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UM10360_0
© NXP B.V. 2009. All rights reserved.
User manual
Rev. 00.06 — 5 June 2009
30 of 808
NXP Semiconductors
UM10360
Chapter 4: LPC17xx Clocking and power control
There are additional dividers at the output of PLL0 to bring the frequency down to what is
needed for the CPU, peripherals, and potentially the USB subsystem. PLL0 output
dividers are described in the Clock Dividers section following the PLL0 description. A
block diagram of PLL0 is shown in
PLL activation is controlled via the PLL0CON register. PLL0 multiplier and divider values
are controlled by the PLL0CFG register. These two registers are protected in order to
prevent accidental alteration of PLL0 parameters or deactivation of the PLL. Since all chip
operations, including the Watchdog Timer, could be dependent on PLL0 if so configured
(for example when it is providing the chip clock), accidental changes to the PLL0 setup
values could result in unexpected or fatal behavior of the microcontroller. The protection is
accomplished by a feed sequence similar to that of the Watchdog Timer. Details are
provided in the description of the PLL0FEED register.
PLL0 is turned off and bypassed following a chip Reset and by entering Power-down
mode. PLL0 must be configured, enabled, and connected to the system by software.
It is important that the setup procedure described in
is followed or PLL0 might not operate at all!
5.1.1 PLL0 and startup/boot code interaction
When there is no valid user code (determined by the checksum word) in the user flash or
the ISP enable pin (P2.10) is pulled low on startup, the ISP mode will be entered and the
boot code will setup the PLL with the IRC. Therefore it can not be assumed that the PLL is
disabled when the user opens a debug session to debug the application code. The user
startup code must follow the steps described in this chapter to disconnect the PLL.
5.2 PLL0 register description
PLL0 is controlled by the registers shown in
. More detailed descriptions follow.
Warning: Improper setting of PLL0 values may result in incorrect operation of the
device!
Table 18.
PLL0 registers
Name
Description
Access Reset
value
Address
PLL0CON
PLL0 Control Register. Holding register for
updating PLL0 control bits. Values written to this
register do not take effect until a valid PLL0 feed
sequence has taken place.
R/W
0
0x400F C080