MPC9350 データシートの表示（PDF） - Motorola => Freescale

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MPC9350

Low Voltage PLL Clock Driver

Motorola => Freescale 

MPC9350

Freescale Semiconductor, Inc.

The waveform plots in Figure 5. “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 MPC9350 output buffer is more than

sufficient to drive 50Ω 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

MPC9350. The output waveform in Figure 5. “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

36Ω 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 = 17Ω

VL = 3.0 ( 25 ÷ (18+17+25)

= 1.25V

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).

3.0

OutA

2.5

tD = 3.8956

OutB

tD = 3.9386

2.0

In

1.5

1.0

0.5

0

2

4

6

8

10 12 14

TIME (nS)

Figure 5. 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 6. “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.

MPC9350

OUTPUT

BUFFER

14Ω

RS = 22Ω ZO = 50Ω

RS = 22Ω ZO = 50Ω

14Ω + 22Ω k 22Ω = 50Ω k 50Ω

25Ω = 25Ω

Figure 6. Optimized Dual Line Termination

MOTOROLA

For More Informa8tion On This Product,

Go to: www.freescale.com

TIMING SOLUTIONS

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