Serial cable voltage should actually be 3.0V, with mod!

[jhiesey]

I have two thoughts here.

Firstly, the 5V cable adapter in the Pine store is wrong for the PBP because it is the wrong voltage and the wrong chipset.

Secondly, I have doubts the correct voltage is 3.0V   Unlike 3.3 and 5V,  3V isn't a standard voltage for logic.  Does anyone even make a 3.0V adapter?

I agree that 3V is unusual, and nonstandard logic levels are almost never exposed on ports a user can access. But for some reason the RK3399 supports a choice between 3.0V and 1.8V (which is much more common but also not usually user-exposed) on the relevant pins, and the PBP engineers decided to use 3.0V.

Normally products that use nonstandard voltages internally would have logic level translators built in to give something standard externally, but I expect since the serial port is only a development/debug feature that was omitted to save on cost. So ideally Pine64 would make a custom cable to go with it.

That being said, given the series 100 ohm resistors in the PBP, using a 3.3V cable is unlikely to cause a problem in practice. So recommending people use 3.3V cables is probably OK.