SST89E554 SST [Silicon Storage Technology, Inc], SST89E554 Datasheet - Page 39

• Manufacturer Part Number: SST89E554
• Description: FlashFlex51 MCU
• Manufacturer: SST [Silicon Storage Technology, Inc]

FlashFlex51 MCU
SST89E564 / SST89V564 / SST89E554 / SST89V554
Preliminary Specifications
7.0 WATCHDOG TIMER
The device offers a programmable Watchdog Timer (WDT)
for fail safe protection against software deadlock and auto-
matic recovery.
To protect the system against software deadlock, the user
software must refresh the WDT within a user-defined time
period. If the software fails to do this periodical refresh, an
internal hardware reset will be initiated if enabled (WDRE=
1). The software can be designed such that the WDT times
out if the program does not work properly.
The WDT in the device uses the system clock (XTAL1) as
its time base. So strictly speaking, it is a watchdog counter
rather than a watchdog timer. The WDT register will incre-
ment every 344064 crystal clocks. The upper 8-bits of the
time base register (WDTD) are used as the reload register
of the WDT.
©2001 Silicon Storage Technology, Inc.
FIGURE
Ext. RST
CLK (XTAL1)
7-1: B
WDTC
LOCK
D
IAGRAM OF
Counter
P
ROGRAMMABLE
344064
clks
WDT Upper Byte
39
WDTD
W
ATCHDOG
The WDTS flag bit is set by WDT overflow and is not
changed by WDT reset. User software can clear WDTS by
writing “1” to it.
Figure 7-1 provides a block diagram of the WDT. Two SFRs
(WDTC and WDTD) control watchdog timer operation.
During idle mode, WDT operation is temporarily sus-
pended, and resumes upon an interrupt exit from idle.
The time-out period of the WDT is calculated as follows:
where WDT is the value loaded into the WDT register and
f
OSC
is the oscillator frequency.
Period = (255 - WDT) * 344064 * 1/f
T
WDT Reset
IMER
384 ILL F10.2
Internal Reset
S71181-03-000 9/01
OSC
384