mc68336 Freescale Semiconductor, Inc, mc68336 Datasheet - Page 9

• Manufacturer Part Number: mc68336
• Description: An Introduction To The Mc68331 And Mc68332
• Manufacturer: Freescale Semiconductor, Inc

circuit values. The crystal manufacturer makes a tacit assumption that the amplifier has enough drive capa-
bility to handle the required load, so that the output voltage levels of the amplifier are not affected.
Parameters related to suppression of harmonics and overtones are generally not specified by the crystal
manufacturer. Harmonics are integer multiples of the fundamental frequency. The first overtone is approx-
imately 1.7 times the fundamental frequency. Since a typical oscillator circuit forms a low pass filter, the
three db roll-off point should be set at about 1.5 times the fundamental frequency of the crystal. This should
cause no attenuation at the fundamental but should cause significant attenuation at the first overtone and
even greater attenuation at the first harmonic. When figuring the reactance of the entire circuit, it is most
important to use the typical parameters of the crystal, the input and output capacitance of the amplifier and
the remainder of the external components in the calculation.
Many companies make crystals. Most re-sell their products through electronics distributors that are listed in
the EITD Electronic Industry Telephone Directory . Refer to 6 SOURCES OF INFORMATION for ordering
information.
Four crystal manufacturers are:
2.5.2.2 Grit and Grime
Oscillators are quite sensitive to dirt, solder flux, grease, condensation due to high humidity, and other con-
ducting materials on the circuit board. These materials can allow a very high resistance leakage path from
one of the amplifier pins to either ground or the positive terminal of the power supply. When the oscillator
has power applied but has not started, the crystal and bypass capacitors appear as DC open circuits. An
oscillator in a DC condition would appear as shown in Figure 5.
The resistor, Rd, represents a high resistance leakage path, somewhere in the range of five to 20 M . The
feedback resistor, Rf, is also in this range. Assuming that Rd and Rf are both 10 M , the voltage at point A
is half the voltage difference between points B and C. Thus, if the XTAL pin is at a logic one (4.5 volts) and
point C is at ground, the voltage at point A (EXTAL pin) will be 2.25 volts. If point B is at a logic zero (0.5
volts) and point C is at ground, the voltage at point A is 0.25 volts. Thus, the voltage at point A may be in-
terpreted as a logic zero regardless of whether the XTAL pin is a logic one or a logic zero. This depends on
the threshold of the inverter whose input is connected to point A. Likewise, if point C is connected to five
volts, point A may be interpreted as a logic one regardless of the state of the XTAL pin. A circuit with this
problem will not oscillate.
The only way to diagnose this problem is to remove the external circuit components as well as the MCU
from the board and use an Ohm meter to check the resistance from points A and B to ground and five volts.
Anything other than a completely open circuit is a sign of trouble. The obvious solution is to clean the printed
MC68331/332
M68331/332TUT/D
ECS — [phone]
The part number for a surface mount 32.768 kHz crystal with a temperature range of –40 to +85 degrees
Celsius is ECX205. This crystal also comes in other packages.
Fox — [phone]
The part number for a surface mount 32.768 kHz crystal with a temperature range of –40 to +85 degrees
Celsius is FSM327. This crystal also comes in other packages.
KDS (Daishinku) — [phone]
The part number for a surface mount 32.768 kHz crystal with a temperature range of –40 to +85 degrees
Celsius is DMX-38. This crystal comes in other packages.
Statek — [phone]
The part number for a surface mount 32.768 kHz crystal that can be used at 25 degrees Celsius is CX-
1VS-SMI 32.768 kHz. For a temperature range of –40 to 85 degrees Celsius, the part number is CX-
1VS-SMI 32.768 kHz A/I.
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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