LPC1766_0.02
© NXP B.V. 2008. All rights reserved.
Objective data sheet
Rev. 00.02 — 12 August 2008
28 of 70
NXP Semiconductors
LPC1766
Fast communication chip
7.26 System tick timer
The ARM Cortex-M3 includes a system tick timer (SYSTICK) that is intended to generate
a dedicated SYSTICK exception at a 10 ms interval. In the LPC1766, this timer can be
clocked from the internal AHB clock or from a device pin.
7.27 Watchdog timer
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.
7.27.1 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 prescaler.
• Selectable time period from (Tcy(WDCLK) × 256 × 4) to (Tcy(WDCLK) × 232 × 4) in
multiples of Tcy(WDCLK) × 4.
• The Watchdog Clock (WDCLK) source can be selected from the Internal RC oscillator
(IRC) or the APB peripheral clock. This gives a wide range of potential timing choices
of Watchdog operation under different power reduction conditions. It also provides the
ability to run the WDT from an entirely internal source that is not dependent on an
external crystal and its associated components and wiring for increased reliability.
• Includes lock/safe feature.
7.28 RTC and backup registers
The RTC is a set of counters for measuring time when system power is on, and optionally
when it is off. The RTC on the LPC1766 is designed to have extremely low power
consumption, i.e. less than 1
μA. The RTC will typically run from the main chip power
supply, conserving battery power while the rest of the device is powered up. When
operating from a battery, the RTC will continue working down to 2.1 V. Battery power can
be provided from a standard 3 V Lithium button cell.
An ultra-low power 32 kHz oscillator will provide a 1 Hz clock to the time counting portion
of the RTC, moving most of the power consumption out of the time counting function.
The RTC includes a calibration mechanism to allow fine-tuning the count rate in a way
that will provide less than 1 second per day error when operated at a constant voltage and
temperature. A clock output function (see Section 7.29.4) makes measuring the oscillator
rate easy and accurate.
The RTC contains a small set of backup registers (64 bytes) for holding data while the
main part of the LPC1766 is powered off.
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