LPC540xx
NXP Semiconductors
32-bit ARM Cortex-M4 microcontroller
13.6 XTAL oscillator
In the XTAL oscillator circuit, only the crystal (XTAL) and the capacitances CX1 and CX2
need to be connected externally on XTALIN and XTALOUT. See Figure 48.
LPCxxxx
L
XTALIN
XTALOUT
C
R
C
P
=
L
XTAL
S
C
C
X2
X1
aaa-025725
Fig 48. XTAL 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. After selecting the proper crystal, the
external load capacitor CX1 and CX2 values can also be generally determined by the
following expression:
C
X1 = CX2 = 2CL (CPad + CParasitic
)
Where:
CL - Crystal load capacitance
C
C
Pad - Pad capacitance of the XTALIN and XTALOUT pins (~3 pF).
Parasitic – 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, measure the clock on the
XTALOUT pin and optimize the values of external load capacitors for minimum frequency
deviation.
LPC540xx
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© NXP Semiconductors N.V. 2018. All rights reserved.
Product data sheet
Rev. 1.8 — 22 June 2018
151 of 168