XC2200 Derivatives
System Units (Vol. 1 of 2)
System Control Unit (SCU)
User’s Manual
6-5
V2.1, 2008-08
SCU, V1.13
6.1.3.2
External Crystal Mode
An external oscillator load circuitry must be used, connected to both pins, XTAL1 and
XTAL2. It consists normally of the two load capacitances C1 and C2. For some crystals
a series damping resistor might be necessary. The exact values and related operating
range depend on the crystal and have to be determined and optimized together with the
crystal vendor using the negative resistance method.
Figure 6-4
XC2200 External Crystal Mode Circuitry for the High-Precision
Oscillator
6.1.4
Phase-Locked Loop (PLL) Module
The PLL can convert a low-frequency external clock signal to a high-speed system clock
for maximum performance. The PLL also has fail-safe logic that detects degenerate
external clock behavior such as abnormal frequency deviations or a total loss of the
external clock. It can execute emergency actions if it loses its lock on the external clock.
This module is a phase locked loop for integer frequency synthesis. It allows the use of
input and output frequencies of a wide range by varying the different divider factors.
6.1.4.1
Features
Here is a brief overview of the functions that are offered by the PLL.
•
VCO lock detection
•
4-bit input divider
P
: (divide by PDIV+1)
•
6-bit feedback divider
N
: (multiply by NDIV+1)
•
10-bit output divider
K2
: (divide K2DIV+1)
•
10-bit VCO bypass divider
K1
: (divide by either by K1DIV+1)
•
Oscillator run detection and Watchdog
•
Different operating modes
– Prescaler Mode
CGU_OSC _HP_Crystal.vsd
OSC_HP
f
OSC
XTAL1
XTAL2
C1
C2
VSS
Fundamental Mode
Crystal
