Motorola M68CPU32BUG Manuel d'utilisateur Page 8

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MOTOROLA MC68332TUT/D
8
NOTE
Some older versions of the MC68332 require different components. These mask
sets are 1C17P, 1C32J, OC53T, and 1C53T. See
Figure 4
for an illustration.
Figure 3 System Clock with a 32.768 kHz Reference Crystal
Figure 4 Oscillator Circuit for Mask Sets 1C17P, 1C32J, OC53T, and 1C53T
For the most accurate oscillator frequency, use the Pierce version of the crystal (rather than the series res-
onant version) with C1 and C2 values to match the specified load capacitance of the crystal. As a side note,
start-up time is inversely proportional to frequency. A 32 kHz crystal may take several hundred milliseconds
to start up. Be aware: all crystals are not created equal nor are they closely related. In fact, there are several
different types of tuning elements that can be used for low frequency oscillators in the range of 32.768 kHz.
While there are many characteristics of the various tuning elements that can be precisely measured, there
are other characteristics that are extremely difficult to measure and express in a useful way.
Maximum power dissipation of a crystal is generally specified by the manufacturer of the device. Crystal
power dissipation is a function of the reactance of the combined input and output capacitance of the internal
amplifier of the microcontroller and of the external circuit components including the crystal itself. The man-
ufacturer specifies this value and also specifies a circuit, usually one like that of
Figure 3
, along with the
circuit values. The crystal manufacturer makes a tacit assumption that the amplifier has enough drive ca-
pability 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 3
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
332TUT XTAL CONN 1
EXTAL
XTAL
10M
330K
22 PF*
22 PF*
V
SSI
RESISTANCE AND CAPACITANCE BASED ON A TEST CIRCUIT CONSTRUCTED WITH A DAISHINKU DMX-38 32.768-KHZ CRYSTAL.
SPECIFIC COMPONENTS MUST BE BASED ON CRYSTAL TYPE. CONTACT CRYSTAL VENDOR FOR EXACT CIRCUIT.
*
RS
C1
C2
R
f
332TUT XTAL CONN 2
EXTAL
XTAL
10M
100
22 PF*
22 PF*
V
SSI
RESISTANCE AND CAPACITANCE BASED ON A TEST CIRCUIT CONSTRUCTED WITH A DAISHINKU DMX-38 32.768-KHZ CRYSTAL.
SPECIFIC COMPONENTS MUST BE BASED ON CRYSTAL TYPE. CONTACT CRYSTAL VENDOR FOR EXACT CIRCUIT.
*
RS
C1
C2
R
f
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