As a start to our investigation of the Pine64 I have added some links on this page. Hopefully as we progress more information will surface and be included.

fet-logic-level-shifter-schematic-for-i2cBi-directional level shifter, connects two different voltage sections of an I2C bus system

* calculated by taking the second letter of the pin number, converting to its alphabetic number equivalent, subtracting one, multiplying by 32, and then adding the numerical part of the pin number. e.g. PC13 = (C-1)*32+13 = (3-1)*32+13 = 77. See this thread.

C=3rd bank of GPIO connections.
13=GPIO in that specific bank.