# I can't figure this out!

Discussion in 'Homework Help' started by Joppy, Apr 30, 2015.

1. ### Joppy Thread Starter New Member

Apr 30, 2015
2
0
The image uploaded is a Linear Feedback shift register with equation X3 = X1 ⊕ X0. From looking through my textbook and online i have discovered how LFSR's work and what they are used for, however i seem to be missing some sort of fundamental understanding of how they work due to my inability to solve the following problem.

The questions are as follows...

The second XOR gate (denoted with a '*') can be used to invert the value at X3, how?

The question im struggling with more is...

What happens if the counter is in an initial state of "000"? How can the second XOR help?

Any help is much appreciated!

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2. ### Papabravo Expert

Feb 24, 2006
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If you look at a truth table for the XOR function with one input always at 0 you see that the output is a copy of the input. What happens if you take that grounded input to a logic 1 (high). What does the output do then?

3. ### Joppy Thread Starter New Member

Apr 30, 2015
2
0
Thanks for your insight! Yes I realise that an XOR with one input tied high becomes an inverter. I will have to clarify with someone else if the grounded input is supposed to remain as a ground ( I know it seems silly... But perhaps it's a trivial solution ). Just confuses me why they would make that input grounded to start with.

Thanks again

I will remove the thread as I haven't stated the question clearly ( my misunderstanding of course ).