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UM10360_0
© NXP B.V. 2009. All rights reserved.
User manual
Rev. 00.06 — 5 June 2009
28 of 808
NXP Semiconductors
UM10360
Chapter 4: LPC17xx Clocking and power control
Since chip operation always begins using the Internal RC Oscillator, and the main
oscillator may not be used at all 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
. The main oscillator provides a status flag (the OSCSTAT bit in the SCS
register) so that software can determine when the oscillator is running and stable. At that
point, software can control switching to the main oscillator as a clock source. Prior to
starting the main oscillator, a frequency range must be selected by configuring the
OSCRANGE bit in the SCS register.
3.3 RTC oscillator
The RTC oscillator provides a 1 Hz clock to the RTC and a 32 kHz clock output that can
be used as the clock source for PLL0 and CPU and/or the watchdog timer.
4.
Clock source selection multiplexer
Several clock sources may be chosen to drive PLL0 and ultimately the CPU and on-chip
peripheral devices. The clock sources available are the main oscillator, the RTC oscillator,
and the Internal RC oscillator.
The clock source selection can only be changed safely when PLL0 is not connected. For a
detailed description of how to change the clock source in a system using PLL0 see
Section 4–5.13 “PLL0 setup sequence”
Note the following restrictions regarding the choice of clock sources:
Table 15.
Recommended values for C
X1/X2
in oscillation mode (crystal and external
components parameters) low frequency mode (OSCRANGE = 0, see
Fundamental
oscillation frequency
F
OSC
Crystal load
capacitance C
L
Maximum crystal
series resistance R
S
External load
capacitors C
X1
,
CX2
1 MHz - 5 MHz
10 pF
< 300
Ω
18 pF, 18 pF
20 pF
< 300
Ω
39 pF, 39 pF
30 pF
< 300
Ω
57 pF, 57 pF
5 MHz - 10 MHz
10 pF
< 300
Ω
18 pF, 18 pF
20 pF
< 200
Ω
39 pF, 39 pF
30 pF
< 100
Ω
57 pF, 57 pF
10 MHz - 15 MHz
10 pF
< 160
Ω
18 pF, 18 pF
20 pF
< 60
Ω
39 pF, 39 pF
15 MHz - 20 MHz
10 pF
< 80
Ω
18 pF, 18 pF
Table 16.
Recommended values for C
X1/X2
in oscillation mode (crystal and external
components parameters) high frequency mode (OSCRANGE = 1, see
Fundamental
oscillation frequency
F
OSC
Crystal load
capacitance C
L
Maximum crystal
series resistance R
S
External load
capacitors C
X1
,
CX2
15 MHz - 20 MHz
10 pF
< 180
Ω
18 pF, 18 pF
20 pF
< 100
Ω
39 pF, 39 pF
20 MHz - 25 MHz
10 pF
< 160
Ω
18 pF, 18 pF
20 pF
< 80
Ω
39 pF, 39 pF