Z8 Encore! XP® F08xA Series
Product Specification
161
Table 101. Oscillator Configuration and Selection
Clock Source
Characteristics
Required Setup
Internal Precision
RC Oscillator
• 32.8KHz or 5.53MHz
• ± 4% accuracy when trimmed
• No external components required
• Unlock and write Oscillator Control
Register (OSCCTL) to enable and
select oscillator at either 5.53MHz or
32.8KHz
External Crystal/
Resonator
• 32KHz to 20MHz
• Configure Flash option bits for correct
external oscillator mode
• Very high accuracy (dependent on
crystal or resonator used)
• Requires external components
• 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
• 32KHz to 4MHz
• 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 20MHz
• 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
• 10KHz nominal
• ± 40% accuracy; no external
components required
• Enable WDT if not enabled and wait
until WDT Oscillator is operating.
• Unlock and write Oscillator Control
Register (OSCCTL) to enable and
select oscillator
• Low power consumption
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.
Caution:
OSC Control Register Unlocking/Locking
To write the oscillator control register, unlock it by making two writes to the OSCCTL
register with the values E7Hfollowed by 18H. A third write to the OSCCTL register
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.
PS024705-0405
P R E L I M I N A R Y
Oscillator Control
ZILOG [ ZILOG, INC. ]
