Datasheet

LPC2388_0 © NXP B.V. 2007. All rights reserved.
Preliminary data sheet Rev. 00.01 — 23 October 2007 31 of 57
NXP Semiconductors
LPC2388
Fast communication chip
• 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 LPC2388 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 LPC2388 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 LPC2388 uses the IRC as the clock source.
Software may later switch to one of the other available clock sources.
7.25.1.2 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 24 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 7.25.2 for additional information.
7.25.1.3 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.
7.25.2 PLL
The PLL accepts an input clock frequency in the range of 32 kHz to 50 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.
The PLL input, in the range of 32 kHz to 50 MHz, may initially be divided down by a value
‘N’, which may be in the range of 1 to 256. This input division provides a wide range of
output frequencies from the same input frequency.