what is the difference between the following 3 sign extensions
wire signed [7:0] a;
wire signed [10:0] a_sxtnd1;
wire signed [10:0] a_sxtnd2;
wire signed [10:0] a_sxtnd3;
assign a_sxtnd1 = $signed(a);
assign a_sxtnd2 = $signed( { {3{a[7]}} ,a} );
assign a_sxtnd3 = $signed( { 3'd0 ,a} );
What are the differences and anyone of them will fail?
There is no need for $signed in all three assignments.
In the first assignment to a_sxtnd1, both the LHS and RHS are signed, to a will be implicitly sign extended from 8 bits to 11 bits
In the second assignment to a_sxtnd2, you are explicitly sign extending a to an 11-bit result.
In the third assignment to a_sxtnd3, you are explicitly zero extending a to an 11-bit result.
So the third assignment is not like the other two, if that is what you mean by "fail"
