LPC55S2X datasheet(110/140 Pages) NXP

110 / 140 page

LPC55S2x/LPC552x

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Product data sheet

Rev. 2.2 — 8 December 2022

110 of 139

NXP Semiconductors

LPC55S2x/LPC552x

32-bit ARM Cortex-M33 microcontroller

integrated load capacitor for the crystal oscillators. The capacitor banks on each crystal

pin can tune the frequency for crystals with a Capacitive Load (CL) between 6 to 10pF

(IEC equivalent).

Simple APIs to configure the Capacitor Banks based on the crystal Capacitive Load (CL)

and measured PCB parasitic capacitances on XIN and XOUT pins.

In the crystal oscillator circuit, only the crystal (XTAL) needs to be connected with the

option to connect capacitances CX1 and CX2 on XTAL32M_P and XTAL32M_N pins.

Depending upon the computation of the required Capacitance Load, there is no need to

add some capacitance on PCB if computation is less than 20 pF (10 pF equivalent IEC),

and if computation is greater than 20 pF (10 pF equivalent IEC), then additional

capacitance is on PCB required. See Figure 27 and refer to the “Cap Bank API” chapter in

the user manual.

In bypass mode, an external clock (maximum frequency of up to 25 MHz) can also be

connected to XTAL32M_P if XTAL32M_N is left open. External [0 – VH] square signal can

be applied on the XTAL32M_P pin from 0 V to 850 mV.

For best results, it is very critical to select a matching crystal for the on-chip oscillator.

Load capacitance (CL), series resistance (RS), and drive level (DL) are important

parameters to consider while choosing the crystal.

13.3.1 Crystal 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 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.

Fig 27. Crystal oscillator components

aaa-018147-x

LPC

XTAL32M_P

XTAL32M_N

CX2

CX1

XTAL

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