In the past blog, I discussed the onewire circuit. Here I’d like to talk about a specific power rail, that powers the onewire circuit, and its importance.
The PP3V42_G3H power rail is one of many power rails on a Macbook Pro logic board. ignore the confusing nonsense lik PPV and all that – PP3V42_G3H means 3.42v.
PP3V42_G3H is an important rail because it starts before all the others. It is the only rail that can work off of two different power lines – it can be created by PPBUS_G3H from the battery, when working off of the battery, or the 18.5v rail from the charger when the charger is plugged in.
The onewire circuit, which is the circuit responsible for telling the SMC that the charger is plugged in and allowing the charger to work, runs off of the PP3V42-G3H rail. So obviously it is important that this rail start first. This rail does not have to be on first when using the BATTERY to create PPBUS_G3H, because the battery is always present without onewire protection. However, the PPBUS_G3H rail being created by the charger needs the onewire protection to be active, so thus needs PP3V42_G3H to be present!
Even if your PPBUS_G3H circuit is fully functional with no issues, it will not turn on without the onewire circuit being activated, and the onewire circuit will not be activated without PP3V42_G3H. So, if you have no PPBUS_G3H, stop troubleshooting PPBUS_G3H if you have not yet checked PP3V42_G3H!
PP3V42_G3H is the FIRST rail to check on any system if it has no power. If you have a green light on the charger, you can cross PP3V42_G3H off the list of rails to check as you know it is already working. The green light would not turn on if the onewire circuit were not working, and the onewire circuit would not be working without PP3V42_G3H.
Here is a video on the subject.