Z8F1680SH020SG Datasheet, PDF (343/412 Pages) Zilog, Inc. – High-Performance 8-Bit Microcontrollers

Z8 Encore! XP^® F1680 Series
Product Specification
318
The Watchdog Timer oscillator failure-detection circuit counts system clocks while look-
ing for a Watchdog Timer clock. The logic counts 8004 system clock cycles before deter-
mining that a failure has occurred. The system clock rate determines the speed at which
the Watchdog Timer failure can be detected. A very slow system clock results in very slow
detection times.
Caution: It is possible to disable the clock failure detection circuitry as well as all functioning
clock sources. In this case, the Z8 Encore! XP F1680 Series device ceases functioning
and can only be recovered by Power-on reset.
24.2. Peripheral Clock
The peripheral clock is based on a low-frequency/low-power 32 kHz secondary oscillator
that can be used with an external watch crystal. The peripheral clock is only available for
driving Timer and associated noise filter operation. It is not supported for other
peripherals. The dedicated peripheral clock source allows Timer operation when the
device is in STOP Mode.
Table 169 summarizes peripheral clock source features and usage.
Table 169. Peripheral Clock Source and Usage
Peripheral Clock Source
Secondary Oscillator
Characteristics
• Optimized for use with a
32 kHz Watch Crystal
• Very high accuracy
• Dedicated XTAL pins
• No external components
Required Setup
• Unlock and write OSCCTL1 to enable the
secondary oscillator
• Select the peripheral clock at the timer
clock source in the TxCTL2 Register
24.3. Oscillator Control Register Definitions
The Oscillator Control registers enable and disable the various oscillator circuits, enable
and disable the failure detection and recovery circuitry, and select the primary oscillator,
which becomes the system clock.
24.3.1. Oscillator Control 0 Register
The Oscillator Control 0 Register (OSCCTL0) must be unlocked before writing. Unlock
the Oscillator Control 0 Register by writing the two-step sequence of E7H followed by
PS025015-1212
PRELIMINARY
Oscillator Control