HELP! VHDL real TYPE produces infinite decimal. How to approximateor reduce precision?

Discussion in 'VHDL' started by adam.kimura, Jun 4, 2013.

  1. adam.kimura

    adam.kimura Guest

    Hello

    I am fairly new to VHDL and have hit a wall in my code.

    Basically, I am wanting to convert a 50% duty cycle frequency signal into aperiod with microsecond units. In VHDL, I perform a 1/frequency operationin order to get the period and then convert it to seconds. I declared theperiod as a real type. The problem I have is when I end up with a frequency that creates an infinite decimal for the period calculation (Example: 1/3 = 0.333333..... )My simulator cannot handle the infinite decimal.

    Is there a way to limit the precision of a REAL type number in VHDL? Should I use a floating point? I appreciate the help!
     
    adam.kimura, Jun 4, 2013
    #1
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  2. adam.kimura

    adam.kimura Guest

    Hello Alan,

    I think I got it working. I honestly don't know what happened. I was trying to re-create the error that I was getting, but it seemed to work fine now. Is there a limit on the number of decimal places a REAL type can have? From what I understood from the error message, the decimal places were infinite; which caused the simulation error. I understood this to mean that VHDL cannot represent such a number beyond a certain number of decimal places. In languages like C++, a number like 1/3 cannot be accurately represented since it is 0.3333333.... (re-occurring 3.) I have the situation where I am dividing 1 by decimal numbers... hence the long decimals.

    Thanks again for your help.

    ak
     
    adam.kimura, Jun 5, 2013
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  3. adam.kimura

    rickman Guest

    Yes, the REAL data type in VHDL has limited precision just like most
    data types in programming languages. I'm not sure what error you might
    have seen. I can't think of anything in VHDL that would have a problem
    with your use of a REAL data type unless you were trying to synthesize
    it. To turn 1/3 into a real number you must first convert the 1 and the
    3 into reals and then divide them as real. No, it won't give you
    exactly 1/3, but it will not give you an error.

    I think the error must have been related to how you were trying to
    convert them.

    Rick



     
    rickman, Jun 5, 2013
    #3
  4. adam.kimura

    adam.kimura Guest

    Thank you Rick... very helpful information. Thanks for taking the time to respond. I sincerely appreciate it!

    Best,

    Adam.
     
    adam.kimura, Jun 5, 2013
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