Datasheet

LPC2131_32_34_36_38 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2011. All rights reserved.
Product data sheet Rev. 5.1 — 29 July 2011 36 of 45
NXP Semiconductors
LPC2131/32/34/36/38
Single-chip 16/32-bit microcontrollers
12. Application information
12.1 Crystal oscillator XTAL input and component selection
The input voltage to the on-chip oscillators is limited to 1.8 V. If the oscillator is driven by a
clock in slave mode, it is recommended that the input be coupled through a capacitor with
C
i
= 100 pF. To limit the input voltage to the specified range, choose an additional
capacitor to ground C
g
which attenuates the input voltage by a factor C
i
/ (C
i
+ C
g
). In
slave mode, a minimum of 200 mV (RMS) is needed.
In slave mode the input clock signal should be coupled by means of a capacitor of 100 pF
(Figure 13
), with an amplitude between 200 mV (RMS) and 1000 mV (RMS). This
corresponds to a square wave signal with a signal swing of between 280 mV and 1.4 V.
The XTAL2 pin in this configuration can be left unconnected.
External components and models used in oscillation mode are shown in Figure 14
and in
Table 10
and Table 11. 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 14 represents the parallel package capacitance and should
not be larger than 7 pF. Parameters F
OSC
, C
L
, R
S
and C
P
are supplied by the crystal
manufacturer.
Fig 13. Slave mode operation of the on-chip oscillator
LPC2xxx
XTAL1
C
i
100 pF
C
g
002aae718