Motorola MC68340 manual Clock Block Diagram for Crystal Operation

Freescale Semiconductor, Inc.

using crystal mode, the system clock frequency is programmable (using the W, X, and Y bits in the SYNCR) over the range specified in Section 11 Electrical Characteristics (see Table 4-2.).

20 pF

EXTAL

K	20 pF
330
20 M
XTAL	XFC PIN

VDDSYN

XFC 1

0.1∝F

VDDSYN

0.1∝F

0.01∝F

Semiconductor, Inc.

CRYSTAL

OSCILLATOR

	PHASE		LOW-PASS			VCO
COMPARATOR			FILTER
	⎟64
			MUX
	MODULUS		1	⎟4
	DIVIDER		0			⎟8
			SEL
6
		0
	$3F
	Y				FEEDBACK DIVIDER
			W

		MUX
		1
		0	CLKOUT
⎟2
		SEL
	0
		X

Freescale

NOTE 1: Must be low-leakage capacitor.

Figure 4-4. Clock Block Diagram for Crystal Operation

	60 kΩ
EXTAL	XTAL

.60 kΩ

Figure 4-5. MC68340 Crystal Oscillator

A separate power pin (VCCSYN) is used to allow the clock circuits to run with the rest of the device powered down and to provide increased noise immunity for the clock circuits.

The source for VCCSYN should be a quiet power supply with adequate external bypass capacitors placed as close as possible to the VCCSYN pin to ensure a stable operating

frequency. Figure 4-4 shows typical values for the bypass and PLL external capacitors. The crystal manufacturer's documentation should be consulted for specific recommendations for external components.

4- 10MC68340 USER’S MANUALMOTOROLA

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