LPC540xx
NXP Semiconductors
32-bit ARM Cortex-M4 microcontroller
13.5 RTC oscillator
In the RTC oscillator circuit, only the crystal (XTAL) and the capacitances CX1 and CX2
need to be connected externally on RTCXIN and RTCXOUT. See Figure 47.
LPC
L
RTCXIN
RTCXOUT
C
R
C
P
=
L
XTAL
S
C
C
X2
X1
aaa-029083
Fig 47. RTC 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 RTCXIN and RTCXOUT 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, output the RTC Clock to the
CLOCKOUT 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
149 of 168
NXP [ NXP ]
