LPC2921 datasheet(74/84 Pages) NXP

74 / 84 page

LPC2921_23_25_3

All information provided in this document is subject to legal disclaimers.

Product data sheet

Rev. 03 — 14 April 2010

74 of 84

NXP Semiconductors

LPC2921/2923/2925

ARM9 microcontroller with CAN, LIN, and USB

10.4 XIN_OSC input

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. For more details see the LPC29xx User

manual UM10316.

10.5 XIN_OSC 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 and 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 plain. 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 31. Slave mode operation of the on-chip oscillator

LPC29xx

XIN_OSC

100 pF

002aae730

LPC2921 датащи(PDF) 74 Page - NXP Semiconductors