LPC1763 datasheet(68/79 Pages) NXP

68 / 79 page

LPC1769_68_67_66_65_64_63

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Product data sheet

Rev. 7 — 5 April 2011

68 of 79

NXP Semiconductors

LPC1769/68/67/66/65/64/63

32-bit ARM Cortex-M3 microcontroller

14.2 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 33), 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 XTALOUT pin in this configuration can be left unconnected.

External components and models used in oscillation mode are shown in Figure 34 and in

Table 22 and Table 23. 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 34 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 32. USB device port configuration

LPC17xx

USB-B

connector

002aad943

USB_UP_LED

USB_CONNECT

VDD

D+

−

USB_D+

USB_D

−

VBUS

VSS

Fig 33. Slave mode operation of the on-chip oscillator

LPC1xxx

XTAL1

100 pF

002aae835

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