Datasheet

Product data sheet Rev. 8.5 — 24 June 2014 29 of 80

NXP Semiconductors

LPC1759/58/56/54/52/51

32-bit ARM Cortex-M3 microcontroller

8.28 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 LPC1759/58/56/54/52/51 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 LPC1759/58/56/54/52/51 is powered off.

The RTC includes an alarm function that can wake up the LPC1759/58/56/54/52/51 from

all reduced power modes with a time resolution of 1 s.

8.28.1 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.

• RTC power supply is isolated from the rest of the chip.

8.29 Clocking and power control

8.29.1 Crystal oscillators

The LPC1759/58/56/54/52/51 include three independent oscillators. These are the main

oscillator, the IRC oscillator, and the RTC oscillator. Each oscillator can be used for more

than one purpose as required in a particular application. Any of the three clock sources

can be chosen by software to drive the main PLL and ultimately the CPU.

Following reset, the LPC1759/58/56/54/52/51 will operate from the Internal RC oscillator

until switched by software. This allows systems to operate without any external crystal and

the bootloader code to operate at a known frequency.

See Figure 4

for an overview of the LPC1759/58/56/54/52/51 clock generation.