Xilinx Synthesis Technology User Guide

XST User Guide
If there is more than a single blocking/non blocking error, only the
first one will be reported.
In some cases, the line number for the error might be incorrect (as
there might be multiple lines where the signal has been assigned).
Integer Handling
There are several cases where XST handles integers differently from
other synthesis tools, and so they must be coded in a particular way.
In case statements, do not use unsized integers in case item
expressions, as this will cause unpredictable results. In the following
example, the case item expression “4” is an unsized integer that will
cause unpredictable results. To avoid problems, size the “4” to 3 bits
as shown below.
reg [2:0] condition1;
always @(condition1)
begin
case(condition1)
4 : data_out = 2; // < will generate bad logic
3'd4 : data_out = 2; // < will work
endcase
end
In concatenations, do not use unsized integers, as this will cause
unpredictable results. If you must use an expression that results in an
unsized integer, assign the expression to a temporary signal, and use
the temporary signal in the concatenation as shown below.
reg [31:0] temp;
assign temp = 4’b1111 % 2;
assign dout = {12/3,temp,din};
7-34 Xilinx Development System