LPC2131 datasheet(38/45 Pages) PHILIPS

38 / 45 page

LPC2131_32_34_36_38

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Product data sheet

Rev. 5.1 — 29 July 2011

38 of 45

NXP Semiconductors

LPC2131/32/34/36/38

Single-chip 16/32-bit microcontrollers

12.2 RTC 32 kHz oscillator component selection

The RTC external oscillator circuit is shown in Figure 15. 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 12 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 12

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.

12.3 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.

Fig 15. RTC oscillator modes and models: oscillation mode of operation and external

crystal model used for CX1/CX2 evaluation

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

002aaf495

LPC2xxx

RTCX1

RTCX2

CX2

CX1

32 kHz XTAL

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