LPC55S2X datasheet(101/132 Pages) NXP

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NXP Semiconductors

LPC55S2x/LPC552x

32-bit Arm Cortex®-M33 microcontroller

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.

13.4 RTC oscillator

The crystal oscillator has embedded capacitor bank where it can be used as an 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 XTAL32K_P and XTAL32K_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 23 and refer to the "Cap Bank API" chapter in the user manual.

In bypass mode, an external clock (maximum frequency of up to 100 kHz) can also be connected to

XTAL32K_P if XTAL32K_N is left open. External [0 – VH] square signal can be applied on the XTAL32K_P pin

with 1.1 V +/-10%

A external signal below 1.0 V or above 1.2 V cannot be applied

LPC

XTAL32K_P

XTAL32K_N

CX2

CX1

XTAL

Figure 29. RTC oscillator components

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.

13.4.1 RTC 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.

LPC55S2X_LPC552X

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

Data sheet

Rev. 2.3 — 29 August 2024

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