MPC9772

TIMING SOLUTIONS

13

MOTOROLA

The waveform plots in Figure 13. “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 MPC9772 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

MPC9772. The output waveform in Figure 13. “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:

= 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).

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 14. “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 12. Single versus Dual Transmission Lines

14

Ω

IN

MPC9772

OUTPUT

BUFFER

OutA

14

Ω

IN

MPC9772

OUTPUT

BUFFER

OutB0

OutB1

Figure 13. 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

OutA

In

Figure 14. Optimized Dual Line Termination

14

Ω

MPC9772

OUTPUT

BUFFER

14

Ω + 22Ω || 22Ω = 50Ω || 50Ω

25

Ω = 25Ω

Figure 15. CCLK MPC9772 AC Test Reference

Pulse

Generator

Z = 50

Ω

MPC9772 DUT