User`s manual

MC68332TUT/D MOTOROLA
9
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 re-
mainder 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:
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 32.768kHz. For a temperature range of -40 to 85 degrees Celsius, the part number is CX-1VS-
SMI 32.768kHz A/I.
2.5.2.2 Grit and Grime
Oscillators are quite sensitive to dirt, solder flux, grease and other conducting 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 5 to 20 M Ω . The
feedback resistor, Rf, is also in this range. Assuming that Rd and Rf are both 10 M Ω , the voltage at point A
is half the voltage difference between points B and C. Thus, if the XTAL pin is at a logic 1 (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 0 (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 interpreted
as a logic 0 regardless of whether the XTAL pin is a logic 1 or a logic 0. This depends on the threshold of
the inverter whose input is connected to point A. Likewise, if point C is connected to 5 volts, point A may be
interpreted as a logic 1 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. Anything other
than a completely open circuit is a sign of trouble. The obvious solution is to clean the printed circuit board.
If the dirt or grime that form the high resistance path is on an inner layer of the printed circuit board, the
board is unusable.