Engineering Bulletin

EB413

10

MOTOROLA

A power-on reset (POR) bit will not usually register a partial power loss

voltage dropped before reset occurred.

Once the reset signal is applied, the MCU pins will mostly go to high-

impedance state and the oscillator will stop running, thus making the

system power consumption lower and this can allow a more graceful

power down of the system.

HC05 Clock Source

Different types and frequencies of oscillator circuits take differing lengths

of time to start up and stabilize. A 4-MHz crystal takes about 5 ms while

a 4-MHz ceramic resonator will usually start up very quickly (normally

less than 1 ms). As the frequency drops, the time taken to start up and

stabilize increases. A 32-kHz crystal can take as long as 500 ms to start

up. The internal clock count delay accommodates these differences

since it simply counts the oscillations and it is assumed that 4064 clocks

is enough under most conditions. If a different amount of time is required

then of course an external reset control is needed.

An RC (resistor-capacitor) option rather than external crystal or

resonator has much poorer accuracy of oscillator frequency but has

virtually instantaneous start up and is very cheap.

Some applications provide an external clock source rather than use the

crystal oscillator option. In these cases the clock source may be present

but the MCU may be in reset. When the reset signal is released the

application may be able to operate more quickly and so some HC05

devices have options for the delay time for system operation after reset

is released. Since 4064 cycles is 2 ms with a 4-MHz clock, shorter delays

are often required.

Freescale Semiconductor, Inc.

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