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UM10360_0
© NXP B.V. 2009. All rights reserved.
User manual
Rev. 00.06 — 5 June 2009
26 of 808
NXP Semiconductors
UM10360
Chapter 4: LPC17xx Clocking and power control
3.
Oscillators
The LPC17xx includes three independent oscillators. These are the Main Oscillator, the
Internal RC Oscillator, and the RTC oscillator. Each oscillator can be used for more than
one purpose as required in a particular application. This can be seen in
.
Following Reset, the LPC17xx will operate from the Internal RC Oscillator until switched
by software. This allows systems to operate without any external crystal, and allows the
boot loader code to operate at a known frequency.
3.1 Internal RC oscillator
The Internal RC Oscillator (IRC) may be used as the clock source for the watchdog timer,
and/or as the clock that drives PLL0 and subsequently the CPU. The precision of the IRC
does not allow for use of the USB interface, which requires a much more precise time
base in order to comply with the USB specification. Also, the IRC should not be used with
the CAN1/2 block if the CAN baud rate is higher than 100 kbit/s.The nominal IRC
frequency is 4 MHz.
Upon power-up or any chip reset, the LPC17xx uses the IRC as the clock source.
Software may later switch to one of the other available clock sources.
Table 14.
Summary of system control registers
Name
Description
Access
Reset value Address
Clock source selection
CLKSRCSEL
Clock Source Select Register
R/W
0
0x400F C10C
Phase Locked Loop
(PLL0, Main PLL)
PLL0CON
PLL0 Control Register
R/W
0
0x400F C080
PLL0CFG
PLL0 Configuration Register
R/W
0
0x400F C084
PLL0STAT
PLL0 Status Register
RO
0
0x400F C088
PLL0FEED
PLL0 Feed Register
WO
NA
0x400F C08C
Phase Locked Loop (PLL1, USB PLL)
PLL1CON
PLL1 Control Register
R/W
0
0x400F C0A0
PLL1CFG
PLL1 Configuration Register
R/W
0
0x400F C0A4
PLL1STAT
PLL1 Status Register
RO
0
0x400F C0A8
PLL1FEED
PLL1 Feed Register
WO
NA
0x400F C0AC
Clock dividers
CCLKCFG
CPU Clock Configuration Register
R/W
0
0x400F C104
USBCLKCFG
USB Clock Configuration Register
R/W
0
0x400F C108
IRCTRIM
IRC Trim Register
R/W
0xA0
0x400F C1A4
PCLKSEL0
Peripheral Clock Selection register 0.
R/W
0
0x400F C1A8
PCLKSEL1
Peripheral Clock Selection register 1.
R/W
0
0x400F C1AC
Power control
PCON
Power Control Register
R/W
0
0x400F C0C0
PCONP
Power Control for Peripherals Register
R/W
0x03BE
0x400F C0C4
Utility
CLKOUTCFG
Clock Output Configuration Register
R/W
0
0x400F C1C8