Datasheet

Product data sheet Rev. 1.7 — 27 February 2018 78 of 97

NXP Semiconductors

LPC84x

32-bit Arm Cortex-M0+ microcontroller

14.2.1 XTAL Printed Circuit Board (PCB) design guidelines

• Connect the crystal and external load capacitors on the PCB as close as possible to

the oscillator input and output pins of the chip.

• The length of traces in the oscillation circuit should be as short as possible and must

not cross other signal lines.

• Ensure that the load capacitors CX1, CX2, and CX3, in case of third overtone crystal

usage, have a common ground plane.

• Loops must be made as small as possible to minimize the noise coupled in through

the PCB and to keep the parasitics as small as possible.

• Lay out the ground (GND) pattern under crystal unit.

• Do not lay out other signal lines under crystal unit for multi-layered PCB.

14.2.2 XTAL input

The input voltage to the on-chip oscillators is limited to 1.95 V. If the oscillator is driven by

a clock in slave mode, it is recommended to couple the input through a capacitor with C

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

ground C

which attenuates the input voltage by a factor C

/(C

+ C

). In slave mode, a

minimum of 200 mV(RMS) is needed.

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

(Figure 38

), 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.

14.3 Connecting power, clocks, and debug functions

Figure 39 shows the basic board connections used to power the LPC84x, connect the

external crystal, and provide debug capabilities via the serial wire port.

Fig 38. Slave mode operation of the on-chip oscillator

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