Datasheet

Product data sheet Rev. 5.1 — 16 October 2013 28 of 66

NXP Semiconductors

LPC2387

Single-chip 16-bit/32-bit MCU

• 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.

• 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.

7.24 Clocking and power control

7.24.1 Crystal oscillators

The LPC2387 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 LPC2387 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.

7.24.1.1 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 LPC2387 uses the IRC as the clock source.

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

7.24.1.2 Main oscillator

The main oscillator can be used as the clock source for the CPU, with or without using the

PLL. The main oscillator operates at frequencies of 1 MHz to 25 MHz. This frequency can

be boosted to a higher frequency, up to the maximum CPU operating frequency, by the

PLL. The clock selected as the PLL input is PLLCLKIN. The ARM processor clock

frequency is referred to as CCLK elsewhere in this document. The frequencies of

PLLCLKIN and CCLK are the same value unless the PLL is active and connected. The

clock frequency for each peripheral can be selected individually and is referred to as

PCLK. Refer to Section 7.24.2

for additional information.

7.24.1.3 RTC oscillator

The RTC oscillator can be used as the clock source for the RTC and/or the WDT. Also, the

RTC oscillator can be used to drive the PLL and the CPU.

7.24.2 PLL

The PLL accepts an input clock frequency in the range of 32 kHz to 25 MHz. The input

frequency is multiplied up to a high frequency, then divided down to provide the actual

clock used by the CPU and the USB block.