Datasheet

LPC408X_7X All information provided in this document is subject to legal disclaimers. © NXP Semiconductors N.V. 2014. All rights reserved.
Product data sheet Rev. 3.1 — 1 September 2014 70 of 138
NXP Semiconductors
LPC408x/7x
32-bit ARM Cortex-M4 microcontroller
7.33.1 Features
• Internally resets chip if not periodically reloaded during the programmable time-out
period.
• Optional windowed operation requires reload to occur between a minimum and
maximum time period, both programmable.
• Optional warning interrupt can be generated at a programmable time prior to
watchdog time-out.
• Enabled by software but requires a hardware reset or a watchdog reset/interrupt to be
disabled.
• Incorrect feed sequence causes reset or interrupt if enabled.
• Flag to indicate watchdog reset.
• Programmable 24-bit timer with internal prescaler.
• Selectable time period from (T
cy(WDCLK)
 256  4) to (T
cy(WDCLK)
 2
24
 4) in
multiples of T
cy(WDCLK)
 4.
• The Watchdog Clock (WDCLK) source is a dedicated watchdog oscillator, which is
always running if the watchdog timer is enabled.
7.34 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 LPC408x/7x is designed to have extremely low power
consumption, i.e. less than 1 A. 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.
The RTC contains a small set of backup registers (20 bytes) for holding data while the
main part of the LPC408x/7x is powered off.
The RTC includes an alarm function that can wake up the LPC408x/7x from all reduced
power modes with a time resolution of 1 s.
7.34.1 Features
• Measures the passage of time to maintain a calendar and clock.
• Ultra low power design to support battery powered systems.
• Provides Seconds, Minutes, Hours, Day of Month, Month, Year, Day of Week, and
Day of Year.
• Dedicated power supply pin can be connected to a battery or to the main 3.3 V.
• Periodic interrupts can be generated from increments of any field of the time registers.
• Backup registers (20 bytes) powered by VBAT.