NXP Semiconductors LPC1751, User Manual, Page: 532 / 808

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UM10360_0 © NXP B.V. 2009. All rights reserved.
User manual Rev. 00.06 — 5 June 2009 532 of 808
1. Basic configuration
The RTC is configured using the following registers:
1. Power: In the PCONP register ( Table 4–46 ), set bits PCRTC.
Remark: On reset, the RTC is enabled. See Section 27–7 for power saving options.
2. Clock: The RTC uses the 1 Hz clock output from the RTC oscillator as the only clock
source. The peripheral clock rate for accessing registers is CCLK/8.
3. Interrupts: See Section 27–6.1 for RTC interrupt handling. Interrupts are enabled in
the NVIC using the appropriate Interrupt Set Enable register.
2. Features
•
Measures the passage of time to maintain a calendar and clock. Provides seconds,
minutes, hours, day of month, month, year, day of week, and day of year.
•
Ultra-low power design to support battery powered systems. Less than 1 microamp
required for battery operation. Uses power from the CPU power supply when it is
present.
•
20 bytes of Battery-backed storage and RTC operation when power is removed from
the CPU.
•
Dedicated 32 kHz ultra low power oscillator.
•
Dedicated battery power supply pin.
•
RTC power supply is isolated from the rest of the chip.
•
Calibration counter allows adjustment to better than
±
1 sec/day with 1 sec resolution.
•
Periodic interrupts can be generated from increments of any field of the time registers
and selected fractional second values.
•
Alarm interrupt can be generated for a specific date/time.
3. Description
The Real Time Clock (RTC) is a set of counters for measuring time when system power is
on, and optionally when it is off. It uses very little power when its registers are not being
accessed by the CPU, especially reduced power modes. On the LPC17xx, the RTC is
clocked by a separate 32 kHz oscillator that produces a 1 Hz internal time reference. The
RTC is powered by its own power supply pin, VBAT, which can be connected to a battery,
externally tied to a 3V supply, or left floating.
The RTC power domain is shown in conceptual form in Figure 27–128 . A detailed view of
the time keeping portion of the RTC is shown in Figure 27–129 .
UM10360
Chapter 27: LPC17xx Real-Time Clock (RTC) and backup
registers
Rev. 00.06 — 5 June 2009 User manual