LPC2377 datasheet(29/55 Pages) NXP

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LPC2377_78_4

Product data sheet

Rev. 04 — 19 November 2008

29 of 55

NXP Semiconductors

LPC2377/78

Single-chip 16-bit/32-bit microcontroller

based on the APB clock. Also, 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

and resume operation.

7.24.1 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 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 mode, or

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 data SRAM powered by VBAT.

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

7.25 Clocking and power control

7.25.1 Crystal oscillators

The LPC2377/78 includes three independent oscillators. These are the Main Oscillator,

the Internal RC 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 PLL and ultimately the CPU.

Following reset, the LPC2377/78 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.

7.25.1.1

Internal RC oscillator

The IRC may be used as the clock source for the WDT, and/or as the clock that drives the

PLL and subsequently the CPU. The nominal IRC frequency is 4 MHz. The IRC is

trimmed to 1 % accuracy.

Upon power-up or any chip reset, the LPC2377/78 uses the IRC as the clock source.

Software may later switch to one of the other available clock sources.

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