LPC2131 datasheet(36/45 Pages) PHILIPS

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LPC2131_32_34_36_38

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Product data sheet

Rev. 5.1 — 29 July 2011

36 of 45

NXP Semiconductors

LPC2131/32/34/36/38

Single-chip 16/32-bit microcontrollers

12. Application information

12.1 Crystal oscillator XTAL input and component selection

The input voltage to the on-chip oscillators is limited to 1.8 V. If the oscillator is driven by a

clock in slave mode, it is recommended that the input be coupled through a capacitor with

Ci = 100 pF. To limit the input voltage to the specified range, choose an additional

capacitor to ground Cg which attenuates the input voltage by a factor Ci / (Ci + Cg). In

slave mode, a minimum of 200 mV (RMS) is needed.

In slave mode the input clock signal should be coupled by means of a capacitor of 100 pF

(Figure 13), with an amplitude between 200 mV (RMS) and 1000 mV (RMS). This

corresponds to a square wave signal with a signal swing of between 280 mV and 1.4 V.

The XTAL2 pin in this configuration can be left unconnected.

External components and models used in oscillation mode are shown in Figure 14 and in

Table 10 and Table 11. Since the feedback resistance is integrated on chip, only a crystal

and the capacitances CX1 and CX2 need to be connected externally in case of

fundamental mode oscillation (the fundamental frequency is represented by L, CL and

RS). Capacitance CP in Figure 14 represents the parallel package capacitance and should

not be larger than 7 pF. Parameters FOSC, CL, RS and CP are supplied by the crystal

manufacturer.

Fig 13. Slave mode operation of the on-chip oscillator

LPC2xxx

XTAL1

100 pF

002aae718

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