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Hardware description
A.5 System resources
A.5.2.4
Watchdog timer (WDT) register (read/write)
The procedures for operating the watchdog timer (WDT) are described below.
Note: Always use read-modify-write operations so that reserved bits are not overwritten. This
can lead to impairment of the functionality of the device.
Enter countdown time and start timer
1. Write the desired value for the WDT Count Mode (minutes or seconds) into the WDT control
register (285h).
2. Write the desired countdown time into the WDT counter register (286h).
The countdown starts. When the countdown reaches 0, the WDT expires. If the WDT has
expired, the countdown will not restart until the WDT status "Expired" is reset.
Reset WDT countdown
1. Write the initial or new countdown time into the WDT counter register (286h)
The WDT starts counting down. When the countdown reaches 0, the WDT expires.
Stop WDT
1. Write 0 into the WDT counter register to stop the WDT completely.
Set behavior when WDT expires
1. Write the desired value for the WDT system reset (On or Off) into the GPIO2 data register
(282h)
– If On, the system restarts when the WDT expires. The WDT status is set to "Expired" and
– If Off, the system will NOT reboot when the WDT expires. The WDT status is set to
Read WDT status
1. Read the current WDT status (active or expired) from the WDT status register (287h).
Reset WDT Status "Expired"
1. Write 0 into the WDT status register (287h) to switch the WDT to "Active" again.
This step resets the "PC ON/WD" LED from pulsing red to its normal state (lit green).
The WDT starts counting down. To prevent this behavior, stop the WDT before resetting the
status to "Active".
102
the "PC ON/WD" LED starts pulsing red.
"Expired" and the "PC ON/WD" LED starts pulsing red.
Operating Instructions, 07/2022, A5E50715883-AA
SIMATIC IPC BX-39A
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