LPC2361_62 NXP Semiconductors, LPC2361_62 Datasheet - Page 54

• Manufacturer Part Number: LPC2361_62
• Description: The LPC2361/2362 microcontrollers are based on a 16-bit/32-bit ARM7TDMI-S CPU with real-time emulation that combines the microcontroller with up to 128 kB of embedded high-speed flash memory
• Manufacturer: NXP Semiconductors

NXP Semiconductors
LPC2361_62
Product data sheet
14.3 RTC 32 kHz oscillator component selection
14.4 XTAL and RTCX Printed Circuit Board (PCB) layout guidelines
The RTC external oscillator circuit is shown in
integrated on chip, only a crystal, the capacitances C
externally to the microcontroller.
Table 17
capacitance of the crystal and is usually specified by the crystal manufacturer. The actual
C
different load capacitance, the circuit will oscillate at a slightly different frequency
(depending on the quality of the crystal) compared to the specified one. Therefore for an
accurate time reference it is advised to use the load capacitors as specified in
that belong to a specific C
this table are calculated from the internal parasitic capacitances and the C
from PCB and package are not taken into account.
Table 17.
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
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
accordingly to the increase in parasitics of the PCB layout.
Crystal load capacitance
C
11 pF
13 pF
15 pF
Fig 24. RTC oscillator modes and models: oscillation mode of operation and external
L
L
influences oscillation frequency. When using a crystal that is manufactured for a
gives the crystal parameters that should be used. C
crystal model used for C
Recommended values for the RTC external 32 kHz oscillator C
All information provided in this document is subject to legal disclaimers.
RTCX1
C X1
LPC2xxx
32 kHz XTAL
Rev. 5 — 25 October 2011
L
Maximum crystal series
resistance R
< 100 k
< 100 k
< 100 k
. The value of external capacitances C
RTCX2
C X2
X1
/C
X2
S
evaluation
x1
Figure
and C
X1
=
24. Since the feedback resistance is
x2
External load capacitors C
18 pF, 18 pF
22 pF, 22 pF
27 pF, 27 pF
and C
should be chosen smaller
Single-chip 16-bit/32-bit MCU
L
X2
is the typical load
x1
LPC2361/62
need to be connected
, C
L
C L
R S
X1
x2
and C
002aaf495
, and C
© NXP B.V. 2011. All rights reserved.
X1
C P
/C
L
X2
. Parasitics
X2
specified in
x3
components
Table 17
in case of
X1
/C
54 of 64
X2