The left-hand-side of continuous assignment is illegal

Question

My input consists of a parameterized number of units. The output I need is to remove the first bit of each unit. For example if the input has units of size 3-bits each and the input value was 011011, the output should be 1111

Here is the solution i used for this:

parameter data_in_size = 11;
parameter data_out_size = 10;
parameter units = 4;
parameter skip_bits = 1;

input [data_in_size * units - 1 : 0] data_in;
output [data_in_size * units - 1 : 0] data_out;

genvar i;
generate
for (i = 0; i < units; i = i + 1) begin
    assign data_out[data_out_size * i +: data_out_size] = data_in [(data_in_size * i + skip_bits) +: data_out_size];

end
endgenerate

But I get the following error The left-hand-side of continuous assignment is illegal

How can i get through this error and why i am getting it ?

I don't see a problem (other than a type in the declaration of data_out). — dave_59

sharvil111 sharvil111 · Accepted Answer · 2016-01-07T03:54:19

Your code works perfectly well. Might be a simulator issue.

But I would like to focus on logic implementation in your code. The logic seems to be wrong.

First error seems to be in declaration of data_out and the slicing logic : data_in [(data_in_size * i + skip_bits) +: data_out_size] must be replaced with: data_in [(data_in_size * i + skip_bits) +: data_in_size]

Lets suppose i=0; data_out_size=3; data_in_size=4 then the LHS evaluates to data_out[2:0] = data_in[4:1]. When i=1, data_out[5:3] = data_in[8:5]. As you can see the bit slicing seems to be incorrect. Since LSB is sliced.

I think you might need to have following logic for bit slicing:

assign data_out[data_out_size * i +: data_out_size] = data_in[(data_in_size * i) +: (data_in_size - skip_bits)];

This will slice the MSB bit of every unit chunk, keeping the rest of bits as it is.

I simulated your code at EdaPlayground here. As an example, you will have following data_out and data_in values. Note the sliced MSB from each unit.

data_int = 1100101010010101
data_out = 100010001101

The left-hand-side of continuous assignment is illegal

1 Answers