st62t62b STMicroelectronics, st62t62b Datasheet - Page 21

• Manufacturer Part Number: st62t62b
• Description: 8-bit Otp/eprom Mcus With A/d Converter, Auto-reload Timer And Eeprom
• Manufacturer: STMicroelectronics

3.3 DIGITAL WATCHDOG
The digital Watchdog consists of a reloadable
downcounter timer which can be used to provide
controlled recovery from software upsets.
The Watchdog circuit generates a Reset when the
downcounter reaches zero. User software can
prevent this reset by reloading the counter, and
should therefore be written so that the counter is
regularly reloaded while the user program runs
correctly. In the event of a software mishap (usu-
ally caused by externally generated interference),
the user program will no longer behave in its usual
fashion and the timer register will thus not be re-
loaded periodically. Consequently the timer will
decrement down to 00h and reset the MCU. In or-
der to maximise the effectiveness of the Watch-
dog function, user software must be written with
this concept in mind.
Watchdog behaviour is governed by two options,
known as “WATCHDOG ACTIVATION” (i.e.
HARDWARE or SOFTWARE) and “EXTERNAL
STOP MODE CONTROL” (see
mended Option
In the SOFTWARE option, the Watchdog is disa-
bled until bit C of the DWDR register has been set.
Table 7. Recommended Option Choices
Stop Mode & Watchdog
Choices).
Functions Required
Stop Mode
Watchdog
Table 7 Recom-
“EXTERNAL STOP MODE” & “HARDWARE WATCHDOG”
When the Watchdog is disabled, low power Stop
mode is available. Once activated, the Watchdog
cannot be disabled, except by resetting the MCU.
In the HARDWARE option, the Watchdog is per-
manently enabled. Since the oscillator will run
continuously, low power mode is not available.
The STOP instruction is interpreted as a WAIT in-
struction, and the Watchdog continues to count-
down.
However, when the EXTERNAL STOP MODE
CONTROL option has been selected low power
consumption may be achieved in Stop Mode.
Execution of the STOP instruction is then gov-
erned by a secondary function associated with the
NMI pin. If a STOP instruction is encountered
when the NMI pin is low, it is interpreted as WAIT,
as described above. If, however, the STOP in-
struction is encountered when the NMI pin is high,
the Watchdog counter is frozen and the CPU en-
ters STOP mode.
When the MCU exits STOP mode (i.e. when an in-
terrupt is generated), the Watchdog resumes its
activity.
“HARDWARE WATCHDOG”
“SOFTWARE WATCHDOG”
Recommended Options
ST62T52B ST62T62B/E62B
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