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MOTOROLA
MC68341 USER’S MANUAL
4- 11
capacitors. The crystal manufacturer's documentation should be consulted for specific
recommendations for external components.
4.2.3.2 EXTERNAL CLOCK MODE. To use an external clock source (see Figure 4-6), the
operating clock frequency can be driven directly into the EXTCLK pin. The V-bit in the
SYNCR allows running the VCO at either the same speed or half the speed of the external
clock. This approach results in a system clock and CLKOUT that are the same as the
input signal frequency, but not tightly coupled to it. The external clock frequency can
optionally be divided by two using the V-bit in SYNCR. To enable this mode, MODCK
must be held low during reset, and VCCSYN held at 0 V while the chip is in operation.
÷2
CLKOUT
PHASE
COMPARATOR
LOW-PASS
FILTER
VCO
CRYSTAL
OSCILLATOR
SYSTEM
CLOCK
CLOCK CONTROL
XFC PIN
VCCSYN
XFC
VCCSYN
.01 F
0.1 F
NOTES:
1. Must be low-leakage capacitor.
FEEDBACK
DIVIDER
EXTAL
XTAL
1
EXTERNAL
CLOCK
MUX
EXTCLK
MODCK
V
1
0
1
Figure 4-6. Block Diagram for External Clock Operation
Alternatively, an external clock signal can be directly driven into EXTCLK using the on-
chip PLL. To enable this mode, MODCK must be held low during reset, and V CCSYN
should be connected to a quiet VCC source. (See Table 4-1).
In external clock mode, the V, W, X, Y, and Z bits in the SYNCR can program the system
frequency over the range specified in Section 12 Electrical Characteristics.
4.2.3.3 LIMP MODE. If an input signal loss for either of the clock modes using the PLL
occurs, chip operation can continue in limp mode with the VCO running at approximately
one-half the operating speed (affected by the value of the X-bit in the SYNCR), using an
internal voltage reference. The SLIMP bit in the SYNCR indicates that a loss of input
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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