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Altera_Forum's avatar
Altera_Forum
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8 years ago

Asynchronous counter

Hey!

I'm doing a school project and I got problem which I have hard to solve so I hope for your help.

Basicly when I try to test my block diagram on my SW[2] output I get undefined value X.

I really don't know what is wrong in it. But I will add picture of how is diagram looking and what result do I get.

I also will add a ling to my diagram I wanted to look http://tinyurl.com/yaejw9a9

Thanks for all help and I also hope I used right subforum.

https://alteraforum.com/forum/attachment.php?attachmentid=14401&stc=1 https://alteraforum.com/forum/attachment.php?attachmentid=14402&stc=1

3 Replies

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    8 years ago

    The simulator doesn't know what value, '0' or '1', to give each of your flip flops at the start of simulation. So, it will give each the value of 'Z'. The result of any combinational logic when any of the input values is 'Z' will be 'X'.

    In practice (on real hardware) this won't be an issue since all your flip flops will power up as either a '0' or '1' and everything will work.

    So, I suggest you modify your code/schematic and introduce a 'reset' signal. When you assert this reset all the flip flops will take a known value ('0' or '1'). You may need to use a slightly different flip flop primitive. The one you've chosen doesn't appear to have a reset. However, there should be one that does.

    Cheers,

    Alex
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    8 years ago

    Hey

    Thanks for your help. First I tried with reset but I actually got same results as before. But I kept in my mind that simulator doesn't know what value it should give at the start of simulation.

    So I added extra PIN in the beginning with '1' value and that solved my problem. I guess it is hard way of doing it but at least it works well now ;)

    Just adding the picture of diagram and results I wanted to get (modulo 10).

    https://www.alteraforum.com/forum/attachment.php?attachmentid=14415 https://www.alteraforum.com/forum/attachment.php?attachmentid=14416