MITO 8M SOM/MITO 8M Hardware/Power and Reset/Reset scheme and control signals
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Contents
Reset scheme and control signalsEdit
The following picture shows the simplified block diagram of reset scheme and voltage monitoring.
NVCC_VSNVSEdit
Some signals that are related to reset circuitry are pulled-up to NVCC_VSNVS (aka 3.3VIN).
Hence it is recommended that system designer takes into account these factors in order to properly manage these signals at carrier board level.
EXT_RESETEdit
EXT_RESET is internally pulled-up with a 90kΩ to NVCC_VSNVS. Connect EXT_RESET signal to GND (for example with a button or an open-collector circuit) causes the internal supervisor IC to assert its RESETn output.
This will power down the VDD_SOC_0V9 and make PMIC_ON pulled low by PG (Power Good) of VDD_SOC_0V9 regulator.
Since PMIC_ON serves as the enable signal of PMIC and other discrete power supply chips, all the power supplies except for the SNVS domain will be OFF.
The RESETn will keep asserted for 270 ms after EXT_RESET is released, thus providing enough time for the power supplies to be completely powered down. During this time, the POR_B driven by the PMIC will also keep asserted (low).
After RESETn is released, the power supplies will start to ramp up in defined sequence. When all the power supplies have reached their operating voltages, POR_B will be de-asserted, and the CPU may begin booting from reset.
CPU_PORnEdit
PMIC can assert this active-low signal. Other internal IC, such as ethernet PHY or boot memory devices, could be connected to this signal. This guarantees it is in a known state when reset signal is released.
CPU_ONOFFEdit
CPU_ONOFF is internally pulled-up with a 100kΩ to NVCC_VSNVS. This input signal is connected directly to the ONOFF input of the CPU.
BOARD_PGOODEdit
BOARD_PGOOD is directly related to the internal NVCC_3V3 rail (I/O pins supply) presence and must be used as power enable for all the electronics on MITO 8M carrier board.
When the I/O pins power rail on MITO 8M is not ready (BOARD_PGOOD low) all the integrated circuits connected to the CPU must be powered off in order to avoid back-powering or other issue related to a wrong power-up sequence.
BOOT_MODE_SELEdit
BOOT_MODE_SEL is internally pulled-up with a 100kΩ to NVCC_3V3.
When connected to GND, select the external microSD as the boot device.
Handling CPU-initiated software resetEdit
By default, MX8 processor does not assert any external signal when it initiates a software reset sequence. Also default software reset implementation does not guarantee that all processor registers are reset properly.
For these reasons it is strongly recommended to use a different approach that, in combination with the use of a processor's watchdog timer (WDT), provides a full hardware reset in case a software reset is issued.
This technique is implemented in DESK-MX8. At the software level, U-Boot and Linux kernel software reset routines make use of the processor's WDT to assert the WDOG_B signal. This signal in turn is routed to the GPIO1_IO02 pad. At the hardware level, this signal is AC-coupled to the master reset pin of the internal supervisor IC. It acts as a complete hardware reset by the assertion of its RESETn output (on the same way of EXT_RESET pin).