MITO 8M Nano SOM/MITO 8M Nano Hardware/Power and Reset/Power Supply Unit (PSU) and recommended power-up sequence

History
Issue Date	Notes
2020/12/29	First release

Power Supply Unit (PSU) and recommended power-up sequence

Implementing correct power-up sequence for iMX8M Mini/Nano processors is not a trivial task because several power rails are involved.

MITO 8M Mini SOM simplifies this task by embedding all the needed circuitry. The following picture shows a simplified block diagram of PSU/voltage monitoring circuitry:

The PSU is composed of two main blocks:

• power management integrated circuit
• additional generic power management circuitry that completes PMIC functionalities

The PSU:

• generates the proper power-up sequence required by the SOC processor and surrounding memories and peripherals
• synchronizes the powering up of carrier board in order to prevent back power

Power-up sequence

The typical power-up sequence is the following:

1. 3.3VIN main power supply rail is powered
2. SNVS domain signals are pulled-up (unless carrier board circuitry keeps this signal low for any reason)
3. CPU_PORn (active-low) is driven low by PMIC
4. RTC_RESET_B are internally released after 10ms
5. PMIC initiates power-up sequence needed by iMX8M Mini/Nano processor
6. BOARD_PGOOD goes up when all CPU I/O power rail is ready
7. CPU_PORn is deasserted after the last regulator to bring the processor out of reset

Note on BOARD_PGOOD usage

BOARD_PGOOD is generally used on carrier board to drive loads such as DC/DC enable inputs or switch on/off control signals.

Depending on the kind of such loads, BOARD_PGOOD might not be able to drive them properly because it has a 20mA output current absolute maximum rating.

In these cases a simple 2-input AND port can be used to address this issue. The following picture depicts a principle schematic showing this solution.

Additionally, we suggest using an IC with Schmitt trigger input ports.

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