mc145146dw2 Lansdale Semiconductor, Inc., mc145146dw2 Datasheet - Page 8

• Manufacturer Part Number: mc145146dw2
• Description: 4?bit Data Bus Input Pll Frequency Synthesizer
• Manufacturer: Lansdale Semiconductor, Inc.

ML145146
CRYSTAL OSCILLATOR CONSIDERATIONS
ence frequency to Motorola’s CMOS frequency synthesizers.
The most desirable is discussed first.
Use of a Hybrid Crystal Oscillator
oscillators (TCXOs) or crystal–controlled data clock oscilla-
tors provide very stable reference frequencies. An oscillator
capable of sinking and sourcing 50 µA at CMOS logic levels
may be direct or DC coupled to OSC in . In general, the highest
frequency capability is obtained utilizing a direct coupled
square wave having a rail–to–rail (V DD to V SS ) voltage
swing. If the oscillator does not have CMOS logic levels on the
outputs, capacitive or AC coupling of OSC in may be used.
OSC out , an unbuffered output, should be left floating.
oscillators, please consult the latest version of the eem
Electronic Engineers Master Catalog, the Gold Book, or simi-
lar publications.
Design an Off–Chip Reference
ICs specifically developed for crystal oscillator applications,
such as the ML12061 MECL device. The reference signal from
the MECL device is AC coupled to OSC in . For large ampli-
tude signals (standard CMOS logic levels), DC coupling is
used. OSC out , an unbuffered output, should be left floating. In
general, the highest frequency capability is obtained with a
direct–coupled square wave having rail–to–rail voltage swing.
Use of the On–Chip Oscillator Circuitry
appropriate crystal may be used to provide a reference source
frequency. A fundamental mode crystal, parallel resonant at the
desired operating frequency, should be connected as shown in
Figure 8.
ing capacitance. C L , which does not exceed 32 pF for frequen-
cies to approximately 8.0 MHz, 20 pF for frequencies in the
area of 8.0 to 15 MHz, and 10 pF for higher frequencies. These
are guidelines that provide a reasonable compromise between
IC capacitance, drive capability, swamping variations stray in
IC input/output capacitance, and realistic C L values. The shunt
load capacitance, C L , presented across the crystal can be esti-
mated to be:
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The following options may be considered to provide a refer-
Commercially available temperature–compensated crystal
For additional information about TCXOs and data clock
The user may design and off–chip crystal oscillator using
The on–chip amplifier (a digital inverter) along with an
For V DD = 5.0 V, the crystal should be specified for a load-
where
C in = 5.0pF (See Figure 9)
C out = 6.0pF (See Figure 9)
C a = 1.0pF (See Figure 9)
C O = the crystal’s holder capacitance (See Figure 10)
C1 and C2 = external capacitors (See Figure 8)
DESIGN CONSIDERATIONS
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portion or all of C1 variable. The crystal and associated com-
ponents must be located as close as possible to the OSC in and
OSC out pins to minimize distortion, stray capacitance, stray
inductance, and startup stabilization time. In some cases, stray
capacitance should be added to the value for C in and C out .
crystal, R e . In Figure 10 The drive level specified by the crys-
tal manufacturer is the maximum stress that a crystal can with-
stand without damaging or excessive shift in frequency. R1 in
Figure 8 limits the drive level. The use of R1 may not be nec-
essary in some cases (i.e. R1 = 0 ohms).
overdrive the crystal, monitor the output frequency as a func-
tion of voltage at OSC out . (care should be taken to minimize
loading.) the frequency should increase very slightly as the dc
supply voltage is increased. An overdriven crystal will decrease
in frequency or become unstable with an increase in supply
voltage. The operating supply voltage must be reduced or R1
must be increased in value if the overdrive condition exists.
The user should note that the oscillator start–up time is propor-
tional to the value of R1.
CMOS inverters, many crystal manufacturers have developed
expertise in CMOS oscillator design with crystals. Discussions
with such manufacturers can prove very helpful. See Table 1.
The oscillator can be “trimmed” on–frequency by making a
Power is dissipated in the effective series resistance of the
To verify that the maximum DC supply voltage does not
Through the process of supplying crystals for use with
LANSDALE Semiconductor, Inc.
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