LPC2141_42_44_46_48 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2011. All rights reserved.
Product data sheet Rev. 5 — 12 August 2011 38 of 45
NXP Semiconductors
LPC2141/42/44/46/48
Single-chip 16-bit/32-bit microcontrollers
12.3 RTC 32 kHz oscillator component selection
The RTC external oscillator circuit is shown in Figure 14. Since the feedback resistance is
integrated on chip, only a crystal, the capacitances C
X1
and C
X2
need to be connected
externally to the microcontroller.
Table 12
gives the crystal parameters that should be used. C
L
is the typical load
capacitance of the crystal and is usually specified by the crystal manufacturer. The actual
C
L
influences oscillation frequency. When using a crystal that is manufactured for a
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 12
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 11. 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 10. 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
Fig 14. RTC oscillator modes and models: oscillation mode of operation and external
crystal model used for C
X1
/C
X2
evaluation
002aaf495
LPC2xxx
RTCX1 RTCX2
C
X2
C
X1
32 kHz XTAL
=
C
L
C
P
R
S
L