Datasheet

LPC15XX All information provided in this document is subject to legal disclaimers. © NXP B.V. 2014. All rights reserved.
Product data sheet Rev. 1 — 19 February 2014 87 of 99
NXP Semiconductors
LPC15xx
32-bit ARM Cortex-M3 microcontroller
14.3 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.
5 MHz to 10 MHz 10 pF < 300  18 pF, 18 pF
20 pF < 200  39 pF, 39 pF
30 pF < 100  57 pF, 57 pF
10 MHz to 15 MHz 10 pF < 160  18 pF, 18 pF
20 pF < 60  39 pF, 39 pF
15 MHz to 20 MHz 10 pF < 80  18 pF, 18 pF
Table 33. 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
Table 32. Recommended values for C
X1
/C
X2
in oscillation mode (crystal and external
components parameters) low 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