LPC5411x All information provided in this document is subject to legal disclaimers. © NXP Semiconductors N.V. 2018. All rights reserved.
Product data sheet Rev. 2.1 — 9 May 2018 90 of 105
NXP Semiconductors
LPC5411x
32-bit ARM Cortex-M4/M0+ microcontroller
13.5 RTC oscillator
In the RTC oscillator circuit, only the crystal (XTAL) and the capacitances C
X1
and C
X2
need to be connected externally on RTCXIN and RTCXOUT. See Figure 33
.
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 C
X1
and C
X2
values can also be generally determined by the
following expression:
C
X1
= C
X2
= 2C
L
(C
Pad
+ C
Parasitic
)
Where:
C
L
- Crystal load capacitance
C
Pad
- Pad capacitance of the RTCXIN and RTCXOUT pins (~3 pF).
C
Parasitic
– 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.
Fig 33. RTC oscillator components
aaa-018147
LPC5411x
RTCXIN RTCXOUT
C
X2
C
X1
XTAL
=
C
L
C
P
R
S
L
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