NXP Semiconductors LPC24XX UM10237 - Manual (Page 647)

UM10237_4 © NXP B.V. 2009. All rights reserved.
User manual Rev. 04 — 26 August 2009 647 of 792
1. Basic configuration
The RTC is configured using the following registers:
1. Power: In the PCONP register ( Table 4–63 ), set bit PCRTC.
Remark: On reset, the RTC is enabled. See Section 26–7 for power saving options.
2. Clock: Select clock source in Table 26–569 . If the peripheral clock is selected, select
PCLK_RTC in the PCLK_SEL0 register ( Table 4–56 ). For the RTC, the peripheral
clock must be scaled (see Section 26–6.7 ).
3. Interrupts: See Section 26–6.1 for RTC interrupt handling. Interrupts are enabled in
the VIC using the VICIntEnable register ( Section 7–3.4 ).
2. 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 32 kHz oscillator or programmable prescaler from APB clock.
• Dedicated power supply pin can be connected to a battery or to the main 3.3 V.
• An alarm output pin is included to assist in waking up from power-down modes and
when the chip has had power removed to all functions except the RTC and Battery
RAM.
• Periodic interrupts can be generated from increments of any field of the time registers
and selected fractional second values.
• 2 kB static RAM powered by VBAT.
• RTC and Battery RAM power supply is isolated from the rest of the chip.
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 little power in Power-down or Deep power-down
modes. On the LPC2400, the RTC can be clocked by a separate 32.768 kHz oscillator or
by a programmable prescale divider based on the APB clock. The RTC is powered by its
own power supply pin, VBAT, which can be connected to a battery or to the same 3.3 V
supply used by the rest of the device.
The VBAT pin supplies power only to the RTC and the Battery RAM. These two functions
require a minimum of power to operate, which can be supplied by an external battery.
When the CPU and the rest of chip functions are stopped and power removed, the RTC
can supply an alarm output that can be used by external hardware to restore chip power
UM10237
Chapter 26: LPC24XX Real-Time Clock (RTC) and battery RAM
Rev. 04 — 26 August 2009 User manual