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| LPC2468_11 Datasheet, PDF (75/85 Pages) NXP Semiconductors – Single-chip 16-bit/32-bit micro; 512 kB flash, Ethernet, CAN, ISP/IAP, USB 2.0 device/host/OTG, external memory interface | |||
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NXP Semiconductors
LPC2468
Single-chip 16-bit/32-bit micro
Table 21. Recommended values for CX1/CX2 in oscillation mode (crystal and external
components parameters): high frequency mode
Fundamental oscillation Crystal load
frequency FOSC
capacitance CL
Maximum crystal
series resistance RS
External load
capacitors CX1, CX2
15 MHz to 20 MHz
10 pF
< 180 ï
18 pF, 18 pF
20 pF
< 100 ï
39 pF, 39 pF
20 MHz to 25 MHz
10 pF
< 160 ï
18 pF, 18 pF
20 pF
< 80 ï
39 pF, 39 pF
14.3 RTC 32 kHz oscillator component selection
LPC2xxx
RTCX1
RTCX2
32 kHz XTAL
CX1
CX2
L
=
CL
CP
RS
002aaf495
Fig 29. RTC oscillator modes and models: oscillation mode of operation and external
crystal model used for CX1/CX2 evaluation
The RTC external oscillator circuit is shown in Figure 29. Since the feedback resistance is
integrated on chip, only a crystal, the capacitances CX1 and CX2 need to be connected
externally to the microcontroller.
Table 22 gives the crystal parameters that should be used. CL is the typical load
capacitance of the crystal and is usually specified by the crystal manufacturer. The actual
CL influences oscillation frequency. When using a crystal that is manufactured for a
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 Table 22
that belong to a specific CL. The value of external capacitances CX1 and CX2 specified in
this table are calculated from the internal parasitic capacitances and the CL. Parasitics
from PCB and package are not taken into account.
Table 22. Recommended values for the RTC external 32 kHz oscillator CX1/CX2 components
Crystal load capacitance Maximum crystal series
CL
resistance RS
11 pF
< 100 kï
External load capacitors CX1/CX2
18 pF, 18 pF
13 pF
< 100 kï
22 pF, 22 pF
15 pF
< 100 kï
27 pF, 27 pF
LPC2468
Product data sheet
All information provided in this document is subject to legal disclaimers.
Rev. 6.1 â 6 September 2011
© NXP B.V. 2011. All rights reserved.
75 of 85
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