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UM10360_0
© NXP B.V. 2009. All rights reserved.
User manual
Rev. 00.06 — 5 June 2009
543 of 808
1.
Features
•
Internally resets chip if not periodically reloaded.
•
Debug mode.
•
Enabled by software but requires a hardware reset or a Watchdog reset/interrupt to be
disabled.
•
Incorrect/Incomplete feed sequence causes reset/interrupt if enabled.
•
Flag to indicate Watchdog reset.
•
Programmable 32-bit timer with internal pre-scaler.
•
Selectable time period from (T
WDCLK
×
256
×
4) to (T
WDCLK
×
2
32
×
4) in multiples of
T
WDCLK
×
4.
•
The Watchdog clock (WDCLK) source can be selected from the Internal RC oscillator
(IRC), the APB peripheral clock (PCLK, see
), or the RTC oscillator. This
gives a wide range of potential timing choices for Watchdog operation under different
power reduction conditions. For increased reliability, it also provides the ability to run
the Watchdog timer from an entirely internal source that is not dependent on an
external crystal and its associated components and wiring.
•
The Watchdog timer can be configured to run in Deep Sleep mode when using the
IRC as the clock source.
2.
Applications
The purpose of the Watchdog is to reset the microcontroller within a reasonable amount of
time if it enters an erroneous state. When enabled, the Watchdog will generate a system
reset if the user program fails to "feed" (or reload) the Watchdog within a predetermined
amount of time.
For interaction of the on-chip watchdog and other peripherals, especially the reset and
boot-up procedures, please read
Section 3–4 “Reset” on page 17
of this document.
3.
Description
The Watchdog consists of a divide by 4 fixed pre-scaler and a 32-bit counter. The clock is
fed to the timer via a pre-scaler. The timer decrements when clocked. The minimum value
from which the counter decrements is 0xFF. Setting a value lower than 0xFF causes 0xFF
to be loaded in the counter. Hence the minimum Watchdog interval is (T
WDCLK
×
256
×
4)
and the maximum Watchdog interval is (T
WDCLK
×
2
32
×
4) in multiples of (T
WDCLK
×
4).
The Watchdog should be used in the following manner:
•
Set the Watchdog timer constant reload value in WDTC register.
•
Setup the Watchdog timer operating mode in WDMOD register.
•
Enable the Watchdog by writing 0xAA followed by 0x55 to the WDFEED register.
UM10360
Chapter 28: LPC17xx Watchdog Timer (WDT)
Rev. 00.06 — 5 June 2009
User manual