NXP Semiconductors LPC24XX UM10237 - Manual (Page 43)

UM10237_4 © NXP B.V. 2009. All rights reserved.
User manual Rev. 04 — 26 August 2009 43 of 792
NXP Semiconductors
UM10237
Chapter 4: LPC24XX Clocking and power control
2. Oscillators
The LPC2400 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.
Following Reset, the LPC2400 will operate from the Internal RC Oscillator until switched
by software. This allows systems to operate without any external crystal, and allows the
Boot Loader code to operate at a known frequency. When Boot Block will branch to a user
program, there could be an option to activate the main oscillator prior to entering user
code.
2.1 Internal RC oscillator
The Internal RC Oscillator (IRC) may be used as the clock source for the watchdog timer,
and/or as the clock that drives the PLL and subsequently the CPU. The precision of the
IRC does not allow for use of the USB interface, which requires a much more precise time
base. Also, do not use the IRC for the CAN1/2 block if the CAN baud rate is higher than
100 kbit/s.The nominal IRC frequency is 4 MHz.
Upon power up or any chip reset, the LPC2400 uses the IRC as the clock source.
Software may later switch to one of the other available clock sources.
2.2 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 oscillator output is called oscclk. The clock selected as the PLL input is pllclkin
and the ARM processor clock frequency is referred to as cclk for purposes of rate
equations, etc. elsewhere in this document. The frequencies of pllclkin and cclk are the
same value unless the PLL is active and connected. Refer to the PLL description in this
chapter for details.
The onboard oscillator in the LPC24xx can operate in one of two modes: slave mode and
oscillation mode.
In slave mode the input clock signal should be coupled by means of a capacitor of 100 pF
(C
i
in Figure 4–13 , drawing a), with an amplitude of at least 200 mV(RMS). The XTAL2 pin
in this configuration can be left not connected.
External components and models used in oscillation mode are shown in Figure 4–13 ,
drawings b and c, and in Table 4–39 and Table 4–40 . Since the feedback resistance is
integrated on chip, only a crystal and the capacitances C
X1
and C
X2
need to be connected
externally in case of fundamental mode oscillation (the fundamental frequency is
represented by L, C
L
and R
S
). Capacitance C
P
in Figure 4–13 , drawing c, represents the
parallel package capacitance and should not be larger than 7 pF. Parameters F
C
, C
L
, R
S
and C
P
are supplied by the crystal manufacturer.