lpc1759 NXP Semiconductors, lpc1759 Datasheet - Page 64

• Manufacturer Part Number: lpc1759
• Description: Lpc17xx Device Highlight
• Manufacturer: NXP Semiconductors

NXP Semiconductors
LPC1759_58_56_54_52_51_4
Product data sheet
14.3 RTC 32 kHz oscillator component selection
14.4 XTAL Printed-Circuit Board (PCB) layout guidelines
The RTC external oscillator circuit is shown in
integrated on chip, only a crystal, the capacitances C
externally to the microcontroller.
Table 21
capacitance of the crystal and is usually specified by the crystal manufacturer. The actual
C
different load capacitance, the circuit will oscillate at a slightly different frequency
(depending on the quality of the crystal) compared to the specified one. Therefore for an
accurate time reference it is advised to use the load capacitors as specified in Table
Table 21
specified in this table are calculated from the internal parasitic capacitances and the C
Parasitics from PCB and package are not taken into account.
Table 21.
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
third overtone crystal usage have a common ground plane. The external components
must also be connected to the ground plain. 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. Values of C
accordingly to the increase in parasitics of the PCB layout.
Crystal load capacitance
C
11 pF
13 pF
15 pF
Fig 33. RTC oscillator modes and models: oscillation mode of operation and external
L
L
influences oscillation frequency. When using a crystal that is manufactured for a
gives the crystal parameters that should be used. C
that belong to a specific C
crystal model used for C
Recommended values for the RTC external 32 kHz oscillator C
All information provided in this document is subject to legal disclaimers.
RTCX1
C X1
LPC1xxx
32 kHz XTAL
Rev. 04.02 — 1 April 2010
Maximum crystal series
resistance R
< 100 kΩ
< 100 kΩ
< 100 kΩ
RTCX2
C X2
X1
/C
L
. The value of external capacitances C
X2
S
LPC1759/58/56/54/52/51
evaluation
x1
Figure
and C
32-bit ARM Cortex-M3 microcontroller
X1
=
33. Since the feedback resistance is
x2
External load capacitors C
18 pF, 18 pF
22 pF, 22 pF
27 pF, 27 pF
and C
should be chosen smaller
L
X2
is the typical load
x1
need to be connected
, C
L
C L
R S
x2
002aaf427
, and C
© NXP B.V. 2010. All rights reserved.
X1
C P
/C
X1
X2
x3
and C
components
in case of
X1
/C
64 of 74
X2
X2
L
.