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UM10360_0
© NXP B.V. 2009. All rights reserved.
User manual
Rev. 00.06 — 5 June 2009
40 of 808
NXP Semiconductors
UM10360
Chapter 4: LPC17xx Clocking and power control
Remember that when a frequency below about 1 MHz is used as the PLL0 clock source,
not all multiplier values are available. As it turns out, all of the rounded M values found in
of this example are supported, as may be confirmed in
If PLL0 calculations suggest use of unsupported multiplier values, those values must be
disregarded and other values examined to find the best fit. Multiplier values one count off
from calculated values may also be good possibilities.
The value written to PLL0CFG for the second table entry would be 0x12254
(N - 1 = 1 = 0x1; M - 1 = 8788 = 0x2254).
5.13 PLL0 setup sequence
The following sequence must be followed step by step in order to have PLL0 initialized
and running:
1. Disconnect PLL0 with one feed sequence if PLL0 is already connected.
2. Disable PLL0 with one feed sequence.
3. Change the CPU Clock Divider setting to speed up operation without PLL0, if desired.
4. Write to the Clock Source Selection Control register to change the clock source if
needed.
5. Write to the PLL0CFG and make it effective with one feed sequence. The PLL0CFG
can only be updated when PLL0 is disabled.
6. Enable PLL0 with one feed sequence.
7. Change the CPU Clock Divider setting for the operation with PLL0. It is critical to do
this before connecting PLL0.
8. Wait for PLL0 to achieve lock by monitoring the PLOCK0 bit in the PLL0STAT register,
or using the PLOCK0 interrupt, or wait for a fixed time when the input clock to PLL0 is
slow (i.e. 32 kHz). The value of PLOCK0 may not be stable when the PLL reference
frequency (FREF, the frequency of REFCLK, which is equal to the PLL input
frequency divided by the pre-divider value) is less than 100 kHz or greater than
20 MHz. In these cases, the PLL may be assumed to be stable after a start-up time
has passed. This time is 500 µs when FREF is greater than 400 kHz and 200 / FREF
seconds when FREF is less than 400 kHz.
9. Connect PLL0 with one feed sequence.
It is very important not to merge any steps above. For example, do not update the
PLL0CFG and enable PLL0 simultaneously with the same feed sequence.
6.
PLL1 (Phase Locked Loop 1)
PLL1 receives its clock input from the main oscillator only and can be used to provide a
fixed 48 MHz clock only to the USB subsystem. This is an option in addition to the
possibility of generating the USB clock from PLL0.
PLL1 is disabled and powered off on reset. If PLL1 is left disabled, the USB clock can be
supplied by PLL0 if everything is set up to provide 48 MHz through that route. If PLL1 is
enabled and connected via the PLL1CON register (see
), it is automatically