Z8F1680SH020SG Datasheet, PDF (341/412 Pages) Zilog, Inc. – High-Performance 8-Bit Microcontrollers

Z8 Encore! XP^® F1680 Series
Product Specification
316
Table 168. Oscillator Configuration and Selection
Clock Source Characteristics
Required Setup
Internal Precision
RC Oscillator
• Selectable frequency 0.0432MHz,
0.0864 MHz, 0.3456 MHz, 
0.6912 MHz, 1.3824 MHz, 
2.7648 MHz, 5.5296 MHz and
11.0592 MHz
• ± 4% accuracy when trimmed
• No external components required
• Unlock and write Oscillator Control Register
(OSCCTL0) to enable and select oscillator
frequency.
External Crystal/
Resonator
• 32kHz to 20MHz
• Very high accuracy (dependent on
crystal or resonator used)
• External components required
• Configure Flash option bits for correct
external oscillator mode
• Unlock and write OSCCTL0 to enable crystal
oscillator, wait for it to stabilize and select as
system clock (if the EXTL_AO option bit has
been deasserted, 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 OSCCTL0 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 OSCCTL0 to select
external system clock
• Apply external clock signal to GPIO
Internal WDT
Oscillator
• 10kHz nominal
• ± 40% 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 Register
(OSCCTL0) to enable and select oscillator
Caution: Unintentional accesses to the Oscillator Control Register can actually stop the chip by
switching to a nonfunctioning oscillator. To prevent this condition, the oscillator control
block employs a register unlocking/locking scheme.
24.1.1.1. OSC Control Register Unlocking/Locking
To write to the Oscillator Control Register (OSCCTL0 and OSCCTL1), unlock it by mak-
ing two writes to the OSCCTLx Register with the value E7H followed by the value 18H. A
third write to the OSCCTLx Register changes the value of the actual register and returns it
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 con-
PS025015-1212
PRELIMINARY
Oscillator Control