Z8F021ASH020SC datasheet(200/276 Pages) ZILOG

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PS022815-0206

Oscillator Control

Z8 Encore! XP® 4K Series

Product Specification

181

Unintentional accesses to the oscillator control register can actually stop the chip by

switching to a non-functioning oscillator. To prevent this condition, the oscillator con-

trol block employs a register unlocking/locking scheme.

OSC Control Register Unlocking/Locking

To write the oscillator control register, unlock it by making two writes to the OSCCTL

changes the value of the actual register and returns the register to a locked state. Any other

sequence of oscillator control register writes has no effect. The values written to unlock

the register must be ordered correctly, but are not necessarily consecutive. It is possible to

write to or read from other registers within the unlocking/locking operation.

Table 111. Oscillator Configuration and Selection

Clock Source

Characteristics

Required Setup

Internal Precision

RC Oscillator

• 32.8 KHz or 5.53 MHz

• High accuracy when trimmed

• No external components required

• Unlock and write Oscillator Control

select oscillator at either 5.53 MHz or

32.8 KHz

External Crystal/

Resonator

• 32 KHz to 20 MHz

• Very high accuracy (dependent on

crystal or resonator used)

• Requires external components

• Configure Flash option bits for correct

external oscillator mode

• Unlock and write OSCCTL to enable

crystal oscillator, wait for it to stabilize

and select as system clock (if the

XTLDIS option bit has been de-

asserted, no waiting is required)

External RC

Oscillator

• 32 KHz to 4 MHz

• Accuracy dependent on external

components

• Configure Flash option bits for correct

external oscillator mode

• Unlock and write OSCCTL to enable

crystal oscillator and select as system

clock

External Clock

Drive

• 0 to 20 MHz

• Accuracy dependent on external clock

source

• Write GPIO registers to configure PB3

pin for external clock function

• Unlock and write OSCCTL to select

external system clock

• Apply external clock signal to GPIO

Internal Watchdog

Timer Oscillator

• 10 KHz nominal

• Low accuracy; no external components

required

• Low power consumption

• Enable WDT if not enabled and wait

until WDT Oscillator is operating.

• Unlock and write Oscillator Control

select oscillator

Caution:

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