Datasheet

LPC408X_7X All information provided in this document is subject to legal disclaimers. © NXP Semiconductors N.V. 2014. All rights reserved.
Product data sheet Rev. 3.1 — 1 September 2014 116 of 138
NXP Semiconductors
LPC408x/7x
32-bit ARM Cortex-M4 microcontroller
13.3 XTAL Printed-Circuit Board (PCB) layout guidelines
The crystal should be connected 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. The external components
must also be connected to the ground plane. Loops must be made as small as possible in
order to keep the noise coupled in via the PCB as small as possible. Also parasitics
should stay as small as possible. Smaller values of C
x1
and C
x2
should be chosen
according to the increase in parasitics of the PCB layout.
13.4 Standard I/O pin configuration
Figure 42 shows the possible pin modes for standard I/O pins with analog input function:
• Digital output driver: Open-drain mode enabled/disabled.
• Digital input: Pull-up enabled/disabled.
• Digital input: Pull-down enabled/disabled.
• Digital input: Repeater mode 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.
Table 35. Recommended values for C
X1
/C
X2
in oscillation mode (crystal and external
components parameters): high frequency mode
Fundamental oscillation
frequency F
OSC
Crystal load
capacitance C
L
Maximum crystal
series resistance R
S
External load
capacitors C
X1
, C
X2
15 MHz to 20 MHz 10 pF < 180  18 pF, 18 pF
20 pF < 100  39 pF, 39 pF
20 MHz to 25 MHz 10 pF < 160  18 pF, 18 pF
20 pF < 80  39 pF, 39 pF