参数资料
型号: ST72F623F2B1
厂商: STMICROELECTRONICS
元件分类: 微控制器/微处理器
英文描述: 8-BIT, FLASH, 8 MHz, MICROCONTROLLER, PDIP20
封装: 0.300 INCH, LEAD FREE, SHRINK, PLASTIC, DIP-20
文件页数: 76/139页
文件大小: 1264K
代理商: ST72F623F2B1
ST7262xxx
41/139
Doc ID 6996 Rev 5
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
Programmable free-running downcounter (64
increments of 65536 CPU cycles)
Programmable reset
Reset (if watchdog activated) when the T6 bit
reaches zero
Hardware Watchdog selectable by option byte
10.1.3 Functional Description
The counter value stored in the CR register (bits
T[6:0]), is decremented every 65,536 machine cy-
cles, and the length of the timeout period can be
programmed by the user in 64 increments.
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 by driving low the reset pin for 30s.
The application program must write in the CR reg-
ister at regular intervals during normal operation to
prevent an MCU reset. This downcounter is free-
running: it counts down even if the watchdog is di-
abled The value to be stored in the CR register
must be between FFh and C0h (see Table 14):
– The WDGA bit is set (watchdog enabled)
– The T6 bit is set to prevent generating an imme-
diate reset
– The T[5:0] bits contain the number of increments
which represents the time delay before the
watchdog produces a reset.
Table 14.Watchdog Timing (fCPU = 8 MHz)
Figure 30. Watchdog Block Diagram
CR Register
initial value
WDG timeout period
(ms)
Max
FFh
524.288
Min
C0h
8.192
RESET
WDGA
7-BIT DOWNCOUNTER
fCPU
T6
T0
CLOCK DIVIDER
WATCHDOG CONTROL REGISTER (CR)
÷65536
T1
T2
T3
T4
T5
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