UM10237_2 © NXP B.V. 2008. All rights reserved.
User manual Rev. 02 — 19 December 2008 44 of 792
NXP Semiconductors UM10237
Chapter 4: LPC24XX Clocking and power control
Since chip operation always begins using the Internal RC Oscillator, and the main oscillator may never be used in some applications, it will only be started by software request. This is accomplished by setting the OSCEN bit in the SCS register, as described in Table3–29. The main oscillator provides a status flag (the OSCSTAT bit in the SCS
Fig 13. Oscillator modes and models: a) slave mode of operation, b) oscillation mode of operation, c) external
crystal model used for CX1/X2 evaluation
LPC24xx LPC24xx
Clock
CC
CX1 CX2
CLCP
L
RS
< = >
a) b) c)
Xtal
XTAL1 XTAL2 XTAL1 XTAL2
Table 39. Recommended values for CX1/X2 in oscillation mode (crystal and external
components parameters) low frequency mode (OSCRANGE = 0, see Table3–29)
Fundamental
oscillation frequency
FOSC
Crystal load
capacitance CL
Maximum crystal
series resistance RS
External load
capacitors CX1, CX2
1MHz - 5MHz 10 pF < 300Ω18 pF, 18 pF
20 pF < 300 Ω39 pF, 39 pF
30 pF < 300 Ω57 pF, 57pF
5 MHz - 10 MHz 10 pF < 300 Ω18 pF, 18 pF
20 pF < 200 Ω39 pF, 39 pF
30 pF < 100 Ω57 pF, 57 pF
10 MHz - 15 MHz 10 pF < 160Ω18 pF, 18 pF
20 pF < 60 Ω39 pF, 39 pF
15 MHz - 20 MHz 10 pF < 80Ω18 pF, 18 pF
Table 40. Recommended values for CX1/X2 in oscillation mode (crystal and external
components parameters) high frequency mode (OSCRANGE = 1, see Table3–29)
Fundamental
oscillation frequency
FOSC
Crystal load
capacitance CL
Maximum crystal
series resistance RS
External load
capacitors CX1, CX2
15 MHz - 20 MHz 10 pF < 180Ω18 pF, 18 pF
20 pF < 100 Ω39 pF, 39 pF
20 MHz - 25 MHz 10 pF < 160Ω18 pF, 18 pF
20 pF < 80 Ω39 pF, 39 pF