Truth Table Statement (AHDL)

The Truth Table Statement is used to specify combinational logic or state machine behavior. In an AHDL truth table, each entry in the table contains a combination of input values that will produce specified output values. These output values can be used as feedback to specify state transitions and outputs of state machines.

The following example shows a Truth Table Statement:

TABLE 
   a0,  f[4..1].q    =>   f[4..1].d, control;
   0,   B"0000"      =>   B"0001",   1;
   0,   B"0100"      =>   B"0010",   0;
   1,   B"0XXX"      =>   B"0100",   0;
   X,   B"1111"      =>   B"0101",   1;
END TABLE;

The Truth Table Statement has the following characteristics:

• The truth table heading consists of the keyword TABLE, followed by a comma-separated list of table inputs, an arrow symbol (=>), and a comma-separated list of table outputs. The heading ends with a semicolon (;).
• Truth table inputs are Boolean expressions; truth table outputs are variables. In the example shown above, the input signals are a0 and f[4..1].q; the output signals are f[4..1].d and control.
• The body of the table consists of one or more entries, each spanning one or more lines and ending with a semicolon.
• An entry consists of a comma-separated list of inputs and a comma-separated list of numerical outputs. The inputs and outputs are separated by =>.
• Each signal has a one-to-one correspondence with the values in each entry. Thus, the first entry in the example shown above signifies that when a0 has the value 0 and f[4..1].q has the value B"0000", f[4..1].d has the value B"0001", and control has the value 1.
• Input and output values can be numbers, predefined constants VCC or GND, symbolic constants (that is, symbolic names used as constants), or groups of numbers or constants. Input values can also be X (don't care).
• Input and output values correspond to the inputs and outputs of the table heading.
• The keywords END TABLE, followed by a semicolon (;), end the truth table.