Quote from: sunk818 on October 27, 2017, 02:45:59 PM

Thanks for the instructions. I touched on each side of the capacitor. At rest, all three are 0.5 (furthest away from USB), and 0.75. Running, I'm seeing 0.62, 0.63.

Good work. That suggests your trimpot is still in the default position as configured by sidehack during the initial test. At that voltage it should reliably run at 100 MHz, and possibly a bit higher.

Quote from: sunk818 on October 27, 2017, 02:45:59 PM

I set multimeter on DCV 20. Centech: https://manuals.harborfreight.com/manuals/98000-98999/98025.pdf - page 5

Good choice.

Quote from: sunk818 on October 27, 2017, 02:45:59 PM

Even if I change the frequency from 100, 150, 200, or 250 Mhz the reading is always 0.62 for both. So, this value only changes if I turn the trim pot clockwise? What is the role of voltage? Allow 2Pac to run more without HW errors in the higher Mhz?

Yes, the trimpot configures the voltage regulator stage on the PCB, and the voltage regulator controls the voltage supplied to each ASIC. You move the pot, the voltage changes, and this happens irrespective of the clockspeed you're running at.

At a given voltage, increasing the clock speed will increase the heat output of the chip, and put more load on your cooling solution. My understanding is that each time a transistor in a high density integrated circuit like a CPU switches from one state to the other, it loses or wastes a bit of electrical energy as heat. The more times per second you have the switches oscillating (ie. the higher the clock speed) the more times per second the transistors have to lose a unit of heat energy. So, running at a higher voltage but low clock speed will put less strain on the heat sink than running at that same voltage and a higher clockspeed.
Running at low clockspeed and low voltage will produce less waste heat than running at low speed with high voltage, so we choose the former condition if we can get reliable operation that way.

Generally speaking, CPUs require a lower supply voltage to run at a lower clockspeed. Modern PCs have power saving methods that automatically ramp down your computer's clock speed and voltage when the OS detects that the CPU isn't doing much. A bit like your car engine idling at traffic lights; no point running at redline if you're not doing any work with the engine.

When the workload increases, your PC will ramp up the CPU voltage and frequency, producing more waste heat, and consuming more power. The voltage is increased because operating at a higher clock speed needs a higher voltage supply to ensure reliable switching of the transistors in the CPU. I think this is due to inherent resistance and capacitance in the chip's die, but it's been a long time since I studied this and I am a barely educated buffoon in this discipline.

Your 2Pac stick does not have space age fancy automated voltage control systems which would increase cost and complexity. It does have a trimpot that lets you alter the ASIC voltage between a safe minimum and maximum range, letting you tune each stick for the minimum voltage at which it will stably run at your chosen clock speed. You need to make sure you can keep the chips cool at the voltage and clock speed you've chosen, because a hot CPU wears out more quickly, and a too hot CPU will just fry its delicate electronic circuitry.

Armed with your multimeter, a thermometer, and cgminer logs, you can determine the minimum stable voltage that your 2Pac will continuously and stably run at. Any higher voltage and you're wasting power and have to deal with more heat. Any lower voltage and your stick won't run continuously without errors.

It's possible to run a stick, and dial down the voltage until cgminer starts reporting hardware errors. (HW:0 in the cgminer status line will start increasing to HW:1 and above.) When you see this, you increase the voltage a bit until the HW errors stop incrementing. Then run it for a few hours and ensure you still have no additional HW errors, and you'll probably have found your stable voltage for that clock speed.