
DR
AFT
DR
AFT
DRAFT
DR
D
RAFT
DRAFT
DRA
FT DRAF
D
RAFT DRAFT DRAFT DRAFT DRAFT D
DRAFT
D
RAFT DRA
FT DRAFT DRAFT DRAFT DRA
UM10360_0
© NXP B.V. 2009. All rights reserved.
User manual
Rev. 00.06 — 5 June 2009
48 of 808
NXP Semiconductors
UM10360
Chapter 4: LPC17xx Clocking and power control
7.2 USB Clock Configuration register (USBCLKCFG - 0x400F C108)
This register is used only if the USB PLL (PLL1) is not connected (via the PLLC1 bit in
PLL1CON). If PLL1 is connected, its output is automatically used as the USB clock
source, and PLL1 must be configured to supply the correct 48 MHz clock to the USB
subsystem. If PLL1 is not connected, the USB subsystem will be driven by PLL0 via the
USB clock divider.
The USBCLKCFG register controls the division of the PLL0 output before it is used by the
USB subsystem.The PLL0 output must be divided in order to bring the USB clock
frequency to 48 MHz with a 50% duty cycle. A 4-bit divider allows obtaining the correct
USB clock from any even multiple of 48 MHz (i.e. any multiple of 96 MHz) within the PLL
operating range.
Remark:
The Internal RC clock should not be used to drive PLL0 when the USB is using
PLL0 as a clock source because a more precise clock is needed for USB specification
compliance (see
).
7.3 IRC Trim Register (IRCTRIM - 0x400F C1A4)
This register is used to trim the on-chip 4 MHz oscillator.
7.4 Peripheral Clock Selection registers 0 and 1 (PCLKSEL0 -
0x400F C1A8 and PCLKSEL1 - 0x400F C1AC)
A pair of bits in a Peripheral Clock Selection register controls the rate of the clock signal
that will be supplied to the corresponding peripheral as specified in
and
Remark:
The peripheral clock for the RTC block is fixed at CCLK/8.
Table 38.
USB Clock Configuration register (USBCLKCFG - address 0x400F C108) bit
description
Bit Symbol
Value
Description
Reset
value
3:0 USBSEL
Selects the divide value for creating the USB clock from the
PLL0 output. Only the values shown below can produce even
number multiples of 48 MHz from the PLL0 output.
Warning:
Improper setting of this value will result in incorrect
operation of the USB interface.
0
5
PLL0 output is divided by 6. PLL0 output must be 288 MHz.
7
PLL0 output is divided by 8. PLL0 output must be 384 MHz.
9
PLL0 output is divided by 10. PLL0 output must be 480 MHz.
7:4 -
Reserved, user software should not write ones to reserved bits.
The value read from a reserved bit is not defined.
NA
Table 39.
IRC Trim register (IRCTRIM - address 0x400F C1A4) bit description
Bit
Symbol
Description
Reset
value
7:0
IRCtrim
IRC trim value. It controls the on-chip 4 MHz IRC frequency.
0xA0
15:8
-
Reserved. Software must write 0 into these bits.
NA