LPC2142 datasheet(38/45 Pages) NXP

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LPC2141_42_44_46_48

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Product data sheet

Rev. 5 — 12 August 2011

38 of 45

NXP Semiconductors

LPC2141/42/44/46/48

Single-chip 16-bit/32-bit microcontrollers

12.3 RTC 32 kHz oscillator component selection

The RTC external oscillator circuit is shown in Figure 14. 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

5 MHz to 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 to 15 MHz

10 pF

< 160



18 pF, 18 pF

20 pF

< 60



39 pF, 39 pF

15 MHz to 20 MHz

10 pF

< 80



18 pF, 18 pF

Table 11.

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 to 20 MHz

10 pF

< 180



18 pF, 18 pF

20 pF

< 100



39 pF, 39 pF

20 MHz to 25 MHz

10 pF

< 160



18 pF, 18 pF

20 pF

< 80



39 pF, 39 pF

Table 10.

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

Fig 14. RTC oscillator modes and models: oscillation mode of operation and external

crystal model used for CX1/CX2 evaluation

002aaf495

LPC2xxx

RTCX1

RTCX2

CX2

CX1

32 kHz XTAL

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