Z8 Microcontrollers
Reset—Watch-Dog Timer ZiLOG
4-8 UM001601-0803
Clock Source for WDT (D4). This bit determines which oscil-
lator source is used to clock the internal POR and WDT counter
chain. If the bit is a 1, the internal RC oscillator is bypassed and
the POR and WDT clock source is driven from the external pin,
XTAL1. The default configuration of this bit is 0, which selects
the internal RC oscillator.
Bits 5, 6 and 7. These bits are reserved.
V
CC
Voltage Comparator. An on-board voltage comparator
checks that V
CC
is at the required level to insure correct opera-
tion of the device. Reset is globally driven if V
CC
is below the
specified voltage. This feature is available in select ROM Z8 de
-
vices. See the device product specification for feature availabil-
ity and operating range.
4.4 POWER-ON-RESET (POR)
A timer circuit clocked by a dedicated on-board RC oscillator is
used for the Power-On Reset (POR) timer (T
POR
) function. The
POR time allows V
CC
and the oscillator circuit to stabilize before
instruction execution begins.
The POR timer circuit is a one-shot timer triggered by one of
three conditions:
1. Power fail to Power OK status (cold start).
2. STOP-Mode Recovery (if bit 5 of SMR=1).
3. WDT timeout.
The POR time is specified as TPOR. On Z8 devices that feature
a Stop-Mode Recovery register (SMR), bit 5 selects whether the
POR timer is used after Stop-Mode Recovery or by-passed. If bit
D5 = 1 then the POR timer is used. If bit 5 = 0 then the POR timer
is by-passed. In this case, the Stop-Mode Recovery source must
be held in the recovery state for 5 T
P
C or 5 crystal clocks to pass
the reset signal internally. This option is used when the clock is
provided with an RC/LC clock. See the device product specifi
-
cation for timing details.
POR (cold start) will always reset the Z8 control and port regis-
ters to their default condition. If a Z8 has a SMR register, the
warm start bit will be reset to a 0 to indicate POR.
Figure 4-6. Example of Z8 with Simple SMR and POR
INT OSC
Chip
POR
Reset
P27
(Stop Mode)
(Cold Start)
VBO
WDT
Delay Line
T
POR
ms
18 CLK
Reset Filter
XTAL OSC
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