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UM10360_0
© NXP B.V. 2009. All rights reserved.
User manual
Rev. 00.06 — 5 June 2009
27 of 808
NXP Semiconductors
UM10360
Chapter 4: LPC17xx Clocking and power control
3.2 Main oscillator
The main oscillator can be used as the clock source for the CPU, with or without using
PLL0. 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 main PLL (PLL0). The oscillator output is called OSC_CLK. The clock selected as the
PLL0 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 PLL0 is active and connected. Refer
to
Section 4–5 “PLL0 (Phase Locked Loop 0)”
for details.
The on-board oscillator in the LPC17xx 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
C
, drawing a), with an amplitude of at least 200 mVrms. The XTAL2 pin in
this configuration can be left unconnected.
External components and models used in oscillation mode are shown in
,
drawings b and c, and in
and
. 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
, 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.
Fig 7.
Oscillator modes and models: a) slave mode of operation, b) oscillation mode of operation, c) external
crystal model used for C
X1
/
X2
evaluation
LPC17xx
LPC17xx
Clock
C
C
C
X1
C
X2
C
L
C
P
L
R
S
< = >
a)
b)
c)
Xtal
XTAL1
XTAL2
XTAL1
XTAL2