UM10503
All information provided in this document is subject to legal disclaimers.
© NXP B.V. 2012. All rights reserved.
User manual
Rev. 1.3 — 6 July 2012
922 of 1269
NXP Semiconductors
UM10503
Chapter 34: LPC43xx Windowed Watchdog timer (WWDT)
34.7.2 Watchdog timer constant register
The TC register determines the time-out value. Every time a feed sequence occurs, the
TC register content is reloaded into the Watchdog timer. This is pre-loaded with the value
0x00 00FF upon reset. Writing values below 0xFF will cause 0x00 00FF to be loaded into
the TC register. Thus the minimum time-out interval is T
WDCLK
256
4.
If the WDPROTECT bit in MOD register is set to one, an attempt to change the value of
TC before the watchdog counter is below the values of WDWARNINT and WDWINDOW
will cause a watchdog reset and set the WDTOF flag.
34.7.3 Watchdog feed register
Writing 0xAA followed by 0x55 to this register will reload the Watchdog timer with the
time-out value in the TC register. This operation will also start the Watchdog if it is enabled
via the MOD register. Setting the WDEN bit in the WDMOD register is not sufficient to
enable the Watchdog. A valid feed sequence must be completed after setting WDEN
before the Watchdog is capable of generating a reset. Until then, the Watchdog will ignore
feed errors. After writing 0xAA to FEED register, access to any Watchdog register other
than writing 0x55 to FEED register causes an immediate reset/interrupt when the
Watchdog is enabled, and sets the WDTOF flag. The reset will be generated during the
second PCLK following an incorrect access to a Watchdog register during a feed
sequence.
Interrupts should be disabled during the feed sequence. An abort condition will occur if an
interrupt happens during the feed sequence.
Table 774. Watchdog operating modes selection
WDEN
WDRESET
Mode of Operation
0
X (0 or 1)
Debug/Operate without the Watchdog running.
1
0
Watchdog interrupt mode: the watchdog warning interrupt will be
generated but watchdog reset will not.
When this mode is selected, the watchdog counter reaching the value
specified by WDWARNINT will set the WDINT flag and the Watchdog
interrupt request will be generated.
1
1
Watchdog reset mode: Both the watchdog interrupt and watchdog reset
are enabled.
When this mode is selected, the watchdog counter reaching the value
specified by WDWARNINT will set the WDINT flag and the Watchdog
interrupt request will be generated. The watchdog counter reaching zero
will reset the microcontroller.
Remark:
Other causes for a watchdog reset are: A watchdog feed or
changing the WDTC value (if the WDPROTECT bit is set in the MOD
register) before reaching the value of WDWINDOW.
Table 775. Watchdog Timer Constant register (TC - 0x4008 0004) bit description
Bit
Symbol
Description
Reset value
23:0
WDTC
Watchdog time-out value.
0x00 00FF
31:24
-
Reserved, user software should not write ones to reserved
bits. The value read from a reserved bit is not defined.
NA