MC68331TS datasheet(22/84 Pages) FREESCALE

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MC68331TS/D

Figure 7 System Clock Block Diagram

3.3.1 Clock Sources

The state of the clock mode (MODCLK) pin during reset determines the clock source. When MODCLK

is held high during reset, the clock synthesizer generates a clock signal from either a crystal oscillator

or an external reference input. Clock synthesizer control register SYNCR determines operating frequen-

cy and various modes of operation. When MODCLK is held low during reset, the clock synthesizer is

disabled, and an external system clock signal must be applied. When the synthesizer is disabled, SYN-

CR control bits have no effect.

A reference crystal must be connected between the EXTAL and XTAL pins to use the internal oscillator.

Use of a 32.768-kHz crystal is recommended. These crystals are inexpensive and readily available. If

an external reference signal or an external system clock signal is applied through the EXTAL pin, the

XTAL pin must be left floating. External reference signal frequency must be less than or equal to max-

imum specified reference frequency. External system clock signal frequency must be less than or equal

to maximum specified system clock frequency.

When an external system clock signal is applied (i.e., the PLL is not used), duty cycle of the input is

critical, especially at near maximum operating frequencies. The relationship between clock signal duty

cycle and clock signal period is expressed:

Minimum external clock period =

minimum external clock high/low time

50% — percentage variation of external clock input duty cycle

SYS CLOCK

BLOCK 32KHZ

CLKOUT

EXTAL

PHASE

COMPARATOR

LOW-PASS

FILTER

VCO

CRYSTAL

OSCILLATOR

SYSTEM

CLOCK

SYSTEM CLOCK CONTROL

XTAL

XFC PIN

VDDSYN

XFC

0.1

µF

0.01

µF

0.1

µF

FEEDBACK DIVIDER

22 pF

10M

330K

1. MUST BE LOW-LEAKAGE CAPACITOR (INSULATION RESISTANCE 30,000 M

Ω OR GREATER).

2. RESISTANCE AND CAPACITANCE BASED ON A TEST CIRCUIT CONSTRUCTED WITH A DAISHINKU DMX-38 32.768-kHz CRYSTAL.

SPECIFIC COMPONENTS MUST BE BASED ON CRYSTAL TYPE. CONTACT CRYSTAL VENDOR FOR EXACT CIRCUIT.

VDDSYN

VSSI

32.768 KHz

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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