MPC9772 datasheet(13/17 Pages) IDT

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MPC9772 REVISION 7 JANUARY 8, 2013

MPC9772 Data Sheet

3.3V 1:12 LVCMOS PLL CLOCK GENERATOR

Figure 12. Single versus Dual Transmission Lines

The waveform plots in Figure 13 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

 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 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 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+17+25)

=1.31 V

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 4.0 ns).

Figure 13. 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 14 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 14. Optimized Dual Line Termination

Figure 15. CCLK MPC9772 AC Test Reference



MPC9772

Output

Buffer

RS = 36 

ZO = 50 

OutA



MPC9772

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

OutB

tD = 3.9386

OutA

tD = 3.8956



MPC9772

Output

Buffer

RS = 22 

ZO = 50 

RS = 22 

ZO = 50 

 + 22  || 22  = 50  || 50 

 = 25 

Pulse

Generator

Z = 50



RT = 50

ZO = 50

RT = 50

ZO = 50

MPC9772 DUT

VTT

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