MPC9653 REVISION 4 FEBRUARY 4, 2013

8

©2013 Integrated Device Technology, Inc.

MPC9653 Data Sheet

3.3 V 1:8 LVCMOS PLL CLOCK GENERATOR

Figure 6. Single versus Dual Transmission Lines

The waveform plots in Figure 7 show the simulation

results of an output driving a single line versus two lines. In

both cases the drive capability of the MPC9653 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 MPC9653. The output waveform

in Figure 7 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

 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 =36  || 36 

R0 =14 

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 2.6 V. It will then increment

towards the quiescent 3.0 V in steps separated by one round

trip delay (in this case 4.0ns).

Figure 7. Single versus Dual Waveforms

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 8 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 8. Optimized Dual Line Termination

14



IN

MPC953

OUTPUT

BUFFER

RS = 36 

ZO = 50 

OutA

14



IN

MPC953

OUTPUT

BUFFER

RS = 36 

ZO = 50 

OutB0

RS = 36 

ZO = 50 

OutB1

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

14



MPC961

OUTPUT

BUFFER

RS = 22 

ZO = 50 

RS = 22 

ZO = 50 

14

+ 22  || 22  =50  || 50 

25

 =25 

Pulse

Generator

Z = 50



RT = 50 

ZO = 50 

RT = 50 

ZO = 50 

MPC0653 DUT

VTT

VTT