Getting access to an I2C bus?

[Arwen]

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So regarding the schematics, what do the blue circles labeled with T## indicate? I’m not familiar with this symbol, and if it indicates a test point, I could reasonably tap into these. For example, the diagram at the bottom right of sheet 19 seems to indicate a pad that can be tapped into for SDA/SDL.

I think that T## are probably test points. Which if on the top of the board, may make access for other uses easier.

Those particular lines are used by the TrackPad & a small EEPROM used for the hardware kill switches. The keyboard controller chip, listed as U23 on that same page, is the parent controller. You probably want something off the main SoC. Trying to re-write the keyboard controller firmware to allow a third device on the TrackPad bus is likely to be pretty hard work. (There is someone who has dis-assembled much, if not all, the keyboard controller's firmware.)

One other note. Many SoCs have multi-function I/O pins, which allow setting their function by software at boot. As an example, some AMD CPUs allow changing pins from PCIe lanes to SATA ports. So even though this SoC, Rockchip RK3399 may have several I2C buses, you need to make sure they are both available as I2C buses, and not used by something critical. Like power control or LCD backlight.