7.10.2.4 Isolated MCU and Main Domains Power- Up Sequencing

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, then 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.

A. T1Time stamp markers:

• T0 – All 3.3-V voltages start supply ramp-up to V_{OPR MIN}. (0 ms)
• T1 – All 1.8-V voltages start supply ramp-up to V_{OPR MIN}. (2 ms)
• T2 – All core voltages start supply ramp-up to V_{OPR MIN}. (3 ms)
• T3 – All RAM array voltages start supply ramp-up to V_{OPR MIN}. (4 ms)
• T4 – OSC1 is stable and PORz/MCU_PORz are de-asserted to release processor from reset. (13 ms)

B. Any MCU or Main dual voltage IO supplies (VDDSHVn_MCU or VDDSHVn) being supplied by 3.3 V to support 3.3-V 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.

C. Any MCU or Main dual voltage IO supplies (VDDSHVn_MCU or VDDSHVn) being supplied by 1.8 V to support 1.8-V digital interfaces. When eMMC memories are used, Main 1.8-V supplies could have delayed start times that aligns to T3 due to PDN designs grouping supplies with VDD_MMC0.

D. VDDSHV5 supports MMC1 signaling for SD memory cards. If compliant UHS-I SD card operation is needed, then an independent, dual voltage (3.3 V/1.8 V) power source and rail are required. The start of ramp-up to 3.3 V will be same as other 3.3-V domains as shown. If SD card is not needed or standard data rates with fixed 3.3-V operation is acceptable, then supply can be grouped with digital IO 3.3-V power rail. If a SD card is capable of operating with fixed 1.8 V, then supply can be grouped with digital IO 1.8-V power rail.

E. VDDA_3P3_USB is 3.3-V 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 3.3 V will be same as other 3.3-V domains as shown. If USB interface is not needed or data bit errors can be tolerated, then supply can be grouped with 3.3-V digital IO power rail either directly or through a supply filter.

F. VDDA_1P8_<clk/pll/ana> are 1.8-V analog domains supporting clock oscillator, PLL and analog circuitry needing a low noise supply for optimal performance. It is not recommended to combine digital VDDSHVn_MCU and VDDSHVn IO domains since high frequency switching noise could negatively impact jitter performance of clock, PLL and DLL signals. Combining analog VDDA_1p8_<phy> domains should be avoided but if grouped, then in-line ferrite bead supply filtering is required.

G. VDDA_1P8_<phy> are 1.8-V analog domains 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 domains can be grouped with digital IO 1.8-V power rail either directly or through an in-line supply filter is allowed.

H. VDDA_0P8_<dll/pll> are 0.8-V analog domains supporting PLL and DLL circuitry needing a low noise supply for optimal performance. It is not recommended to combine these domains with any other 0.8-V domains since high frequency switching noise could negatively impact jitter performance of PLL and DLL signals.

I. 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 0.8-V VDD_CORE at time stamp T2 or 0.85-V RAM array domains (VDDAR_xxx) at time stamp T3.

J. 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.

K. Minimum elapsed time from crystal oscillator circuitry being energized (VDDA_OSC1 at T1) until stable clock frequency is reached depends upon on crystal oscillator, capacitor parameters and PCB parasitic values. A conservative 10 ms 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.

Figure 7-5 Isolated MCU and Main Domains, Primary Power-Up Sequence