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What is WatchDog Timer? How to test it?

2007-10-22 Installation Document No.1-0000293

Solution:

  • What is WatchDog Timer?


    The watchdog timer (WDT) will reset CPU or generates an interrupt if watchdog resident processing comes to a standstill for any reason. This feature helps to ensure system reliability in industrial standalone or unmanned environments. The reset or interrupt service can wake the application up if it goes into endless loops without response.

    Normally there are 2 inputs and 1 output in WDT:
    I1: Time interval setting (or boundled with Activate/Enable switch)
    I2: Clear counting
    O1: Trigger for "system reset" or "interrupt"

    Customers have to write the WDT program as resident service "inside" the industrial application program, or another individual program resident with OS at background. After enabling the WDT service, the resident program needs to "frequently clear" the WDT chip's timer counting so WDT chip won't send trigger out. When the software is alive, WDT is always cleared to start from the beginning (e.g. 63sec) then counts down again.

    If CPU hangs up to make it unable to execute the resident program well, WDT chip can't get clear signal so it continues to count down to "0". That's the time to send trigger signal to reset the whole system, or send an IRQ. Hope to use this feature to wake up the computer system, then execute dedicated application programs introduced in batch file or startup script.

    The possible reasons to cause CPU not being able to respond are as follows:
    1. Hardware malfunctions!
    2. CPU is jamed for unknown software issue (e.g. virus atttack, response time-out in application program to jam CPU processing...).

    For example, there are 2 different WDT designs in Advantech full-size CPU board designs:
    I. Additional chipset which uses I/O ports 0x443h & 0x43h as inputs! (Used on PCA-6168, PCA-6178, PCA-6179)
    II. Built-in WDT function in Super I/O chip! (Winbond 83627 series, used on PCA-6180, PCA-6184, PCA-6186...etc)

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