VHDL Tutorial

Time type in VHDL
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Time type

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Time

 

VHDL has a predefined type called time that is used to represent simulation times and delays.   We can write a time value as a numeric literal followed by a time unit.   For example:

 

5 ns    22 us    471.3 msec

 

Notice that we must include a space before the unit name.   The valid unit names are

 

fs    ps    ns    us    ms    sec    min    hr

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The type time includes both positive and negative values.  VHDL also has a rede- fined subtype of time, delay_length, that only includes non-negative values.

Many  of  the  arithmetic  operators  can  be  applied  to  time  values,  but  with  some restrictions.  The addition, subtraction, identity and negation operators can be applied

to yield results of type time A time value can be multiplied or divided by an integer

or real value to yield a time value, and two time values can be divided to yield an in- teger For example:

 

18 ns / 2.0  =  9 ns,    33 ns / 22 ps  =  1500

 

Finally, the abs operator may be applied to a time value, for example:

 

abs 2 ps  =  2 ps,    abs (2 ps)  =  2 ps

 

 

 

 

 

 

 

 

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