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UM10360_0
© NXP B.V. 2009. All rights reserved.
User manual
Rev. 00.06 — 5 June 2009
32 of 808
NXP Semiconductors
UM10360
Chapter 4: LPC17xx Clocking and power control
PLL0 must be set up, enabled, and Lock established before it may be used as a clock
source. When switching from the oscillator clock to the PLL0 output or vice versa, internal
circuitry synchronizes the operation in order to ensure that glitches are not generated.
Hardware does not insure that PLL0 is locked before it is connected or automatically
disconnect PLL0 if lock is lost during operation. In the event of loss of lock on PLL0, it is
likely that the oscillator clock has become unstable and disconnecting PLL0 will not
remedy the situation.
5.4 PLL0 Configuration register (PLL0CFG - 0x400F C084)
The PLL0CFG register contains PLL0 multiplier and divider values. Changes to the
PLL0CFG register do not take effect until a correct PLL feed sequence has been given
(see
Section 4–5.8 “PLL0 Feed register (PLL0FEED - 0x400F C08C)”
). Calculations for
the PLL frequency, and multiplier and divider values are found in the
Table 19.
PLL Control register (PLL0CON - address 0x400F C080) bit description
Bit
Symbol
Description
Reset
value
0
PLLE0
PLL0 Enable. When one, and after a valid PLL0 feed, this bit will
activate PLL0 and allow it to lock to the requested frequency. See
PLL0STAT register,
0
1
PLLC0
PLL0 Connect. Setting PLLC0 to one after PLL0 has been enabled
and locked, then followed by a valid PLL0 feed sequence causes
PLL0 to become the clock source for the CPU, AHB peripherals, and
used to derive the clocks for APB peripherals. The PLL0 output may
potentially be used to clock the USB subsystem if the frequency is 48
MHz. See PLL0STAT register,
0
7:2
-
Reserved, user software should not write ones to reserved bits. The
value read from a reserved bit is not defined.
NA
Table 20.
PLL0 Configuration register (PLL0CFG - address 0x400F C084) bit description
Bit
Symbol
Description
Reset
value
14:0
MSEL0
PLL0 Multiplier value. Supplies the value "M" in PLL0 frequency
calculations. The value stored here is M - 1. Supported values for M
are 6 through 512 and those listed in
.
Note:
Not all values of M are needed, and therefore some are not
supported by hardware. For details on selecting values for MSEL0
see
Section 4–5.10 “PLL0 frequency calculation”
.
0
15
-
Reserved, user software should not write ones to reserved bits. The
value read from a reserved bit is not defined.
NA
23:16 NSEL0
PLL0 Pre-Divider value. Supplies the value "N" in PLL0 frequency
calculations. The value stored here is N - 1. Supported values for N
are 1 through 32.
Note:
For details on selecting the right value for NSEL0 see
4–5.10 “PLL0 frequency calculation”
0
31:24 -
Reserved, user software should not write ones to reserved bits. The
value read from a reserved bit is not defined.
NA