cancel
Showing results for 
Show  only  | Search instead for 
Did you mean: 
aniket1992
Visitor
Visitor
10,023 Views
Registered: ‎06-24-2012

Reg: Feedback in combinatorial logic

Hi,

 

We are coding a combinatorial circuit with feedback in Verilog. 

 

In first stage there are 2 AND gates and outputs of those two gates are inputs to an XOR gate in the second stage. Then, the output of this is one of the inputs to the first AND gate in first stage. Remaining 3 inputs in first stage are left to the user. (normal non-feedback inputs).

 

How to implement this without getting an error/warning in synthesis report. (The warning we get is that it removes the output port which is used as the feedback).

 

Also, kindly let me know if there is any good material to learn about combinatorial logic circuits that involve feedback.

 

Thanks in advance,

Aniket

0 Kudos
1 Reply
eteam00
Instructor
Instructor
10,021 Views
Registered: ‎07-21-2009

There are quite a few past threads discussing delay chains, ring oscillators, and pulse/glitch generators.  You might want to search the forums to browse through these threads, if you have not already done so.

 

Time for a reality check.  In the real world of FPGA hardware, there are no AND gates or XOR gates.  There are LUTs.  If you are endeavouring to design at the very most basic, primitive level of FPGA hardware, then you need to abandon all levels of abstraction -- including quaint notions of discrete logic gates.

 

-- Bob Elkind

SIGNATURE:
README for newbies is here: http://forums.xilinx.com/t5/New-Users-Forum/README-first-Help-for-new-users/td-p/219369

Summary:
1. Read the manual or user guide. Have you read the manual? Can you find the manual?
2. Search the forums (and search the web) for similar topics.
3. Do not post the same question on multiple forums.
4. Do not post a new topic or question on someone else's thread, start a new thread!
5. Students: Copying code is not the same as learning to design.
6 "It does not work" is not a question which can be answered. Provide useful details (with webpage, datasheet links, please).
7. You are not charged extra fees for comments in your code.
8. I am not paid for forum posts. If I write a good post, then I have been good for nothing.
0 Kudos