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| 型号： | MPC97H74AER2 |
| 厂商： | FREESCALE SEMICONDUCTOR INC |
| 元件分类： | 时钟及定时 |
| 英文描述： | 97H SERIES, PLL BASED CLOCK DRIVER, 14 TRUE OUTPUT(S), 0 INVERTED OUTPUT(S), PQFP52 |
| 封装： | LEAD FREE, LQFP-52 |
| 文件页数： | 11/12页 |
| 文件大小： | 240K |
| 代理商： | MPC97H74AER2 |
Advanced Clock Driver Devices
8
Freescale Semiconductor
MPC97H74
Driving Transmission Lines
The MPC97H74 clock driver was designed to drive high
speed signals in a terminated transmission line environment.
To provide the optimum flexibility to the user the output
drivers were designed to exhibit the lowest impedance
possible. With an output impedance of less than 20
the
drivers can drive either parallel or series terminated
transmission lines. For more information on transmission
lines the reader is referred to Freescale Semiconductor
application note AN1091. 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 terminates the signal at the end of the line
with a 50
resistance to V
CC divided by 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 MPC97H74 clock driver. For the series
terminated case however there is no DC current draw, thus
the outputs can drive multiple series terminated lines. Figure
8. Single versus Dual Transmission Lines illustrates an
output driving a single series terminated line versus two
series terminated lines in parallel. When taken to its extreme
the fanout of the MPC97H74 clock driver is effectively
doubled due to its capability to drive multiple lines.
The waveform plots in Figure 9. Single versus Dual
Waveforms show the simulation results of an output driving a
single line versus two lines. In both cases the drive capability
of the MPC97H74 output buffer is more than sufficient to drive
50
transmission lines on the incident edge. Note from the
delay measurements in the simulations a delta of only 43 ps
exists between the two differently loaded outputs. This
suggests that the dual line driving need not be used
exclusively to maintain the tight output-to-output skew of the
MPC97H74. The output waveform in Figure 9. Single versus
Dual Waveforms shows a step in the waveform, this step is
caused by the impedance mismatch seen looking into the
driver. The parallel combination of the 40
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:
VL =VS ( Z0 ÷ (RS + R0 + Z0))
Z0 =50 || 50
RS =40 || 40
R0 =10
VL = 3.0 ( 25 ÷ (20 + 10 + 25)
=1.36 V
At the load end the voltage will double, due to the near
unity reflection coefficient, to 2.7 V. It will then increment
towards the quiescent 3.0 V in steps separated by one round
trip delay (in this case 4.0 ns).
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 10. Optimized Dual Line Termination
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.
MPC97H74
Output
Buffer
MPC97H74
Output
Buffer
10
Figure 8. Single versus Dual Transmission Lines
IN
RS = 40
ZO = 50
OutA
10
IN
RS = 40
ZO = 50
OutB0
RS = 40
ZO = 50
OutB1
Figure 9. Single versus Dual Waveforms
Time (ns)
Volta
ge
(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
Figure 10. Optimized Dual Line Termination
10
MPC97H74
Output
Buffer
RS = 30
ZO = 50
RS = 30
ZO = 50
10
+ 30 || 30 = 50 || 50
25
= 25
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