UM10208_2
© NXP B.V. 2007. All rights reserved.
User manual
Rev. 02 — 1 June 2007
55 of 362
NXP Semiconductors
UM10208
Chapter 7: LPC2800 CGU
3.2 Main PLL
The main PLL typically uses the fast (12 MHz) oscillator as its input and multiplies it up to
a clock rate at which the processor and core peripherals can operate.
shows
the block diagram of the Main PLL.
describes the registers that are related to the main PLL.
Fig 17. Main PLL Block Diagram
Table 37.
Main PLL registers
Name
Description
Access Reset
value
Address
LPFIN
Input Select Register.
This field selects the main
PLL’s input clock (CLKIN)
0000 32 kHz oscillator
0001 Fast (12 MHz) oscillator
0010 MCLKI pin
0011 BCKI pin
0100 WSI pin
0111 High Speed PLL
(values not shown are reserved and should not be
written)
R/W
0001
0x8000 4CE4
LPPDN
Power Down Register.
When bit 0 of this register is
1, as it is after a reset, the main PLL is powered down.
Write a 0 to this bit after writing the LPMSEL and
LPPSEL registers, to start the main PLL.
R/W
1
0x8000 4CE8
LPMBYP
Multiplier Bypass Register
. When bit 0 of this
register is 1, CLKIN is routed to the Post Divider, the
CCO is powered down, and the Feedback Divider and
the Phase/Frequency Comparator are not used.
R/W
0
0x8000 4CEC
LPLOCK
Lock Status.
A 1 in bit 0 of this read-only register
indicates that the main PLL has achieved
synchronization lock, so that its output can be used for
clocking.
RO
0
0x8000 4CF0
clkin
Phase/
frequency
comparator
Current
Controlled
Oscillator
(CCO)
Feedback
Divider
control
Fcco
M
u
x
LPMBYP
Post
Divider
M
u
x
LPDBYP
clkout
LPMSEL
LPPSEL