User Manual

UM10360 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2010. All rights reserved.
User manual Rev. 2 — 19 August 2010 66 of 840
NXP Semiconductors
UM10360
Chapter 4: LPC17xx Clocking and power control
4.10 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 Figure 12
.
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).
4.10.1 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 12. 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.
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.