
UM10800
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© NXP Semiconductors N.V. 2016. All rights reserved.
User manual
Rev. 1.2 — 5 October 2016
298 of 487
NXP Semiconductors
UM10800
Chapter 17: LPC82x Windowed Watchdog Timer (WWDT)
When the Watchdog Timer is configured so that a watchdog event will cause a reset and
the counter reaches zero, the CPU will be reset, loading the stack pointer and program
counter from the vector table as for an external reset. The Watchdog time-out flag
(WDTOF) can be examined to determine if the Watchdog has caused the reset condition.
The WDTOF flag must be cleared by software.
When the Watchdog Timer is configured to generate a warning interrupt, the interrupt will
occur when the counter matches the value defined by the WARNINT register.
17.5.1 Block diagram
The block diagram of the Watchdog is shown below in the
. The synchronization
logic (PCLK - WDCLK) is not shown in the block diagram.
17.5.2 Clocking and power control
The watchdog timer block uses two clocks: PCLK and WDCLK. PCLK is used for the APB
accesses to the watchdog registers and is derived from the system clock (see
The WDCLK is used for the watchdog timer counting and is derived from the watchdog
oscillator.
The synchronization logic between the two clock domains works as follows: When the
MOD and TC registers are updated by APB operations, the new value will take effect in 3
WDCLK cycles on the logic in the WDCLK clock domain.
When the watchdog timer is counting on WDCLK, the synchronization logic will first lock
the value of the counter on WDCLK and then synchronize it with PCLK, so that the CPU
can read the WDTV register.
Fig 44. Windowed Watchdog timer block diagram
watchdog
interrupt
WDRESET
(MOD
[1])
WDTOF
( MOD
[2])
WDINT
(MOD
[3])
WDEN
(MOD
[0])
chip reset
÷4
feed error
feed ok
wd_clk
enable count
MOD
register
compare
WDTV
compare
in
range
underflow
feed sequence
detect and
protection
FEED
feed ok
fe
ed ok
compare
0
interrupt
compare
24-bit down counter
WDINTVAL
WINDOW
TC
shadow bit
WDPROTECT
(MOD
[ 4])
TC
w
rit
e