is still c03112).

I have not found how to enter a negative offset this way and I'm not willing to experiment any further at this point. Any comments adding more information would be appreciated.