Engineering Bulletin

Power-On Reset (POR) Detection

EB413

MOTOROLA

5

EEPROM On Chip discusses protection for on-chip EEPROM where the

use of an external LVR device is highly recommended.

Power-On Reset (POR) Detection

While the LVR circuit can protect against a partial power loss, the POR

function can only indicate a total loss of power followed by power up. The

is reset correctly. On power up, a bit in one of the HC05 registers (often

the miscellaneous register if it exists) will be set to a logic high state. This

bit once cleared (by reading the bit) will remain in the logic low state even

if an external reset occurs and only be reset to a 1 when the power is

totally lost and returned.

System Power-Up Sequence

When power is applied to the system, it will normally take a finite period

to achieve the designed operating voltage. During this time, the MCU

may be required to maintain a specific state on its output pins and hold

off any execution of code until it is safe to commence running the

application.

Once power is applied to an HC05 device, the oscillator will start running

whether reset is held in the active state or not. Of course the oscillator

takes a short time to start up (4-MHz crystals take about 5 ms to start

and 32-kHz crystals take much longer, often as much as 500 ms), but

once the first couple of clocks have occurred, the active state (logic 0) of

the RESET pin normally configures the port lines to be inputs or to a

known state if they are only able to operate in output mode. In this way,

the external circuitry can assume a known state on the MCU pins just a

short time after power on but before the MCU reset is released.

Please remember that the port data registers are not usually initialized

by power on or reset and so a reset will normally leave the port data

unchanged and simply place the port in input mode.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com