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UM10316_0
© NXP B.V. 2008. All rights reserved.
User manual
Rev. 00.06 — 17 December 2008
300 of 571
NXP Semiconductors
UM10316
Chapter 20: LPC29xx WatchDog Timer (WDT)
4.4 Watchdog timer key register
The Watchdog timer key register contains a protection code to be used when accessing
the other Watchdog timer registers to prevent accidental alteration of these registers. The
value is hard-wired and can only be read, not modified. Writing the key value to this
register restarts the Timer_Counter, but writing other values has no effect. The Watchdog
timer must be periodically triggered by correct writes to this register in order to prevent
generation of a system reset .
4.5 Watchdog time-out register
The Watchdog time-out register holds the time-out value for Watchdog reset generation.
Timer_Counter counts up to this value and then asserts the Watchdog reset. To prevent
this from happening the user must write the key word to the Watchdog_Key register
before Timer_Counter reaches the programmed value. To be able to write to this register it
must be unlocked first. This is done by first writing to this register the key word as stored in
the Watchdog_Key register. Updating the Watchdog_Time_Out register by unlocking and
writing is also possible when the Watchdog timer has already been enabled (i.e. the
COUNTER_ENABLE bit in the WTCR register is set).
4.6 Watchdog debug register
To debug the Watchdog functionality, generation of a system reset when the Watchdog
timer counter reaches the Wd_Time_Out value must be prevented. When it is enabled an
interrupt can be generated instead. Reset generation on time-out can be blocked by
writing a 1 to the Watchdog reset-disable bit Wd_Rst_Dis.
This is only possible when the upper 31 bits of the data written to the Watchdog_Debug
register are identical to the Watchdog_Key. The Wd_Rst_Dis bit must be XOR-ed with the
Watchdog key. In all other cases writes to this register are ignored.
Table 255. WD_KEY register bit description (WD_KEY, address: 0xE004 0038)
* = reset value
Bit
Variable name
Access
Value
Description
31 to 0
WD_KEY_VAL
R/W
251D
8950h*
Key value to be used when accessing
Watchdog-timer control register
Table 256. WD_TIMEOUT register bit description
* = reset value
Bit
Variable name
Access
Value
Description
31 to 0
WD_TIMEOUT_VAL
R/W
00FF
FFFFh*
When the TC matches this value the
Watchdog reset will be asserted