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.
...
Figure 7 shows the port for another output of ISO_LATCH_OUT taken from the SR latch. This controls the 2 N-channel MOSFETS in the HV circuit which in turn controls the P-channel MOSFETs connecting the pre-charge resistors to HV. When ISO_LATCH_OUT outputs a logic high, the first N-channel closes, which opens the second N-channel. This closes the paired P-channel MOSFETs disconnecting HV.
Fig 8. Pre-Charge Logic Sch - Isolated DC/DC Fig Fig 9. Motor Interface Pcb - DC/DC
...