Datasheet
LPC4350_30_20_10 All information provided in this document is subject to legal disclaimers. © NXP Semiconductors N.V. 2014. All rights reserved.
Product data sheet Rev. 4.2 — 18 August 2014 135 of 155
NXP Semiconductors
LPC4350/30/20/10
32-bit ARM Cortex-M4/M0 microcontroller
13.3 RTC oscillator
In the RTC oscillator circuit, only the crystal (XTAL) and the capacitances C
RTCX1
and
C
RTCX2
need to be connected externally. Typical capacitance values for C
RTCX1
and
C
RTCX2
are C
RTCX1/2
= 20 (typical) 4 pF.
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 with a coupling capacitance of
5 pF to 10 pF. V
i(RMS)
must be lower than 450 mV. See Figure 42 for a similar slave-mode
set-up that uses the crystal oscillator.
13.4 XTAL and RTCX Printed Circuit Board (PCB) layout guidelines
Connect the crystal on the PCB as close as possible to the oscillator input and output pins
of the chip. Take care that the load capacitors C
x1
, C
x2
, and C
x3
in case of third overtone
crystal usage have a common ground plane. Also connect the external components to the
ground plain. To keep the noise coupled in via the PCB as small as possible, make loops
and parasitics as small as possible. Choose smaller values of C
x1
and C
x2
if parasitics
increase in the PCB layout.
Ensure that no high-speed or high-drive signals are near the RTCX1/2 signals.
13.5 Standard I/O pin configuration
Figure 45 shows the possible pin modes for standard I/O pins with analog input function:
• Digital output driver enabled/disabled
• Digital input: Pull-up enabled/disabled
• Digital input: Pull-down enabled/disabled
• Digital input: Repeater mode enabled/disabled
• Digital input: Input buffer enabled/disabled
• Analog input
The default configuration for standard I/O pins is input buffer disabled and pull-up
enabled. The weak MOS devices provide a drive capability equivalent to pull-up and
pull-down resistors.
Fig 44. RTC 32 kHz oscillator circuit
002aah148
LPC43xx
RTCX1 RTCX2
C
RTCX2
C
RTCX1
XTAL