Datasheet

LPC2141_42_44_46_48 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2011. All rights reserved.
Product data sheet Rev. 5 — 12 August 2011 39 of 45
NXP Semiconductors
LPC2141/42/44/46/48
Single-chip 16-bit/32-bit microcontrollers
that belong to a specific C
L
. The value of external capacitances C
X1
and C
X2
specified in
this table are calculated from the internal parasitic capacitances and the C
L
. Parasitics
from PCB and package are not taken into account.
12.4 XTAL and RTCX Printed Circuit Board (PCB) layout guidelines
The crystal should be connected on the PCB as close as possible to the oscillator input
and output pins of the chip. Take care that the load capacitors C
x1
, C
x2
, and C
x3
in case of
third overtone crystal usage have a common ground plane. The external components
must also be connected to the ground plane. Loops must be made as small as possible in
order to keep the noise coupled in via the PCB as small as possible. Also parasitics
should stay as small as possible. Values of C
x1
and C
x2
should be chosen smaller
accordingly to the increase in parasitics of the PCB layout.
Table 12. Recommended values for the RTC external 32 kHz oscillator C
X1
/C
X2
components
Crystal load capacitance
C
L
Maximum crystal series
resistance R
S
External load capacitors C
X1
/C
X2
11 pF < 100 k 18 pF, 18 pF
13 pF < 100 k 22 pF, 22 pF
15 pF < 100 k 27 pF, 27 pF