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rf_man

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how to simulate a shift register?
Aug 31^st, 2007, 4:54am

Hello,

As an RF design engineer, I have no experience with digital simulators, and also not quite familiar with digital stuff. I would like to get help from you that how to simulate a shift register. The shift register is an 8-bit register, consiscts of 8 in series connected D-FFs, and the Q output of the 8th D-FF is fed back to the D input of the 1st D-FF. As I know, once a word is write to the register, say 10000000, then the shift register will shift one bit to the right till the right-most bit, and then rotate back to the left most bit...

In the SpectreRF I used vpulse to simulate the register but without feedback, I can see that the bit is shifted to the right, but how to simulate the register with feedback? how to write the initial word into the register, and then let the shift register rotate itself?

Thanks!

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Geoffrey_Coram Senior Fellow Offline Posts: 1999 Massachusetts, USA	Re: how to simulate a shift register? Reply #1 - Aug 31^st, 2007, 8:09am Does your DFF model have a set or reset pin? You may be able to use initial conditions (ic statement in Spectre).
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rf_man Junior Member Offline Posts: 28	Re: how to simulate a shift register? Reply #2 - Sep 3^rd, 2007, 12:50am yes, there are set and clear pins. you mean to use the "INITIAL CONDITION PARAMETERS" option in SpectreRF transient option form? How do you set this option then?
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Geoffrey_Coram

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Posts: 1999
Massachusetts, USA

Re: how to simulate a shift register?
Reply #3 - Sep 4^th, 2007, 4:53am

I run spectre from the command line; I don't know about the option form. I would guess you could put
out=1 fb=0.5
in that box to set node "out" to have an initial condition of 1 volt, etc.

First, try it without feedback, using the ic (rather than the vpulse) to set the first bit of the register and see that it clocks out. Then try setting a bit in the middle to see how the ic fights with the output of the previous DFF.

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If at first you do succeed, STOP, raise your standards, and stop wasting your time.

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