NXP Semiconductors LPC550x, Hardware Design Manuallines

Page: 7 / 24

4.2 Crystal oscillator
In the crystal oscillator circuit, only the crystal (XTAL) and the capacitances, CX1 and CX2, need to be connected externally on
XTAL32M_P
and
XTAL32M_N
pins, as shown in Figure 4 . In bypass mode, an external clock (maximum frequency of up to 24 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.
Figure 4. Reference oscillator circuit
Table 7. Components of the oscillator circuit
Symbol Description
XTAL Quartz crystal/ceramic resonator
CX1 Stabilizing capacitor
CX2 Stabilizing capacitor
For best results, it is very critical to select a matching crystal for the on-chip oscillator. Load Capacitance (CL), Resistance Series
(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:
Where:
NXP Semiconductors
Clock circuitry
Hardware Design Guidelines for LPC55(S)xx Microcontrollers, Rev. 0, 30 October 2020
Application Note 7 / 24