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UM10360_0
© NXP B.V. 2009. All rights reserved.
User manual
Rev. 00.06 — 5 June 2009
36 of 808
NXP Semiconductors
UM10360
Chapter 4: LPC17xx Clocking and power control
5.8 PLL0 Feed register (PLL0FEED - 0x400F C08C)
A correct feed sequence must be written to the PLL0FEED register in order for changes to
the PLL0CON and PLL0CFG registers to take effect. The feed sequence is:
1. Write the value 0xAA to PLL0FEED.
2. Write the value 0x55 to PLL0FEED.
The two writes must be in the correct sequence, and there must be no other register
access in the same address space (0x400F C000 to 0x400F FFFF) between them.
Because of this, it may be necessary to disable interrupts for the duration of the PLL0 feed
operation, if there is a possibility that an interrupt service routine could write to another
register in that space. If either of the feed values is incorrect, or one of the previously
mentioned conditions is not met, any changes to the PLL0CON or PLL0CFG register will
not become effective.
5.9 PLL0 and Power-down mode
Power-down mode automatically turns off and disconnects PLL0. Wake-up from
Power-down mode does not automatically restore PLL0 settings, this must be done in
software. Typically, a routine to activate PLL0, wait for lock, and then connect PLL0 can be
called at the beginning of any interrupt service routine that might be called due to the
wake-up. It is important not to attempt to restart PLL0 by simply feeding it when execution
resumes after a wake-up from Power-down mode. This would enable and connect PLL0
at the same time, before PLL lock is established.
5.10 PLL0 frequency calculation
PLL0 equations use the following parameters:
Table 23.
PLL control bit combinations
PLLC0 PLLE0 PLL Function
0
0
PLL0 is turned off and disconnected. PLL0 outputs the unmodified clock input.
0
1
PLL0 is active, but not yet connected. PLL0 can be connected after PLOCK0 is
asserted.
1
0
Same as 00 combination. This prevents the possibility of PLL0 being
connected without also being enabled.
1
1
PLL0 is active and has been connected as the system clock source.
Table 24.
PLL Feed register (PLL0FEED - address 0x400F C08C) bit description
Bit
Symbol
Description
Reset
value
7:0
PLL0FEED The PLL0 feed sequence must be written to this register in order for
PLL0 configuration and control register changes to take effect.
0x00
Table 25.
PLL frequency parameter
Parameter
Description
F
IN
the frequency of PLLCLKIN from the Clock Source Selection Multiplexer.
F
CCO
the frequency of the PLLCLK (output of the PLL Current Controlled Oscillator)