UM10237_2 © NXP B.V. 2008. All rights reserved.
User manual Rev. 02 — 19 December 2008 661 o f 792
NXP Semiconductors UM10237
Chapter 26: LPC24XX Real-Time Clock (RTC) and battery RAM
Table26–584 gives the crystal parameters that should be used. CL is the typical load
capacitance of the crystal and is usually specified by the crystal manufacturer. The actual
CL 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
Table26–584 that belong to a specific CL. The value of external capacitances CX1 and
CX2 specified in this table are calculated from the internal parasitic capacitances and the
CL. Parasitics from PCB and package are not taken into account.
Fig 136. RTC 32 kHz crystal oscillator circuit
Table 584. Recommended values for the RTC external 32 kHz oscillator CX1/X2 components
Crystal load capacitance
CL
Maximum crystal series
resistance RS
External load capacitors CX1, CX2
11 pF < 100 kΩ18 pF, 18pF
13 pF < 100 kΩ22 pF, 22pF
15 pF < 100 kΩ27 pF, 27pF
RTCX1 RTCX2
LPC24xx
CX1 CX2
32 kHz
Xtal