LPC2142 datasheet(39/45 Pages) NXP

39 / 45 page

LPC2141_42_44_46_48

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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 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.

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 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 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 Cx1 and Cx2 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 CX1/CX2 components

Crystal load capacitance

Maximum crystal series

resistance RS

External load capacitors CX1/CX2

11 pF

< 100 k



18 pF, 18 pF

13 pF

< 100 k



22 pF, 22 pF

15 pF

< 100 k



27 pF, 27 pF

LPC2142 Datasheet(PDF) 39 Page - NXP Semiconductors