The precharge circuit runs entirely based on hardware (i.e. no firmware is required).
...
The first IC on the left of figure 5 is an optoisolator (or optocoupler) used to interface between the HV and LV sides of the board. You can see this in figure 6, the optoisolator spans across a gap where the HV side (top) is isolated from the LV side (bottom). Following the latch previously mentioned, when ISO_LATCH_OUT outputs a logic high, the MOSFET is closed connecting the cathode (CAT) pin to ground. This connects the circuit from the anode (AN) completing the circuit and powering the GaAsP LED inside the IC. When this happens, the Vo will output a logic high and close the MOSFET to the contactor driver. This signals the relay connecting HV directly to the motor controllers to close.
...
The isolated side of the board gets power through the isolated DC/DC, which connects the isolated and non-isolated side. The IC can be seen in the PCB layout as U9, spanning to the isolated side.
I need to update/finish this eventually...