© Koninklijke Philips Electronics N.V. 2006. All rights reserved.
User manual
Rev. 01 — 12 January 2006
224
Philips Semiconductors
UM10161
Volume 1
Chapter 17: RTC
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
that belong to a specific C
L
. The value of external capacitances C
X1
and C
X2
specified in
this table are calculated from the internal parasitic capacitances and the C
L
. Parasitics
from PCB and package are not taken into account.
Fig 59. RTC 32 kHz crystal oscillator circuit
Table 201: Recommended values for the RTC external 32 kHz oscillator C
X1/X2
components
Crystal load capacitance
C
L
Maximum crystal series
resistance R
S
External load capacitors C
X1
,
CX2
11 pF
< 100 k
Ω
18 pF, 18 pF
13 pF
< 100 k
Ω
22 pF, 22 pF
15 pF
< 100 k
Ω
27 pF, 27 pF
RTXC1
RTXC2
LPC2101/02/03
C
X1
C
X2
32 kHz
Xtal