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| 型号： | MPC974FA |
| 厂商： | FREESCALE SEMICONDUCTOR INC |
| 元件分类： | 时钟及定时 |
| 英文描述： | 974 SERIES, PLL BASED CLOCK DRIVER, 14 TRUE OUTPUT(S), 0 INVERTED OUTPUT(S), PQFP52 |
| 封装： | PLASTIC, LQFP-52 |
| 文件页数： | 6/6页 |
| 文件大小： | 294K |
| 代理商： | MPC974FA |
MPC974
MOTOROLA ADVANCED CLOCK DRIVERS DEVICE DATA
215
Driving Transmission Lines
The MPC974 clock driver was designed to drive high speed
signals in a terminated transmission line environment. To pro-
vide the optimum flexibility to the user the output drivers were
designed to exhibit the lowest impedance possible. With an
output impedance of approximately 10
the drivers can drive
either parallel or series terminated transmission lines. For
more information on transmission lines the reader is referred to
application note AN1091 in the Timing Solutions data book
(DL207/D).
Figure 5. Single versus Dual Transmission Lines
7
IN
MPC974
OUTPUT
BUFFER
RS = 43
ZO = 50
OutA
7
IN
MPC974
OUTPUT
BUFFER
RS = 43
ZO = 50
OutB0
RS = 43
ZO = 50
OutB1
In most high performance clock networks point–to–point
distribution of signals is the method of choice. In a point–to–
point scheme either series terminated or parallel terminated
transmission lines can be used. The parallel technique termi-
nates the signal at the end of the line with a 50
resistance to
VCC/2. This technique draws a fairly high level of DC current
and thus only a single terminated line can be driven by each
output of the MPC974 clock driver. For the series terminated
case however there is no DC current draw, thus the outputs
can drive multiple series terminated lines. Figure 5 illustrates
an output driving a single series terminated line vs two series
terminated lines in parallel. When taken to its extreme the fan-
out of the MPC974 clock driver is effectively doubled due to its
capability to drive multiple lines.
The waveform plots of Figure 6 show the simulation results
of an output driving a single line vs two lines. In both cases the
drive capability of the MPC974 output buffers is more than suf-
ficient to drive 50
transmission lines on the incident edge.
Note from the delay measurements in the simulations a delta
of only 43ps exists between the two differently loaded outputs.
This suggests that the dual line driving need not be used exclu-
sively to maintain the tight output–to–output skew of the
MPC974. The output waveform in Figure 6 shows a step in the
waveform, this step is caused by the impedance mismatch
seen looking into the driver. The parallel combination of the
43
series resistor plus the output impedance does not match
the parallel combination of the line impedances. The voltage
wave launched down the two lines will equal:
Figure 6. Single versus Dual Waveforms
TIME (nS)
VOL
TAGE
(V)
3.0
2.5
2.0
1.5
1.0
0.5
0
2
4
6
8
10
12
14
OutB
tD = 3.9386
OutA
tD = 3.8956
In
VL = VS ( Zo / Rs + Ro +Zo) = 3.0 (25/53.5) = 1.40V
At the load end the voltage will double, due to the near unity
reflection coefficient, to 2.8V. It will then increment towards the
quiescent 3.0V in steps separated by one round trip delay (in
this case 4.0ns).
Since this step is well above the threshold region it will not
cause any false clock triggering, however designers may be
uncomfortable with unwanted reflections on the line. To better
match the impedances when driving multiple lines the situation
in Figure 7 should be used. In this case the series terminating
resistors are reduced such that when the parallel combination
is added to the output buffer impedance the line impedance is
perfectly matched.
SPICE level output buffer models are available for engi-
neers who want to simulate their specific interconnect
schemes. In addition IV characteristics are in the process of
being generated to support the other board level simulators in
general use.
Figure 7. Optimized Dual Line Termination
7
MPC974
OUTPUT
BUFFER
RS = 36
ZO = 50
RS = 36
ZO = 50
7
+ 36 k 36 = 50 k 50
25
= 25
2
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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