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UM10316_0
© NXP B.V. 2008. All rights reserved.
User manual
Rev. 00.06 — 17 December 2008
39 of 571
NXP Semiconductors
UM10316
Chapter 3: LPC29xx Clock Generation Unit (CGU)
Feedback-divider ratio programming
The feedback-divider division ratio is controlled by MSEL[4:0] in the PLL_CONTROL
register. The division ratio between the PLL output clock and the input clock is the decimal
value on MSEL[4:0] plus one.
Frequency selection, mode 1 (normal mode)
In this mode the post-divider is enabled, giving a 50% duty cycle clock with the frequency
relations described below:
The output frequency of the PLL is given by the following equation:
To select the appropriate values for M and P:
1. Specify the input clock frequency f
clkin
2. Calculate M to obtain the desired output frequency f
clkout PLL
with M = f
clkout
/f
clkin
3. Find a value for P so that f
cco
= 2
×
P
×
f
clkout
4. Verify that all frequencies and divider values conform to the limits specified.
Frequency selection, mode 2 (direct CCO mode)
In this mode the post-divider is bypassed and the CCO clock is sent directly to the
output(s), leading to the following frequency equation:
To select the appropriate values for M and P:
1. Specify the input clock frequency f
clkin
2. Calculate M to obtain the desired output frequency f
clkout
with M = f
clkout
/f
clkin
3. Verify that all frequencies and divider values conform to the limits specified.
Note that although the post-divider is not used, it still runs in this mode. To reduce current
consumption to the lowest possible value it is recommended to set PSEL[1:0] to ’00’. This
sets the post-divider to divide by two, which causes it to consume the least amount of
current.
fclkoutPLL
Mfclkin
fcco
2 P
⋅
(
)
----------------
=
=
fclkout
Mfclkin
fcco
=
=