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AN574

更新时间： 2022-11-29 07:29:00

品牌	Logo	应用领域
美信 - MAXIM		微控制器
页数	文件大小	规格书
5页	26K
描述
App Note 101: Using the Secure Microcontroller Watchdog Timer

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AN574 数据手册

MICROCONTROLLERS

Aug 10, 2001

App Note 101: Using the Secure Microcontroller

Watchdog Timer

The Secure Microcontroller Family (DS5000FP, DS5001FP, DS5002FP, and

associated modules) integrate an internal watchdog timer to prevent code

execution errors. The watchdog timer uses the high precision crystal oscillator that

is also used by the microcontroller. This eliminates the need for a RC oscillator

while providing greater accuracy.

Overview

Microcontrollers are often used in harsh environments where power supply transients, electromagnetic

interference (EMI), and electrostatic discharge (ESD) are abundant. Program corruption caused by bus

corruption and electromagnetic discharges can cause a microprocessor to execute erroneous

instructions. In these environments, a watchdog timer is a useful peripheral that can help catch and

reset a microcontroller that has gone "out of control."

A watchdog timer is a simple countdown timer which is used to reset a microprocessor after a specific

interval of time. In a properly operating system, software will periodically "pet" or restart the watchdog

timer. After being restarted, the watchdog will begin timing another predetermined interval. When

software or the device is not functioning correctly, software will not restart the watchdog timer before it

times out. When the watchdog timer times out, it will cause a reset of the microcontroller. If the system

software has been designed correctly and there has been no hardware failure, the reset will cause the

system to operate properly again. The reset condition must be a "safe" state. For instance, it would not

be wise to have the reset state of a magnetic stripe card reader enabling the write head.

Many systems have been designed using an external watchdog timer. The Secure Microcontroller family

eliminates the need for external components by incorporating an internal watchdog timer. By moving the

watchdog timer inside the microcontroller, the number of devices in the system is reduced, increasing

the overall system reliability. The watchdog timer can take advantage of the high-precision crystal

oscillator used by the microcontroller, rather than the imprecise RC oscillator used by most independent

watchdog timers. The operation of the watchdog timer is independent of the microcontroller, unless

specifically addressed via the Timed Access procedure. The possibility of an out-of-control

microcontroller accidentally disabling the watchdog timer is less than 1 in 7.2 X 10¹⁶. This application

note describes the features and use of the Secure Microcontroller’s watchdog timer.

General Use of a Watchdog Timer

The primary application of a watchdog timer is as a system monitor to detect and reset an "out of

control" microprocessor. When program execution goes awry it will not properly execute the code that

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