MPC9456

TIMING SOLUTIONS

7

MOTOROLA

APPLICATIONS INFORMATION

Driving Transmission Lines

The MPC9456 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

Ω 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

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

Ω 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 MPC9456 clock driver. For the series terminated

case however there is no DC current draw, thus the outputs

can drive multiple series terminated lines. Figure 3. “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

MPC9456 clock driver is effectively doubled due to its

capability to drive multiple lines.

Figure 3. Single versus Dual Transmission Lines

14

Ω

IN

MPC9456

OUTPUT

BUFFER

RS = 36Ω

ZO = 50Ω

OutA

14

Ω

IN

MPC9456

OUTPUT

BUFFER

RS = 36Ω

ZO = 50Ω

OutB0

RS = 36Ω

ZO = 50Ω

OutB1

The waveform plots in Figure 4. “Single versus Dual Line

Termination Waveforms” show the simulation results of an

output driving a single line versus two lines. In both cases the

drive capability of the MPC9456 output buffer is more than

sufficient to drive 50

Ω 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 exclusively to maintain the tight output-to-output skew

of the MPC9456. The output waveform in Figure 4. “Single

versus Dual Line Termination 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 36

Ω 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 = 50Ω || 50Ω

RS = 36Ω || 36Ω

R0 = 14Ω

VL = 3.0 ( 25 ÷ (18+14+25)

= 1.31V

At the load end the voltage will double, due to the near

unity reflection coefficient, to 2.5V. It will then increment

towards the quiescent 3.0V in steps separated by one round

trip delay (in this case 4.0ns).

Figure 4. Single versus Dual Waveforms

TIME (nS)

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

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 5. “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.

Figure 5. Optimized Dual Line Termination

14

Ω

MPC9456

OUTPUT

BUFFER

RS = 22Ω

ZO = 50Ω

RS = 22Ω

ZO = 50Ω

14

Ω + 22Ω k 22Ω = 50Ω k 50Ω

25

Ω = 25Ω

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com