© Koninklijke Philips Electronics N.V. 2006. All rights reserved.
User manual
Rev. 01 — 12 January 2006
29
Philips Semiconductors
UM10161
Volume 1
Chapter 3: System control block
3.8.7 PLL
Feed
register (PLLFEED - 0xE01F C08C)
A correct feed sequence must be written to the PLLFEED register in order for changes to
the PLLCON and PLLCFG registers to take effect. The feed sequence is:
1. Write the value 0xAA to PLLFEED.
2. Write the value 0x55 to PLLFEED.
The two writes must be in the correct sequence, and must be consecutive APB bus
cycles. The latter requirement implies that interrupts must be disabled for the duration of
the PLL feed operation. If either of the feed values is incorrect, or one of the previously
mentioned conditions is not met, any changes to the PLLCON or PLLCFG register will not
become effective.
3.8.8 PLL and Power-down mode
Power-down mode automatically turns off and disconnects activated PLL(s). Wake-up
from Power-down mode does not automatically restore the PLL settings. This must be
done in software. Typically, a routine to activate the PLL, wait for lock, and then connect
the PLL can be called at the beginning of any interrupt service routine that might be called
due to the wake-up. It is important not to attempt to restart the PLL by simply feeding it
when execution resumes after a wake-up from Power-down mode. This would enable and
connect the PLL at the same time, before PLL lock is established.
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
Table 20:
PLL Feed register (PLLFEED - address 0xE01F C08C) bit description
Bit
Symbol
Description
Reset
value
7:0
PLLFEED
The PLL feed sequence must be written to this register in order for
PLL configuration and control register changes to take effect.
0x00
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