MPC9774

MOTOROLA

TIMING SOLUTIONS

10

Figure 7. Single versus Dual Transmission Lines

14

Ω

IN

MPC9774

OUTPUT

BUFFER

RS = 36Ω

ZO = 50Ω

OutA

14

Ω

IN

MPC9774

OUTPUT

BUFFER

RS = 36Ω

ZO = 50Ω

OutB0

RS = 36Ω

ZO = 50Ω

OutB1

The waveform plots in Figure 8 “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 MPC9774 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 MPC9774. The output waveform in Figure 8 “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 + 17 + 25)

= 1.31V

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

unity reflection coefficient, to 2.6V. It will then increment

towards the quiescent 3.0V in steps separated by one round

trip delay (in this case 4.0ns).

Figure 8. 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 9 “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 9. Optimized Dual Line Termination

14

Ω

MPC9774

OUTPUT

BUFFER

RS = 22Ω

ZO = 50Ω

RS = 22Ω

ZO = 50Ω

14

Ω + 22Ω k 22Ω = 50Ω k 50Ω

25

Ω = 25Ω