MC68340 datasheet(108/441 Pages) FREESCALE

108 / 441 page

MOTOROLA

MC68340 USER’S MANUAL

4- 11

To use an external clock source (see Figure 4-6), the operating clock frequency can be

driven directly into the EXTAL pin (the XTAL pin must be left floating for this case). This

approach results in a system clock and CLKOUT that are the same as the input signal

frequency, but not tightly coupled to it. To enable this mode, MODCK must be held low

during reset, and VCCSYN held at 0 V while the chip is in operation.

CLKOUT

PHASE

COMPARATOR

LOW-PASS

FILTER

VCO

CRYSTAL

OSCILLATOR

SYSTEM

CLOCK

CLOCK CONTROL

XFC PIN

VCCSYN

XFC

VCCSYN

.01 µF

0.1 µF

NOTES:

1. Must be low-leakage capacitor.

2. External mode uses this path only.

FEEDBACK

DIVIDER

EXTAL

XTAL

EXTERNAL

CLOCK

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 configuration 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 should be 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 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 signal reference has

been detected. The RSTEN bit in the SYNCR controls whether an input signal loss causes

a system reset or causes the device to operate in limp mode. The SLOCK bit 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

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