Z8 Microcontrollers
Reset—Watch-Dog Timer
ZiLOG
4-8
UM001600-Z8X0599
Clock Source for WDT (D4).
This bit determines
which oscillator source is used to clock the internal POR
and WDT counter chain. If the bit is a 1, the internal RC os-
cillator is bypassed and the POR and WDT clock source is
driven from the external pin, XTAL1. The default configu-
ration of this bit is 0, which selects the internal RC oscilla-
tor.
Bits 5, 6 and 7.
These bits are reserved.
V
CC
Voltage Comparator
. An on-board voltage com-
parator checks that V
CC
is at the required level to insure
correct operation of the device. Reset is globally driven if
V
CC
is below the specified voltage. This feature is available
in select ROM Z8 devices. See the device product specifi-
cation for feature availability and operating range.
4.4 POWER-ON-RESET (POR)
A timer circuit clocked by a dedicated on-board RC oscil-
lator 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 se-
lects whether the POR timer is used after Stop-Mode Re-
covery 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 re-
set signal internally. This option is used when the clock is
provided with an RC/LC clock. See the device product
specification for timing details.
POR (cold start) will always reset the Z8 control and port
registers to their default condition. If a Z8 has a SMR reg-
ister, 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
Reset
POR
(Cold Start)
P27
(Stop Mode)
VBO
WDT
Delay Line
T
POR
ms
18 CLK
Reset Filter
XTAL OSC