The main power source of D821/D821 is provided by 2 chips which are PM8941 and PM8841. Since MSM8974 only
powers up normally when MSM8974 is properly powered therefore, PM8941 is the actual one to take look at.
Power ON sequence of D821/D821 is below.
1. PM8941 receives a poweron trigger (e.g. KPD_PWR_N press).
2. PM8941 poweron sequence begins (PBS sequence runs). The following take place:
- BMS OCV measurement occurs.
- EXT_REG_EN1 is asserted (VPH_PWR) to enable external supplies.
- PON_OUT is asserted (1.8 V) to enable PM8841.
- A timer starts to allow PM8841 to complete his sequence.
3. PM8841 receives poweron trigger (PON_1 and PS_HOLD are asserted).
4. PM8841 poweron sequence begins (PBS sequence runs). The following take place:
- S1 is turned ON (memory supply).
- S2 is turned ON (core supply).
5. PM8841 PBS sequence completes. Since PM8841 PS_HOLD is already high, the PM8841 sequence completes. PM8841
is on.
6. PBS sequence on PM8941 continues.
- The timer on PM8841 expires.
- The rest of the power sequence (regulators, clocks) are enabled in several steps.
7. The PM8941 PBS sequence completes. PM8941 PON module deasserts PON_RESET_N and sets PS_HOLD timer.
8. Once PON_RESET_N is deasserted, MSM comes out of reset. and PBL will assert PS_HOLD. PM8941 is on.
9. After FLCB, secondary poweron sequence takes place.