ST72F321 STMICROELECTRONICS [STMicroelectronics], ST72F321 Datasheet - Page 52

• Manufacturer Part Number: ST72F321
• Description: 64/44-PIN 8-BIT MCU WITH 32 TO 60K FLASH/ROM, ADC, FIVE TIMERS, SPI, SCI, I2C INTERFACE
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

ST72321
10 ON-CHIP PERIPHERALS
10.1 WATCHDOG TIMER (WDG)
10.1.1 Introduction
The Watchdog timer is used to detect the occur-
rence of a software fault, usually generated by ex-
ternal interference or by unforeseen logical condi-
tions, which causes the application program to
abandon its normal sequence. The Watchdog cir-
cuit generates an MCU reset on expiry of a pro-
grammed time period, unless the program refresh-
es the counter’s contents before the T6 bit be-
comes cleared.
10.1.2 Main Features
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10.1.3 Functional Description
The counter value stored in the Watchdog Control
register (WDGCR bits T[6:0]), is decremented
every 16384 f
length of the timeout period can be programmed
by the user in 64 increments.
Figure 33. Watchdog Block Diagram
52/189
MCC/RTC
Programmable free-running downcounter
Programmable reset
Reset (if watchdog activated) when the T6 bit
reaches zero
Optional
(configurable by option byte)
Hardware Watchdog selectable by option byte
11
MSB
RTC COUNTER
12-BIT MCC
reset
f
DIV 64
OSC2
OSC2
6
5
LSB
cycles (approx.), and the
on
0
HALT
TB[1:0] bits
(MCCSR
Register)
instruction
WDGA
RESET
T6
If the watchdog is activated (the WDGA bit is set)
and when the 7-bit timer (bits T[6:0]) rolls over
from 40h to 3Fh (T6 becomes cleared), it initiates
a reset cycle pulling low the reset pin for typically
500ns.
The application program must write in the
WDGCR register at regular intervals during normal
operation to prevent an MCU reset. This down-
counter is free-running: it counts down even if the
watchdog is disabled. The value to be stored in the
WDGCR register must be between FFh and C0h:
– The WDGA bit is set (watchdog enabled)
– The T6 bit is set to prevent generating an imme-
– The T[5:0] bits contain the number of increments
Following a reset, the watchdog is disabled. Once
activated it cannot be disabled, except by a reset.
The T6 bit can be used to generate a software re-
set (the WDGA bit is set and the T6 bit is cleared).
If the watchdog is activated, the HALT instruction
will generate a Reset.
WATCHDOG CONTROL REGISTER (WDGCR)
diate reset
which represents the time delay before the
watchdog produces a reset (see
proximate Timeout
between a minimum and a maximum value due
to the unknown status of the prescaler when writ-
ing to the WDGCR register (see
T5
6-BIT DOWNCOUNTER (CNT)
WDG PRESCALER
T4
DIV 4
T3
Duration). The timing varies
T2
T1
Figure
Figure 34. Ap-
T0
35).