LPC55S6X datasheet(102/128 Pages) NXP

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LPC55S6x

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Product data sheet

Rev. 1.9 — 10 February 2020

102 of 128

NXP Semiconductors

LPC55S6x

32-bit ARM Cortex-M33 microcontroller

CX1 = CX2 = 2CL  (CPad + CParasitic)

Where:

CL - Crystal load capacitance

CPad - Pad capacitance of the XTAL32M_P and XTAL32M_N pins (~3 pF).

CParasitic – Parasitic or stray capacitance of external circuit.

Although CParasitic can be ignored in general, the actual board layout and placement of

external components influences the optimal values of external load capacitors. Therefore,

it is recommended to fine tune the values of external load capacitors on actual hardware

board to get the accurate clock frequency. For fine tuning, output the RTC Clock to one of

the GPIOs and optimize the values of external load capacitors for minimum frequency

deviation.

13.2.1 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 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.

13.3 RTC oscillator

In the RTC oscillator circuit, only the 32.768 kHz crystal (XTAL) and the capacitances CX1

and CX2 need to be connected externally on XTAL32K_P and XTAL32K_N. See

Figure 24.

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

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