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UM10237_2
© NXP B.V. 2008. All rights reserved.
User manual
Rev. 02 — 19 December 2008
53 of 792
NXP Semiconductors
UM10237
Chapter 4: LPC24XX Clocking and power control
3.2.11 PLL frequency calculation
The PLL equations use the following parameters:
The PLL output frequency (when the PLL is both active and connected) is given by:
F
CCO
= (2
×
M
×
F
IN
) / N
The PLL inputs and settings must meet the following:
•
F
IN
is in the range of 32 kHz to 50 MHz.
•
F
CCO
is in the range of 275 MHz to 550 MHz.
The PLL equation can be solved for other PLL parameters:
M = (F
CCO
×
N) / (2
×
F
IN
)
N = (2
×
M
×
F
IN
) / F
CCO
F
IN
= (F
CCO
×
N) / (2
×
M)
Allowed values for M:
At higher oscillator frequencies, in the MHz range, values of M from 6 through 512 are
allowed. This supports the entire useful range of both the main oscillator and the IRC.
For lower frequencies, specifically when the RTC is used to clock the PLL, a set of 65
additional M values have been selected for supporting baud rate generation, CAN/USB
operation, and attaining even MHz frequencies. These values are shown in
Table 50.
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)
N
PLL Pre-divider value from the NSEL bits in the PLLCFG register (PLLCFG
NSEL field + 1). N is an integer from 1 through 32.
M
PLL Multiplier value from the MSEL bits in the PLLCFG register (PLLCFG
MSEL field + 1). Not all potential values are supported. See below.
F
REF
PLL internal reference frequency, FIN divided by N.
Table 51.
Additional Multiplier Values for use with a Low Frequency Clock Input
Low Frequency PLL Multipliers
4272
4395
4578
4725
4807
5127
5188
5400
5493
5859
6042
6075
6104
6409
6592
6750
6836
6866
6958
7050
7324
7425
7690
7813
7935
8057
8100
8545
8789
9155
9613
10254
10376
10986
11719
12085
12207
12817
13184
13672
13733
13916
14099
14420
14648
15381
15564
15625
15869
16113