Motorola M68CPU32BUG User Manual download pdf (Page 7)

MC68332TUT/D MOTOROLA

2.5 Clock Circuitry

The designer must decide whether to use the internal frequency synthesizer circuit or an external clock to

produce the system clock signal. Both options are discussed in the following paragraphs.

2.5.1 Using the Internal Frequency Synthesizer Circuit

The MCU uses a voltage-controlled oscillator (VCO) and a phase-locked loop (PLL) to generate an internal

high speed clock. This arrangement permits low power operation using only the low frequency oscillator.

Low frequency in CMOS technology translates into low power because power consumption is proportional

to frequency.

The internal frequency synthesizer circuit is enabled when the MODCLK pin is pulled high during reset. The

synthesizer requires a reference frequency in order to operate. There are two reference frequency options:

using a crystal oscillator circuit or using an external clock reference, such as a canned oscillator circuit (a

single package which contains the crystal and buffer circuit) as the input.

2.5.2 Using a Crystal Oscillator Circuit

2.5.2.1 Oscillator Components

The crystal oscillator used is a Pierce oscillator, also known as a parallel resonant crystal oscillator. It is

shown in

Figure 3

. Its components consist of a series resistor, a feedback resistor, a crystal, an inverter,

and two capacitors:

Rs —

Series resistor

Rs must be large enough to appropriately limit current to the crystal and yet small

enough to provide enough current to start it oscillating quickly. The smaller Rs, the faster the oscillator

will start. However, if Rs is too small, the crystal will start up in unpredictable modes or dissipate too

much power. This can cause heating problems. In extreme cases, the crystal may even be damaged

and not work properly again. If Rs is too large, the oscillator will start very slowly or not at all. The best

way to minimize start-up time is to minimize the size of Rs within the guidelines of the maximum power

dissipation.

The crystal manufacturer generally recommends a range of values to use. To ensure that Rs is large

enough to prevent the crystal from being overdriven, observe the output frequency as a function of V

on the CLKOUT pin. If the crystal is overdriven at start-up, the frequency will be very unstable.

Rf —

Feedback resistor Rf is used to bias the inverter between EXTAL and XTAL inside the MCU. Rf

affects the loop gain; lower values reduce gain, while higher values increase gain.

C1 and C2

— The series combination of C1 and C2 provides the parallel load for the crystal. Their val-

ues may be varied to trim frequency. In high frequency applications, C1 and C2 are usually equal. How-

ever, in low frequency applications, C1 can be smaller than C2 (about 5 pF) to provide a higher voltage

at the EXTAL input. A wider voltage swing at this input will result in lower power-supply current. Usually,

the actual capacitances will be smaller than the intended capacitances since circuit and layout capaci-

tances add to the values of C1 and C2.

Inverter —

The inverter is inside the MCU. It provides the 180 degree phase shift necessary for oscil-

lation.

Crystal —

The crystal is made of piezoelectric quartz. It must be a good quality crystal that is capable

of suppressing harmonics and overtones and quickly locking onto the fundamental frequency. If a par-

ticular crystal type or brand is prone to starting with overtones or harmonics, don’t use it. No amount of

circuit design can compensate for a bad or poor quality crystal.

The MCU is designed to use a 32.768 kHz AT-cut crystal to produce an 8.389 MHz CLKOUT signal. The

frequency of the internal clock can be increased or decreased by writing to the SYNCR register.

Figure 3

shows clock circuitry for a Daishinku DMX-38 32.768 KHz crystal, but the circuit will work for most other

32.768 kHz crystals also. To use other crystal values (the allowable range is 20 kHz - 50 kHz), consult the

crystal vendor for analysis of the crystal components needed.

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Motorola M68CPU32BUG User Manual Page 7

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