UM10850
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User manual
Rev. 2.4 — 13 September 2016
227 of 464
NXP Semiconductors
UM10850
Chapter 15: LPC5410x Windowed Watchdog Timer (WWDT)
15.6.2 Watchdog Timer Constant register
The TC register determines the time-out value. Every time a feed sequence occurs the
value in the TC is loaded into the Watchdog timer. The TC resets to 0x00 00FF. Writing a
value below 0xFF will cause 0x00 00FF to be loaded into the TC. Thus the minimum
time-out interval is T
WDCLK
256
4.
If the WDPROTECT bit in WDMOD = 1, 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.
15.6.3 Watchdog Feed register
Writing 0xAA followed by 0x55 to this register will reload the Watchdog timer with the TC
register value. This operation will also start the Watchdog if it is enabled via the WDMOD
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 WDFEED, access to any Watchdog register other than writing 0x55
to WDFEED 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.
It is good practice to disable interrupts around a feed sequence, if the application is such
that an interrupt might result in rescheduling processor control away from the current task
in the middle of the feed, and then lead to some other access to the WDT before control is
returned to the interrupted task.
15.6.4 Watchdog Timer Value register
The TV register is used to read the current value of Watchdog timer counter.
When reading the value of the 24-bit counter, the lock and synchronization procedure
takes up to 6 WDCLK cycles plus 6 PCLK cycles, so the value of TV is older than the
actual value of the timer when it's being read by the CPU.
Table 269. Watchdog Timer Constant register (TC, 0x4003 8004) bit description
Bit
Symbol Description
Reset value
23:0
COUNT Watchdog time-out value.
0x00 00FF
31:24 -
Reserved. Read value is undefined, only zero should be written.
NA
Table 270. Watchdog Feed register (FEED, 0x4003 8008) bit description
Bit
Symbol
Description
Reset value
7:0
FEED
Feed value should be 0xAA followed by 0x55.
NA
31:8
-
Reserved. Read value is undefined, only zero should be written.
NA
Table 271. Watchdog Timer Value register (TV, 0x4003 800C) bit description
Bit
Symbol
Description
Reset value
23:0
COUNT
Counter timer value.
0x00 00FF
31:24
-
Reserved. Read value is undefined, only zero should be written. NA