Multiple constant drivers error?

Hi there,

The problem is that my project is running perfectly on ModelSim but when I've tried to compile it on Quartus 8.1 it gave me multiple constant drivers error.

Here is part of the code in verilog:

wire ob_write = (out_buffer_write == 2'b10);
always @(posedge clk)
  if(SCK_risingedge)
    cs <= ~SSEL;
....
always @(posedge clk ) begin
  if (ob_write)
    byte_data_sent <= ram_data_out;
    
end
always @(posedge clk)
  out_buffer_write <= {out_buffer_write, (com_read_enable | com_set_enable)};
always @(posedge clk) begin
  if (~cs)
    byte_data_sent <= 8'h00;
....

The error is: Error (10028): Can't resolve multiple constant drivers for net "byte_data_sent[7]" at spi.v(124)

Any suggestions?

Thanks in advance :)