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| LPC178X Datasheet, PDF (102/117 Pages) NXP Semiconductors – 32-bit ARM Cortex-M3 microcontroller | |||
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NXP Semiconductors
LPC178x/7x
32-bit ARM Cortex-M3 microcontroller
LPC1xxx
XTALIN
XTALOUT
XTAL
CX1
CX2
L
=
CL
CP
RS
002aaf424
Fig 39. Oscillator modes and models: oscillation mode of operation and external crystal
model used for CX1/CX2 evaluation
Table 32. Recommended values for CX1/CX2 in oscillation mode (crystal and external
components parameters): low frequency mode
Fundamental oscillation
frequency FOSC
1 MHz - 5 MHz
Crystal load
capacitance CL
10 pF
Maximum crystal
series resistance RS
< 300 ï
External load
capacitors CX1/CX2
18 pF, 18 pF
20 pF
< 300 ï
39 pF, 39 pF
30 pF
< 300 ï
57 pF, 57 pF
5 MHz - 10 MHz
10 pF
< 300 ï
18 pF, 18 pF
20 pF
< 200 ï
39 pF, 39 pF
30 pF
< 100 ï
57 pF, 57 pF
10 MHz - 15 MHz
10 pF
< 160 ï
18 pF, 18 pF
20 pF
< 60 ï
39 pF, 39 pF
15 MHz - 20 MHz
10 pF
< 80 ï
18 pF, 18 pF
Table 33. Recommended values for CX1/CX2 in oscillation mode (crystal and external
components parameters): high frequency mode
Fundamental oscillation
frequency FOSC
15 MHz - 20 MHz
Crystal load
capacitance CL
10 pF
Maximum crystal
series resistance RS
< 180 ï
External load
capacitors CX1, CX2
18 pF, 18 pF
20 pF
< 100 ï
39 pF, 39 pF
20 MHz - 25 MHz
10 pF
< 160 ï
18 pF, 18 pF
20 pF
< 80 ï
39 pF, 39 pF
14.3 XTAL 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 Cx1, Cx2, and Cx3 in case of
third overtone crystal usage have a common ground plane. The external components
must also be connected to the ground plain. Loops must be made as small as possible in
LPC178X_7X
Objective data sheet
All information provided in this document is subject to legal disclaimers.
Rev. 2 â 27 May 2011
© NXP B.V. 2011. All rights reserved.
102 of 117
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