The PMU Reference System consists of the following:
- Bandgap (BGAP) and BGAP coarse level checker
- LDO and LDO coarse lever checker
- Supply (VDDA18 and VDD) coarse level checker
- Power on Sequencer and reset generation circuit
- Level Shifters
The REFSYS generates a reset based on PORz and the stability of external voltage rails and provide outputs in three supply domains, VDD, VDDA18, and VDDA33, which are used in different domain logic.
The power up sequence is shown in
AM263x Power Up Sequence and details for the sequence from
the PMU REFSYS reset are shown below:
- The device needs to be
supplied with 3.3-V (VDDS33/VDDA33)and 1.2-V (VDD/VDDARx) supplies externally. As
the external power supplies ramps up, PORz should be asserted low externally
until the supplies are stable. During this time the SOC will be held in a
reset state.
- The device can rely on an
external supervisor to guarantee that the device external supplies (3.3V,
1.2V) are valid
before releasing the reset to the device (PORz de-asserted). An inverted
signal HHV (High Heating Value) is generated internally from PORz to enable
isolation logic during power up for the IOs and PLL logic.
- When the PORz is de-asserted,
i.e PORz transitions from low to High, the PMU will start its sequence by
enabling the Bandgap (BGAP) Voltage and Current reference voltage
generators.
- While the reference voltages
are settling, BGAP voltage is monitored by a coarse level checker and the
BGAP ready signal is generated when the voltage stabilizes. This signal
enables LDO and supply coarse checkers inside the reference system.
- Once the VDD and VDDA18
supplies are deemed ready, the internal signal VDD_OK is asserted. This will
release the reset going to SOC.
- A rising edge on VDD_OK
enables the RC Oscillator (provides the 10MHz RCCLK) and the crystal clock
(XTALCLK).
- Reset and clock
control module checks the presence of RCCLK for 16 clock cycle
before enabling it to rest of the SOC.
- In order to ensure
stability of XTALCLK, a 2ms counter using RCCLK is enabled. Once
XTALCLK is stabilized, internal reset to CPU is released.
- After the RCCLK has been
enabled to the rest of the system and XTALCLK is stabalized, eFuse data is
read and trim values are applied to the analog domain. The internal signal
hhv_mask is asserted to gate the influence of comparator checks and prevent
the SOC from going into reset due to changing trim values. Once the trim is
completed, hhv_mask is de-asserted.
- The externally
applied PORz is not affected by the hhv_mask signal and if PORz is
asserted during trim, the device goes back to reset state.
The trim values provided in the eFuse chain are enabled by a PORz de-assertion. The status of coarse monitors on supplies VDDA18, VDD12, 1.8V LDO, and BGAP can be monitored through TOP_CTRL.PMU_COARSE_STAT register during runtime.