Figure 5. Reference oscillator circuit
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:
• C
L
: Crystal load capacitance
• C
Pad
: Pad capacitance of the XTAL32K_P and XTAL32K_N pins (~3 pF)
• C
Prasitic
: Parasitic or stray capacitance of external circuit
Although C
Parasitic
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.
4.3.1 RTC 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.
NXP Semiconductors
Clock circuitry
Hardware Design Guidelines for LPC55(S)xx Microcontrollers, Rev. 0, 30 October 2020
Application Note 9 / 24
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