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參數(shù)資料
| 型號(hào)： | MC12429FA |
| 廠商： | FREESCALE SEMICONDUCTOR INC |
| 元件分類： | 時(shí)鐘產(chǎn)生/分配 |
| 英文描述： | 400 MHz, OTHER CLOCK GENERATOR, PQFP32 |
| 封裝： | LQFP-32 |
| 文件頁數(shù)： | 7/9頁 |
| 文件大?。?/td> | 173K |
| 代理商： | MC12429FA |
MC12429
358
FREESCALE SEMICONDUCTOR ADVANCED CLOCK DRIVERS DEVICE DATA
APPLICATIONS INFORMATION
Using the On-Board Crystal Oscillator
The MC12429 features a fully integrated on-board crystal
oscillator to minimize system implementation costs. The
oscillator is a series resonant, multivibrator type design as
opposed to the more common parallel resonant oscillator
design. The series resonant design provides better stability and
eliminates the need for large on chip capacitors. The oscillator
is totally self contained so that the only external component
required is the crystal. As the oscillator is somewhat sensitive to
loading on its inputs the user is advised to mount the crystal as
close to the MC12429 as possible to avoid any board level
parasitics. To facilitate co-location surface mount crystals are
recommended, but not required. Because the series resonant
design is affected by capacitive loading on the xtal terminals
loading variation introduced by crystals from different vendors
could be a potential issue. For crystals with a higher shunt
capacitance it may be required to place a resistance across the
terminals to suppress the third harmonic. Although typically not
required it is a good idea to layout the PCB with the provision of
adding this external resistor. The resistor value will typically be
between 500 and 1K
.
The oscillator circuit is a series resonant circuit and thus for
optimum performance a series resonant crystal should be used.
Unfortunately most crystals are characterized in a parallel
resonant mode. Fortunately there is no physical difference
between a series resonant and a parallel resonant crystal. The
difference is purely in the way the devices are characterized. As
a result a parallel resonant crystal can be used with the
MC12429 with only a minor error in the desired frequency. A
parallel resonant mode crystal used in a series resonant circuit
will exhibit a frequency of oscillation a few hundred ppm lower
than specified, a few hundred ppm translates to kHz
inaccuracies. In a general computer application this level of
inaccuracy is immaterial. Table 6 below specifies the
performance requirements of the crystals to be used with the
MC12429.
Power Supply Filtering
The MC12429 is a mixed analog/digital product and as such
it exhibits some sensitivities that would not necessarily be seen
on a fully digital product. Analog circuitry is naturally susceptible
to random noise, especially if this noise is seen on the power
supply pins. The MC12429 provides separate power supplies
for the digital ciruitry (VCC) and the internal PLL (PLL_VCC) of
the device. The purpose of this design technique is to try and
isolate the high switching noise digital outputs from the
relatively sensitive internal analog phase-locked loop. In a
Table 5. AC Characteristics (TA = 0° to 70°C, VCC = 3.3V to 5.0V ±5%)
Symbol
Characteristic
Min
Max
Unit
Condition
FMAXI
Maximum Input Frequency
S_CLOCK
Xtal Oscillator
10
20
MHz
Note 1
1.
10MHz is the maximum frequency to load the feedback divide registers. S_CLOCK can be switched at higher frequencies when used as a test
clock in TEST_MODE 6.
FMAXO
Maximum Output Frequency
VCO (Internal)
FOUT
200
25
400
MHz
Note 2
2.
50
to V
CC – 2.0V pulldown.
tLOCK
Maximum PLL Lock Time
10
ms
tjitter
Period Deviation (Peak-to-Peak)
±25
ps
Note 2,
See Applications Section
ts
Setup Time
S_DATA to S_CLOCK
S_CLOCK to S_LOAD
M, N to P_LOAD
20
ns
th
Hold Time
S_DATA to S_CLOCK
M, N to P_LOAD
20
ns
tpwMIN
Minimum Pulse Width
S_LOAD
P_LOAD
50
ns
Note 2
tr, tf
Output Rise/Fall
FOUT
300
800
ps
20%-80%, Note 2
Table 6. Recommended Crystal Specifications
Parameter
Value
Crystal Cut
Fundamental AT Cut
Resonance
Series Resonance1
1.
See accompanying text for series versus parallel resonant
discussion.
Frequency Tolerance
±75ppm at 25°C
Frequency/Temperature Stability
±150pm 0 to 70°C
Operating Range
0 to 70
°C
Shunt Capacitance
5–7pF
Equivalent Series Resistance (ESR)
50 to 80
Correlation Drive Level
100
W
Aging
5ppm/Yr (First 3 Years)
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