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UM10375
All information provided in this document is subject to legal disclaimers.
© NXP B.V. 2011. All rights reserved.
User manual
Rev. 3 — 14 June 2011
29 of 368
NXP Semiconductors
UM10375
Chapter 3: LPC13xx System configuration
Remark:
When switching clock sources, both clocks must be running before the clock
source is updated. The default clock source for the USB controller is the USB PLL output.
For switching the clock source to the main clock, ensure that the system PLL and the USB
PLL are running to make both clock sources available for switching. The main clock must
be set to 48 MHz and configured with the main PLL and the system oscillator. After the
switch, the USB PLL can be turned off.
3.5.25 USB clock source update enable register
This register updates the clock source of the USB with the new input clock after the
USBCLKSEL register has been written to. In order for the update to take effect, first write
a zero to the USBCLKUEN register and then write a one to USBCLKUEN.
Remark:
When switching clock sources, both clocks must be running before the clock
source is updated.
3.5.26 USB clock divider register
This register allows the USB clock usb_clk to be divided to 48 MHz. The usb_clk can be
shut down by setting the DIV bits to 0x0.
Table 31.
USB clock source select register (USBCLKSEL, address 0x4004 80C0) bit
description
Bit
Symbol
Value
Description
Reset
value
1:0
SEL
USB clock source
0x00
0x0
USB PLL out
0x1
Main clock
0x2
Reserved
0x3
Reserved
31:2
-
-
Reserved
0x00
Table 32.
USB clock source update enable register (USBCLKUEN, address 0x4004 80C4) bit
description
Bit
Symbol
Value
Description
Reset value
0
ENA
Enable USB clock source update
0x0
0
No change
1
Update clock source
31:1
-
-
Reserved
0x00
Table 33.
USB clock divider register (USBCLKDIV, address 0x4004 80C8) bit description
Bit
Symbol
Description
Reset value
7:0
DIV
USB clock divider values.
0: Disable USB clock.
1: Divide by 1.
to
255: Divide by 255.
0x00
31:8
-
Reserved
0x00