© Koninklijke Philips Electronics N.V. 2005. All rights reserved.
User manual
Rev. 01 — 15 August 2005
33
Philips Semiconductors
UM10139
Volume 1
Chapter 3: System Control Block
3.8.9 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:
CCLK = M
×
F
OSC
or CCLK = F
CCO
/ (2
×
P)
The CCO frequency can be computed as:
F
CCO
= CCLK
×
2
×
P or F
CCO
= F
OSC
×
M
×
2
×
P
The PLL inputs and settings must meet the following:
•
F
OSC
is in the range of 10 MHz to 25 MHz.
•
CCLK is in the range of 10 MHz to F
max
(the maximum allowed frequency for the
microcontroller - determined by the system microcontroller is embedded in).
•
F
CCO
is in the range of 156 MHz to 320 MHz.
3.8.10 Procedure for determining PLL settings
If a particular application uses the PLL0, its configuration may be determined as follows:
1. 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
than the processor (see
Section 3.11 “VPB divider” on page 40
2. Choose an oscillator frequency (F
OSC
). CCLK must be the whole (non-fractional)
multiple of F
OSC
.
3. Calculate the value of M to configure the MSEL bits. M = CCLK / F
OSC
. M must be in
the range of 1 to 32. The value written to the MSEL bits in PLLCFG is M
−
1 (see
.
4. Find a value for P to configure the PSEL bits, such that F
CCO
is within its defined
frequency limits. F
CCO
is calculated using the equation given above. P must have one
of the values 1, 2, 4, or 8. The value written to the PSEL bits in PLLCFG is 00 for
P = 1; 01 for P = 2; 10 for P = 4; 11 for P = 8 (see
Important: if a particular application is using the USB peripheral, the PLL1 must be
configured since this is the only available source of the 48 MHz clock required by
the USB. This limits the selection of F
OSC
to either 12 MHz, 16 MHz or 24 MHz.
Table 21:
Elements determining PLL’s frequency
Element
Description
F
OSC
the frequency from the crystal oscillator/external oscillator
F
CCO
the frequency of the PLL current controlled oscillator
CCLK
the PLL output frequency (also the processor clock frequency)
M
PLL Multiplier value from the MSEL bits in the PLLCFG register
P
PLL Divider value from the PSEL bits in the PLLCFG register