
DR
AFT
DR
AFT
DRAFT
DR
D
RAFT
DRAFT
DRA
FT DRAF
D
RAFT DRAFT DRAFT DRAFT DRAFT D
DRAFT
D
RAFT DRA
FT DRAFT DRAFT DRAFT DRA
UM10360_0
© NXP B.V. 2009. All rights reserved.
User manual
Rev. 00.06 — 5 June 2009
46 of 808
NXP Semiconductors
UM10360
Chapter 4: LPC17xx Clocking and power control
USBCLK = M
×
F
OSC
or USBCLK = F
CCO
/ (2
×
P)
The CCO frequency can be computed as:
F
CCO
= USBCLK
×
2
×
P or F
CCO
= F
OSC
×
M
×
2
×
P
The PLL1 inputs and settings must meet the following criteria:
•
F
OSC
is in the range of 10 MHz to 25 MHz.
•
USBCLK is 48 MHz.
•
F
CCO
is in the range of 156 MHz to 320 MHz.
6.9 Procedure for determining PLL1 settings
The PLL1 configuration for USB may be determined as follows:
1. The desired PLL1 output frequency is USBCLK = 48 MHz.
2. Choose an oscillator frequency (F
OSC
). USBCLK must be the whole (non-fractional)
multiple of F
OSC
meaning that the possible values for F
OSC
are 12 MHz, 16 MHz, and
24 MHz.
3. Calculate the value of M to configure the MSEL1 bits. M = USBCLK / F
OSC
. In this
case, the possible values for M = 2, 3, or 4 (F
OSC
= 24 MHz, 16 MHz, or 12 MHz). The
value written to the MSEL1 bits in PLL1CFG is M
−
1 (see
).
4. Find a value for P to configure the PSEL1 bits, such that F
CCO
is within its defined
frequency limits of 156 MHz to 320 MHz. F
CCO
is calculated using F
CCO
= USBCLK
×
2
×
P. It follows that P = 2 is the only P value to yield F
CCO
in the allowed range. The
value written to the PSEL1 bits in PLL1CFG is ‘01’ for P = 2 (see
Table 35.
PLL1 Divider values
Values allowed for using PLL1 with USB are highlighted.
PSEL1 Bits (PLL1CFG bits [6:5])
Value of P
00
1
01
2
10
4
11
8
Table 36.
PLL1 Multiplier values
Values allowed for using PLL1 with USB are highlighted.
MSEL1 Bits (PLL1CFG bits [4:0])
Value of M
00000
1
00001
2
00010
3
00011
4
...
...
11110
31
11111
32