15
Z08617 NMOS Z8
®
8-B
IT
MCU
K
EYBOARD
C
ONTROLLER
FUNCTIONAL DESCRIPTION
(Continued)
Watch-Dog Timer.
The Z08617 is equipped with a hard-
ware Watch-Dog Timer which will be turned on automati-
cally by power-on (Figure 15). The Watch-Dog Timer must
be refreshed at least once every 50 ms by executing the
instruction WDT (Opcode = %5F), otherwise the Z08617
will reset itself if /WDTOUT pin 9 is connected to /RESET
(Pin 6). Figure 16 shows the block diagram of WDT.
The Watch-Dog Timer is automatically enabled upon
power-up of the microcontroller and /RESET going High.
The /WDTOUT pin can be connected to the /RESET pin to
provide an automatic reset upon WDT time-out. Dur-
ing WDT time-out, the /WDTOUT pin goes Low for
approximately 8-15
µ
s.
WDT Hot Bit.
Bit 7 of the Interrupt Request Register
(IRR register FAH) determines whether a hot start or
cold start occurred. A cold start is defined as a rest
occurring from the power-up of the Z08617 (bit 7 is set to
zero upon power-up). A hot start occurs when a WDT time-
out has occurred (bit 7 is set to 1). Bit 7 of the IRQ register
is read-only and is automatically reset to 0 when accessed.
Power-On Reset.
Upon power-up of the microcontroller,
a reset condition is enabled. A delay of 150 ms
±
20%
is used to assist in initializing the microcontroller.
/Reset
VCC
/WDT
Output
Internal
Reset
POR
8-15
µ
s
Reset
Delay
* Reset Delay = POR 150 ms
±
20%
Figure 15. WDT Turn-On Timing After Reset
Reset
Internal
Reset
Counter
Reset Logic
50 ms
10.4 Kohms
Internal RC
WDT
150 ms
Figure 16. WDT Block Diagram
