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UM10360_0
© NXP B.V. 2009. All rights reserved.
User manual
Rev. 00.06 — 5 June 2009
57 of 808
NXP Semiconductors
UM10360
Chapter 4: LPC17xx Clocking and power control
10.
External clock output pin
For system test and development purposes, any one of several internal clocks may be
brought out on the CLKOUT function available on the P1.27 pin, as shown in
.
Clocks that may be observed via CLKOUT are the CPU clock (cclk), the main oscillator
(osc_clk), the internal RC oscillator (irc_osc), the USB clock (usb_clk), and the RTC clock
(rtc_clk).
10.1 Clock Output Configuration register (CLKOUTCFG - 0x400F C1C8)
The CLKOUTCFG register controls the selection of the internal clock that appears on the
CLKOUT pin and allows dividing the clock by an integer value up to 16. The divider can be
used to produce a system clock that is related to one of the on-chip clocks. For most clock
sources, the division may be by 1. When the CPU clock is selected and is higher than
approximately 50 MHz, the output must be divided in order to bring the frequency within
the ability of the pin to switch with reasonable logic levels.
Note: The CLKOUT multiplexer is designed to switch cleanly, without glitches, between
the possible clock sources. The divider is also designed to allow changing the divide value
without glitches.
Fig 11. CLKOUT selection
CLKOUTCFG[3:0]
rtc_clk
irc_osc
osc_clk
usb_clk
cclk
CLKOUT
Divider
CLKOUTCFG[7:4]
010
001
011
100
000
Clock Enable
Syncronizer
CLKOUTCFG[8]
CLKOUT
CLKOUTCFG[9]
Table 47.
Clock Output Configuration register (CLKOUTCFG - 0x400F C1C8) bit description
Bit
Symbol
Value Description
Reset
value
3:0
CLKOUTSEL
Selects the clock source for the CLKOUT function.
0
0000
Selects the CPU clock as the CLKOUT source.
0001
Selects the main oscillator as the CLKOUT source.
0010
Selects the Internal RC oscillator as the CLKOUT source
(default).
0011
Selects the USB clock as the CLKOUT source.
0100
Selects the RTC oscillator as the CLKOUT source.
others Reserved, do not use these settings.