This message appears when the Net or Reg does not have a driver.
WHAT NEXT:
Make surethe Net in question is connectedto a source, so that it can drive the net, and re-run XST.
EXAMPLE:
Consider the following RTL:
module top (din, clk, rst, out);
input din;
input clk;
input rst;
output out;
reg temp_reg;
reg OutData;
always @(posedge clk, posedge rst) begin
if(rst)
OutData <= temp_reg;
else
OutData <= din;
end
assign out = OutData;
endmodule
In the above example,the register 'temp_reg' is not driven by a source (driver).