
M68HC11
REFERENCE MANUAL
PINS AND CONNECTIONS
MOTOROLA
2-15
production. An emulator version of the part will also have slightly different electrical
properties than the masked ROM version of the same part.
To determine the optimum value for R
S
, observe the operating current (I
DD
) of the
MCU as a function of R
S
. The MCU should be held in reset throughout this procedure
because operating current variations during run modes are much greater than the cur-
rent variations due to varying R
S
. Normally, a dip in current will occur. This dip is not
sharp as in many LC circuits but is instead very broad. As the shape of this curve sug-
gests, the exact value of R
S
is not critical.
Finally, verify that the maximum-operating supply voltage does not overdrive the crys-
tal. Observe the output frequency as a function of V
DD
at the buffered E-clock output.
Under proper operating conditions, the frequency should increase a few parts per mil-
lion as supply voltage increases. If the crystal is overdriven, an increase in supply volt-
age will cause a decrease in frequency, or the frequency will become unstable. If
frequency problems arise, supply voltage must be decreased, or the values of R
S
, C1,
and C2 should be increased to reduce the crystal drive.
2.2.4 Crystal Oscillator Application Information
Some crystal oscillator application information is presented in the following para-
graphs.
2.2.4.1 Crystals for Parallel Resonance
Parallel resonance refers to a Pierce oscillator that has the crystal in parallel with an
inverter. Almost all (if not all) CMOS MCUs use this type oscillator. AT-cut crystals are
available as standard devices for both series resonant circuits and Pierce oscillators.
The load capacitance has to be specified for the Pierce version. The series resonant
versions do not require this specification and are more likely to be listed as a standard
product. The type circuit affects the oscillating frequency of the crystal.
Any 4- to 8-MHz AT-cut crystal will normally meet the requirements of an M68HC11.
However, for a very accurate oscillator frequency, use the Pierce version of the crystal
with C1 and C2 values to match the specified load capacitance value for the crystal.
The load capacitance is approximately equal to the series combination of C1 and C2.
2.2.4.2 Using Crystal Oscillator Outputs
The crystal oscillator is actually an RF application. Connecting the crystal pins to other
circuitry is likely to interfere with proper operation of the oscillator. Modern CMOS in-
puts are very high impedance and relatively low capacitance; thus, one of these inputs
can be connected to the oscillator without disturbing the oscillator. The data sheet
shows examples of ways the crystal oscillator can be used to drive other circuits for
crystal frequencies between 4 and 8 MHz.
2.2.4.3 Using External Oscillator
An externally built Pierce oscillator will operate like a crystal connected to the
M68HC11. Use a single inverter and connect the crystal feedback resistor and load