Engineering Bulletin

Device Controls Its Own Power Off

EB413

MOTOROLA

9

The MCU can perform a number of read and writes to EEPROM or RAM

which could be independently backed up and a good practice is to place

the MCU in low-power stop mode, ensuring first that all outputs are in a

low-current state. This way, the device is in a secure state and ready for

reset to be applied with no risk of a partially completed saving of system

data.

Stop mode is ideal for power down as the port lines are all in a known

state and there is a reset delay time (4064 cycles in most devices) before

supply passes below the minimum specified for the current clock

frequency.

Device Controls Its Own Power Off

An application that does not have a battery backup but which is required

to save power may switch itself off by activating an external control of the

power supply. Clearly as the power is being switched off, the MCU could

see external pins change state and interpret the signals wrongly. If care

is not taken to place the MCU in a dormant state before the power supply

voltage drops below the safe level for MCU operation, the MCU could

run away and cause a power on to be initiated again.

Partial Power Loss

The only way to protect against failure of the application during partial

power loss is to install a lower voltage detection and reset circuit. Some

MCU devices have these built into the design and these will force the

RESET pin low (active state) to reset the MCU and any other external

devices that use the reset signal.

Freescale Semiconductor, Inc.

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Go to: www.freescale.com