Use Pspice to Model a Star Connected Heavy Load Digital Board

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Use Pspice to Model a Star Connected Heavy Load Digital Board

This was a project I did before. We had 4 DSP processors to perform high speed image processing. Control and data hub is a high speed Virtex6 FPGA. There is also an SDRAM to buffer the data. Six components need to be inter-connected. So we tried star connection as shown in below Pspice model:




Fig 1. Pspice Model

where all 6 transmission lines are identical with Zo=50ohm and delay as 450ps. Delay of FR4 microstrip is 145ps/inch. So 450ps is about 3 inches. Voltage VPULSE source is set as V1=0, V2=1.8v, TR=TF=0.1ns,  PW=15ns, and PER=30ns.

Sim shows with serial R11 as 45-55ohm, receiver side voltage is good as below.


Fig2. Receiver Waveform with R11=50ohm



But¬†waveform¬†distorts¬†when we increase R11 to 90ohm¬†(Fig3) and 180ohm(Fig4).¬†Changing transmission line delay(aka length) didn’t help. Note DSP and Virtex6 driving strength is 10mA. For 1.8v IO, it is 1.8v/10mA=180ohm. On¬†average it is 180/2=90ohm. So this result shows this topology¬†doesn’t work. Main reason is¬†R11 ofDSP and Virtext6 is¬†too large so it takes¬†a long¬†time to charge all capacitance. We eventually dropped this idea. Share this lesson to show you how we used Pspice model to reveal¬†board design issue¬†BEFORE board¬†was built.



Fig23. Receiver Waveform with R11=90ohm



Fig4. Receiver Waveform with R11=180ohm

If you are interested to give it a try yourself, Pspice schematics file of Fig 1 is attached.


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  1. mazhar 2 months ago

    Great answer, thanks for sharing.


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