Connecting modules using conditional (?:) assign statements

Question

i'm trying to connect 2 modules differently depending on bit_proc_rw signal value. Can't post pictures yet, so:

if bit_proc_rw dataflow: bit_in --> SCR --> CRC --> bit_out

if not, it's a reciever: bit_in --> CRC --> SCR --> bit_out

So I did this in a top module(all wire-type):

assign SCR_input = (bit_proc_rw) ?  bit_proc_in : CRC_output; 
assign SCR_input_dav = (bit_proc_rw) ? bit_proc_in_dav : CRC_output_dav;

assign SCR_output = (bit_proc_rw) ? CRC_input : bit_proc_out;
assign SCR_output_dav = (bit_proc_rw) ? CRC_input_dav : bit_proc_out_dav;

assign CRC_input = (bit_proc_rw) ?  SCR_output : bit_proc_in; 
assign CRC_input_dav = (bit_proc_rw) ? SCR_output_dav : bit_proc_in_dav;

assign CRC_output = (bit_proc_rw) ? bit_proc_out : SCR_input;
assign CRC_output_dav = (bit_proc_rw) ? bit_proc_out_dav : SCR_input_dav;


crc_gen crc_gen_u (
.clk (clk),
.crc_in (CRC_input),
.crc_in_dav (CRC_input_dav),
.crc_out (CRC_output),
.crc_out_dav (CRC_output_dav),
.bit_proc_rw (bit_proc_rw),

);

scr scr_u (
.clk (clk),
.scr_in (SCR_input),
.scr_in_dav (SCR_input_dav),
.scr_out (SCR_output),
.scr_out_dav (SCR_output_dav),
.bit_proc_rw (bit_proc_rw)
);

Testbench shows that only inputs are being driven(ex. rw=0 CRC input works but rest are of unknown value). Is it because i'm assigning both input to output and output to input at the same time? Or is this correct and i should look for bugs in my code? If not, how to do this using only 1 instance of each module?

Hope I made everything clear and thanks for your time!

Greg Greg · Accepted Answer · 2013-11-04T19:43:28

The conditional operator (?:) is 2:1 mono-directional mux. The assign statements also mono-directional. Therefore the assignments are putting additional drivers on signals matching *_output*. The multiple conflicting drivers values are the cause of the unknown values. To correct the error, remove the assignments for outputs and assign a mux to bit_proc_out* with a mux

/* Remove / comment out:
 * assign SCR_output = (bit_proc_rw) ? CRC_input : bit_proc_out;
 * assign SCR_output_dav = (bit_proc_rw) ? CRC_input_dav : bit_proc_out_dav;
 *
 * assign CRC_output = (bit_proc_rw) ? bit_proc_out : SCR_input;
 * assign CRC_output_dav = (bit_proc_rw) ? bit_proc_out_dav : SCR_input_dav;
 */

// Add
assign bit_proc_out = (bit_proc_rw) ? CRC_output : SCR_output;
assign bit_proc_out_dav = (bit_proc_rw) ? CRC_output_dav : SCR_output_dav;

Connecting modules using conditional (?:) assign statements

1 Answers