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4-10
MC68330 USER'S MANUAL
MOTOROLA
CLKOUT
PHASE
COMPARATOR
LOW-PASS
FILTER
VCO
CRYSTAL
OSCILLATOR
SYSTEM
CLOCK
CLOCK CONTROL
XFC PIN
VCCSYN
XFC
VCCSYN
.01
F
0.1
F
NOTE 1: MUST BE LOW-LEAKAGE CAPACITOR.
2: EXTERNAL MODE USES THIS PATH ONLY.
FEEDBACK DIVIDER
EXTAL
XTAL
1
EXTERNAL
CLOCK
2
Figure 4-6. Clock Block Diagram for External Oscillator Operation
Alternatively, an external clock signal can be directly driven into EXTAL (with XTAL left
floating) using the on-chip PLL. This results in an internal clock and CLKOUT signal of
the same frequency as the input signal, with a tight skew between the external clock and
the internal clock and CLKOUT signals. To enable this mode, MODCK must be held low
during reset, and VCCSYN connected to a quiet 5 V source.
If an input signal loss for either of the clock modes utilizing the PLL occurs, chip
operation can continue in limp mode with the VCO running at approximately one-half the
maximum speed (affected by the value of the X-bit in the SYNCR register), using an
internal voltage reference. The limp mode bit (SLIMP) in the SYNCR indicates that a loss
of input signal reference has been detected. The reset enable (RSTEN) bit controls
whether an input signal loss causes a system reset or causes the device to operate in
limp mode. The synthesizer lock bit (SLOCK) in the SYNCR indicates when the VCO has
locked onto the desired frequency, or if an external clock is being used.
4.2.3.1 PHASE COMPARATOR AND FILTER. The phase comparator takes the
output of the frequency divider and compares it to an external input signal reference. The
result of this compare is low-pass filtered and used to control the VCO. The comparator
also detects when the external crystal or oscillator stops running to initiate the limp mode
for the system clock.
The PLL requires an external low-leakage filter capacitor, typically in the range from 0.01
to 0.1
F, connected between the XFC and VCCSYN pins. The XFC capacitor should
provide 50 M
insulation, and should not be electrolytic. Smaller values of the external