XC886/888CLM
Power Supply, Reset and Clock Management
User’s Manual
7-11
V1.3, 2010-02
Power, Reset and Clock, V 1.0
7.3
Clock System
The XC886/888 clock system performs the following functions:
•
Acquires and buffers incoming clock signals to create a master clock frequency
•
Distributes in-phase synchronized clock signals throughout the system
•
Divides a system master clock frequency into lower frequencies for power saving
mode
7.3.1
Clock Generation Unit
The Clock Generation Unit (CGU) in the XC886/888 consists of an oscillator circuit and
a Phase-Locked Loop (PLL). In the XC886/888, the oscillator can be from either of these
two sources: the on-chip oscillator (9.6 MHz) or the external oscillator (4 MHz to
12 MHz). The term “oscillator” is used to refer to both on-chip oscillator and external
oscillator, unless otherwise stated. After the reset, the on-chip oscillator will be used by
default. The external oscillator can be selected via software. The PLL can convert a
low-frequency external clock signal from the oscillator circuit to a high-speed internal
clock for maximum performance.
shows the block diagram of CGU.
Figure 7-6
CGU Block Diagram
PLL
core
lock
detect
N:1
P:1
f
vco
f
n
f
p
osc fail
detect
OSC
f
osc
K:1
f
sys
NDIV
OSCR
LOCK
VCOBYP
1
0
OSCDISC
&
0: Connected
1: Disconnected
>=1
OSCR
*