Datasheet

LPC55S6x All information provided in this document is subject to legal disclaimers. © NXP Semiconductors N.V. 2019. All rights reserved.
Product data sheet Rev. 1.0 — 26 February 2019 108 of 123
NXP Semiconductors
LPC55S6x
32-bit ARM Cortex-M33 microcontroller
external loads connected to the pin.
I/O pins also contribute to the dynamic power consumption when the pins are switching
because the V
DD
supply provides the current to charge and discharge all internal and
external capacitive loads connected to the pin in addition to powering the I/O circuitry.
The contribution from the I/O switching current I
sw
can be calculated as follows for any
given switching frequency f
sw
if the external capacitive load (C
ext
) is known (see Table 21
for the internal I/O capacitance):
I
sw
= V
DD
x f
sw
x (C
io
+ C
ext
)
13.4 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 32.
An external clock can be connected to RTCX1 if RTCX2 is left open. The recommended
amplitude of the clock signal is V
i(RMS)
= 100 mV to 200 mV (<450 mV) with a coupling
capacitance of 5 pF to 10 pF.
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.
Fig 32. RTC oscillator components <tbd>
aaa-018147-x
LPC556x
RTCXIN RTCXOUT
C
X2
C
X1
XTAL
=
C
L
C
P
R
S
L