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UM10601
All information provided in this document is subject to legal disclaimers.
© NXP B.V. 2012. All rights reserved.
Preliminary user manual
Rev. 1.0 — 7 November 2012
148 of 313
NXP Semiconductors
UM10601
Chapter 12: LPC800 Windowed Watchdog Timer (WWDT)
12.5.2 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.
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 16. Windowed Watchdog timer block diagram
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