Z8F0113HJ005EG Datasheet, PDF (186/245 Pages) Zilog, Inc. – High-Performance 8-Bit Microcontrollers

Z8 Encore! XP^® F0823 Series
Product Specification
171
conditions, do not enable the clock failure circuitry (POFEN must be deasserted in the
OSCCTL Register).
Watchdog Timer Failure
In the event of a Watchdog Timer oscillator failure, a similar non-maskable interrupt-like
event is issued. This event does not trigger an attendant clock switch-over, but alerts the
CPU of the failure. After a Watchdog Timer failure, it is no longer possible to detect a pri-
mary oscillator failure. The failure detection circuitry does not function if the Watchdog
Timer is used as the primary oscillator or if the Watchdog Timer oscillator has been dis-
abled. For either of these cases, it is necessary to disable the detection circuitry by deas-
serting the WDFEN bit of the OSCCTL Register.
The Watchdog Timer oscillator failure detection circuit counts system clocks while
searching for a Watchdog Timer clock. The logic counts 8004 system clock cycles before
determining 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 F0823 Series device ceases functioning
and can only be recovered by Power-On Reset.
Oscillator Control Register Definitions
The following section provides the bit definitions for the Oscillator Control Register.
Oscillator Control Register
The Oscillator Control Register (OSCCTL) enables/disables the various oscillator circuits,
enables/disables the failure detection/recovery circuitry and selects the primary oscillator,
which becomes the system clock.
The Oscillator Control Register must be unlocked before writing. Writing the two step
sequence E7H followed by 18H to the Oscillator Control Register unlocks it. The register
is locked at successful completion of a register write to the OSCCTL.
PS024315-1011
P R E L I M I N A R Y Oscillator Control Register Definitions