w79e532 Winbond Electronics Corp America, w79e532 Datasheet - Page 47

• Manufacturer Part Number: w79e532
• Description: 8-bit Microcontroller
• Manufacturer: Winbond Electronics Corp America

The Watchdog timer should first be restarted by using RWT. This ensures that the timer starts from a
known state. The RWT bit is used to restart the watchdog timer. This bit is self clearing, i.e. after
writing a 1 to this bit the software will automatically clear it. The watchdog timer will now count clock
cycles. The time-out interval is selected by the two bits WD1 and WD0 (CKCON.7 and CKCON.6).
When the selected time-out occurs, the Watchdog interrupt flag WDIF (WDCON.3) is set. After the
time-out has occurred, the watchdog timer waits for an additional 512 clock cycles. If the Watchdog
Reset EWT (WDCON.1) is enabled, then 512 clocks after the time-out, if there is no RWT, a system
reset due to Watchdog timer will occur. This will last for two machine cycles, and the Watchdog timer
reset flag WTRF (WDCON.2) will be set. This indicates to the software that the watchdog was the
cause of the reset.
When used as a simple timer, the reset and interrupt functions are disabled. The timer will set the
WDIF flag each time the timer completes the selected time interval. The WDIF flag is polled to detect a
time-out and the RWT allows software to restart the timer. The Watchdog timer can also be used as a
very long timer. The interrupt feature is enabled in this case. Every time the time-out occurs an
interrupt will occur if the global interrupt enable EA is set.
The main use of the Watchdog timer is as a system monitor. This is important in real-time control
applications. In case of some power glitches or electro-magnetic interference, the processor may
begin to execute errant code. If this is left unchecked the entire system may crash. Using the
watchdog timer interrupt during software development will allow the user to select ideal watchdog
reset locations. The code is first written without the watchdog interrupt or reset. Then the watchdog
interrupt is enabled to identify code locations where interrupt occurs. The user can now insert
instructions to reset the watchdog timer which will allow the code to run without any watchdog timer
interrupts. Now the watchdog timer reset is enabled and the watchdog interrupt may be disabled. If
any errant code is executed now, then the reset watchdog timer instructions will not be executed at
the required instants and watchdog reset will occur.
The watchdog time-out selection will result in different time-out values depending on the clock speed.
The reset, when enabled, will occur 512 clocks after the time-out has occurred.
Table 9. Time-out values for the Watchdog timer
The Watchdog timer will de disabled by a power-on/fail reset. The Watchdog timer reset does not
disable the watchdog timer, but will restart it. In general, software should restart the timer to put it into
a known state.
The control bits that support the Watchdog timer are discussed below.
WD1
0
0
1
1
WD0
0
1
0
1
WATCHDOG
INTERVAL
2
2
2
2
17
20
23
26
NUMBER OF
67108864
CLOCKS
1048576
8388608
131072
36408.88 mS
- 47 -
4551.11 mS
@ 1.8432 MHZ
568.89 mS
71.11 mS
TIME
Publication Release Date: November 21, 2005
W79E532/W79L532
6710.89 mS
104.86 mS
838.86 mS
13.11 mS
@ 10 MHZ
TIME
2684.35 mS
335.54 mS
41.94 mS
@ 25 MHZ
5.24 mS
TIME
Revision A5