Isolated MCU and Main voltage domains enable an SoC’s MCU and Main processor sub-systems to operate independently. There are 2 reasons an SoC’s PDN design may need to support independent MCU and Main processor functionality. First is to provide flexibility to enable SoC low power modes that can significant reduce SoC power dissipation when processor operations are not needed. Second is to enable robustness to gain freedom from interference (FFI) of a single fault impacting both MCU and Main processor sub-systems which is especially beneficial if using the SoC’s MCU as the system safety monitoring processor. The number of additional PDN power rails needed is dependent upon number of different MCU IO signaling voltage levels. If only 1.8V IO signaling is used, the only 2 additional power rails could be required. If both 1.8 and 3.3V IO signaling is desired, then 4 additional power rails could be needed. Table 8-2 in Section 8.1, Power Supply Mapping captures recommended device power supplies to power rail mapping summary.

Figure 6-5 Isolated MCU and Main Domains, Primary Power-Up Sequence

1. Time Stamp Markers

   T0 – 3.3V voltages start ramp-up to V_{OPR MIN}. (0ms)

   T1 – 1.8V voltages startramp-up to V_{OPR MIN}. (2ms)

   T2 – Low voltage core supplies start ramp-up to V_{OPR MIN}. (3ms)

   T3 – Low voltage RAM array voltages start ramp-up to V_{OPR MIN}. (4ms)

   T4 – OSC1 is stable and PORz/MCU_PORz are de-asserted to release processor from reset. (13ms)

2. Any MCU or Main dual voltage IO supplies (VDDSHVn_MCU or VDDSHVn) being supplied by 3.3V to support 3.3V digital interfaces. A few supplies could have varying start times between T0 to T1 due to PDN designs using different power resources with varying turn-on & ramp-up time delays.
3. Any MCU or Main dual voltage IO supplies (VDDSHVn_MCU or VDDSHVn) being supplied by 1.8V to support 1.8V digital interfaces. When eMMC memories are used, Main 1.8V supplies could have delayed start times that aligns to T3 due to PDN designs grouping supplies with VDD_MMC0.
4. VDDSHV5 supports MMC1 signaling for SD memory cards. If compliant UHS-I SD card operation is needed, then an independent, dual voltage (3.3V/1.8V) power source and rail are required. The start of ramp-up to 3.3V will be same as other 3.3V domains as shown. If SD card is not needed or standard data rates with fixed 3.3V operation is acceptable, then supply can be grouped with digital IO 3.3V power rail. If a SD card is capable of operating with fixed 1.8V, then supply can be grouped with digital IO 1.8V power rail.
5. VDDA_3P3_USB is 3.3V analog supply used for USB 2.0 differential interface signaling. A low noise, analog supply is recommended to provide best signal integrity for USB data eye mask compliance. The start of ramp-up to 3.3V will be same as other 3.3V domains as shown. If USB interface is not needed or data bit errors can be tolerated, then supply can be grouped with 3.3V digital IO power rail either directly or through a supply filter.
6. VDDA_1P8_<phy> are 1.8V analog supplies supporting multiple serial PHY interfaces. A low noise, analog supply is recommended to provide best signal integrity, interface performance and spec compliance. If any of these interfaces are not needed, data bit errors or non-compliant operation can be tolerated, then supplies can be grouped with digital IO 1.8V power rail either directly or through an in-line supply filter is allowed.
7. VDD_MMC0 is 1.8V digital supply supporting MMC0 signaling for eMMC interface and must ramp up at time stamp T3. Any MCU or Main dual voltage IO operating at 1.8V can be grouped with VDD_MMC0 into a common power rail with a ramp-up at time stamp T3. If MMC0 or eMMC0 interface is not needed, then domain can be grouped with digital IO 1.8V power rail with ramp-up at time stamp T1.
8. VDD_MCU is a digital voltage supply with a wide operational voltage range and power sequencing flexibility, enabling it to be grouped and ramped-up with either 0.8V VDD_CORE at time stamp T2 or 0.85V RAM array domains (VDDAR_xxx) at time stamp T3.
9. VDDA_1P8_<clk/pll/ana> are 1.8V analog supplies supporting clock oscillator, PLL and analog circuitry needing a low noise supply for optimal performance. It is not recommended to combine analog VDDA_1P8_<phy> domains or digital VDDSHVn_MCU and VDDSHVn IO domains since high frequency switching noise could negatively impact jitter performance of clock, PLL and DLL signals.
10. VDDA_0P8_<dll/pll> are 0.8V analog supplies supporting PLL and DLL circuitry needing a low noise supply for optimal performance. It is not recommended to combine these domains with any other 0.8V domains since high frequency switching noise could negatively impact jitter performance of PLL and DLL signals.
11. Minimum set-up and hold times shown with respect to MCU_PORz and PORz asserting high to latch MCU_BOOTMODEn (referenced to MCU_VDDSHV0) and BOOTMODEn (reference to VDDSHV2) settings into registers during power up sequence.
12. Minimum elapsed time from crystal oscillator circuitry being energized (VDDS_OSC1 at T1) until stable clock frequency is reached depends upon on crystal oscillator, capacitor parameters and PCB parasitic values. A conservative 10ms elapsed time defined by (T4 – T1) time stamps is shown. This could be reduced depending upon customer’s clock circuit (that is, crystal oscillator or clock generator) and PCB designs.