LPC55S2X datasheet(111/140 Pages) NXP

111 / 140 page

LPC55S2x/LPC552x

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Product data sheet

Rev. 2.2 — 8 December 2022

111 of 139

NXP Semiconductors

LPC55S2x/LPC552x

32-bit ARM Cortex-M33 microcontroller

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 22 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

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.

• Ensure that the load capacitors have a common ground plane.

Fig 28. RTC oscillator components

aaa-0181472-x

LPC

XTAL32K_P

XTAL32K_N

CX2

CX1

XTAL

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