Datasheet

LPC4370 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
Product data sheet Rev. 2 — 21 October 2013 132 of 150
NXP Semiconductors
LPC4370
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.
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.
13.5 Standard I/O pin configuration
Figure 42 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 with pull-up enabled. The weak
MOS devices provide a drive capability equivalent to pull-up and pull-down resistors.
Fig 41. RTC 32 kHz oscillator circuit
002aah148
LPC43xx
RTCX1 RTCX2
C
RTCX2
C
RTCX1
XTAL