FlashFlex MCU
SST89E516RD2 / SST89E516RD
SST89V516RD2 / SST89V516RD
Data Sheet
7.0 WATCHDOG TIMER
The device offers a programmable Watchdog Timer (WDT)
for fail safe protection against software deadlock and auto-
matic recovery.
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.
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.
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:
Period = (255 - WDTD) * 344064 * 1/fCLK (XTAL1)
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 344,064 crystal clocks. The upper 8-bits of the
time base register (WDTD) are used as the reload register
of the WDT.
where WDTD is the value loaded into the WDTD register
and fOSC is the oscillator frequency.
344064
WDT Reset
clks
CLK (XTAL1)
Internal Reset
Counter
WDT Upper Byte
Ext. RST
WDTC
WDTD
1273 F19.0
FIGURE
7-1: Block Diagram of Programmable Watchdog Timer
©2007 Silicon Storage Technology, Inc.
S71273-03-000
1/07
45
SST [ SILICON STORAGE TECHNOLOGY, INC ]
