NXP Semiconductors LPC24XX UM10237 - Manual (Page 54)

UM10237_4 © NXP B.V. 2009. All rights reserved.
User manual Rev. 04 — 26 August 2009 54 of 792
NXP Semiconductors
UM10237
Chapter 4: LPC24XX Clocking and power control
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 4–51
3.2.12 Procedure for determining PLL settings
PLL parameter determination can be simplified by using a spreadsheet available from
NXP. To determine PLL parameters by hand, the following general procedure may be
used:
1. Determine if the application requires use of the USB interface. The USB requires a
50% duty cycle clock of 48 MHz within a very small tolerance, which means that F
CCO
must be an even integer multiple of 48 MHz (i.e. an integer multiple of 96 MHz), within
a very small tolerance.
2. Choose the desired processor operating frequency (cclk). This may be based on
processor throughput requirements, need to support a specific set of UART baud
rates, etc. Bear in mind that peripheral devices may be running from a lower clock
frequency than that of the processor (see
Section 4–3.3 “Clock dividers” on page 57
and Section 4–3.4 “Power control” on page 60 ). Find a value for F
CCO
that is close to
a multiple of the desired cclk frequency, bearing in mind the requirement for USB
support in [1] above, and that lower values of F
CCO
result in lower power dissipation.
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
16479 17578 18127 18311 19226
19775 20508 20599 20874 21149
21973 23071 23438 23804 24170