UM10462
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© NXP B.V. 2016. All rights reserved.
User manual
Rev. 5.5 — 21 December 2016
371 of 523
NXP Semiconductors
UM10462
Chapter 17: LPC11U3x/2x/1x Windowed Watchdog Timer (WWDT)
17.6 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 wdt_clk in
. Either the IRC or the watchdog oscillator can be used as wdt_clk in Active mode,
Sleep mode, and Deep-sleep modes. In Power-down mode only the watchdog oscillator is
available.
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.
Remark:
Because of the synchronization step, software must add a delay of three
WDCLK clock cycles between the feed sequence and the time the WDPROTECT bit is
enabled in the MOD register. The length of the delay depends on the selected watchdog
clock WDCLK.
Fig 64. Watchdog 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