VHDL Tutorial

Bits type in VHDL
Fundamental concepts
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Elements of behaviour
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Lexical elements
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Scalar type
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Time type
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Character types
Boolean type 
Bits type

Standard logic
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Array types & array operations
Architecture bodies
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Behavioral descriptions 
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Conditional signal assignment 
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Parameterizing behavior
Parameterizing structure




Since VHDL is used to model digital systems, it is useful to have a data type to repre- sent bit values.  The predefined enumeration type bit serves this purpose.  It is defined



type bit is ('0', '1');


The logical operators that we mentioned for Boolean values can also be applied

to values of type bit, and they produce results of type bit The value 0 corresponds

to false, and 1 corresponds to true.   So, for example:


'0' and '1' = '0',    '1' xor '1' = '0'


The difference between the types boolean and bit is that boolean values are used

to  model  abstract  conditions,  whereas  bit  values  are  used  to  model  hardware  logic levels.   Thus, 0 represents a low logic level and 1 represents a high logic level.


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