LPC81XM_13 datasheet(57/71 Pages) NXP

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LPC81xM

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Product data sheet

Rev. 3 — 29 July 2013

57 of 71

NXP Semiconductors

LPC81xM

32-bit ARM Cortex-M0+ microcontroller

14.2 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

Fig 35. Oscillator modes and models: oscillation mode of operation and external crystal

model used for CX1/CX2 evaluation

Table 24.

Recommended values for CX1/CX2 in oscillation mode (crystal and external

components parameters) low frequency mode

Fundamental oscillation

frequency FOSC

Crystal load

capacitance CL

Maximum crystal

series resistance RS

External load

capacitors CX1, CX2

1 MHz - 5 MHz

10 pF

< 300



18 pF, 18 pF

20 pF

< 300



39 pF, 39 pF

30 pF

< 300



57 pF, 57 pF

5 MHz - 10 MHz

10 pF

< 300



18 pF, 18 pF

20 pF

< 200



39 pF, 39 pF

30 pF

< 100



57 pF, 57 pF

10 MHz - 15 MHz

10 pF

< 160



18 pF, 18 pF

20 pF

< 60



39 pF, 39 pF

15 MHz - 20 MHz

10 pF

< 80



18 pF, 18 pF

Table 25.

Recommended values for CX1/CX2 in oscillation mode (crystal and external

components parameters) high frequency mode

Fundamental oscillation

frequency FOSC

Crystal load

capacitance CL

Maximum crystal

series resistance RS

External load

capacitors CX1, CX2

15 MHz - 20 MHz

10 pF

< 180



18 pF, 18 pF

20 pF

< 100



39 pF, 39 pF

20 MHz - 25 MHz

10 pF

< 160



18 pF, 18 pF

20 pF

< 80



39 pF, 39 pF

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