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AT91SAM7X256 Datasheet(PDF) 82 Page - ATMEL Corporation

No. de Pieza. AT91SAM7X256
Descripción  AT91 ARM Thumb-based Microcontrollers
Descarga  664 Pages
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Fabricante  ATMEL [ATMEL Corporation]
Página de inicio  http://www.atmel.com
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AT91SAM7X256 Datasheet(HTML) 82 Page - ATMEL Corporation

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82
6120D–ATARM–02-Feb-06
AT91SAM7X256/128 Preliminary
17.3
Functional Description
The Watchdog Timer can be used to prevent system lock-up if the software becomes trapped in
a deadlock. It is supplied with VDDCORE. It restarts with initial values on processor reset.
The Watchdog is built around a 12-bit down counter, which is loaded with the value defined in
the field WV of the Mode Register (WDT_MR). The Watchdog Timer uses the Slow Clock
divided by 128 to establish the maximum Watchdog period to be 16 seconds (with a typical Slow
Clock of 32.768 kHz).
After a Processor Reset, the value of WV is 0xFFF, corresponding to the maximum value of the
counter with the external reset generation enabled (field WDRSTEN at 1 after a Backup Reset).
This means that a default Watchdog is running at reset, i.e., at power-up. The user must either
disable it (by setting the WDDIS bit in WDT_MR) if he does not expect to use it or must repro-
gram it to meet the maximum Watchdog period the application requires.
The Watchdog Mode Register (WDT_MR) can be written only once. Only a processor reset
resets it. Writing the WDT_MR register reloads the timer with the newly programmed mode
parameters.
In normal operation, the user reloads the Watchdog at regular intervals before the timer under-
flow occurs, by writing the Control Register (WDT_CR) with the bit WDRSTT to 1. The
Watchdog counter is then immediately reloaded from WDT_MR and restarted, and the Slow
Clock 128 divider is reset and restarted. The WDT_CR register is write-protected. As a result,
writing WDT_CR without the correct hard-coded key has no effect. If an underflow does occur,
the “wdt_fault” signal to the Reset Controller is asserted if the bit WDRSTEN is set in the Mode
Register (WDT_MR). Moreover, the bit WDUNF is set in the Watchdog Status Register
(WDT_SR).
To prevent a software deadlock that continuously triggers the Watchdog, the reload of the
Watchdog must occur in a window defined by 0 and WDD in the WDT_MR:
0
≤WDT ≤WDD; writing WDRSTT restarts the Watchdog Timer.
Any attempt to restart the Watchdog Timer in the range [WDV; WDD] results in a Watchdog
error, even if the Watchdog is disabled. The bit WDERR is updated in the WDT_SR and the
“wdt_fault” signal to the Reset Controller is asserted.
Note that this feature can be disabled by programming a WDD value greater than or equal to the
WDV value. In such a configuration, restarting the Watchdog Timer is permitted in the whole
range [0; WDV] and does not generate an error. This is the default configuration on reset (the
WDD and WDV values are equal).
The status bits WDUNF (Watchdog Underflow) and WDERR (Watchdog Error) trigger an inter-
rupt, provided the bit WDFIEN is set in the mode register. The signal “wdt_fault” to the reset
controller causes a Watchdog reset if the WDRSTEN bit is set as already explained in the reset
controller programmer Datasheet. In that case, the processor and the Watchdog Timer are
reset, and the WDERR and WDUNF flags are reset.
If a reset is generated or if WDT_SR is read, the status bits are reset, the interrupt is cleared,
and the “wdt_fault” signal to the reset controller is deasserted.
Writing the WDT_MR reloads and restarts the down counter.
While the processor is in debug state or in idle mode, the counter may be stopped depending on
the value programmed for the bits WDIDLEHLT and WDDBGHLT in the WDT_MR.


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