Question

Which code is better in writing a RAM?

1. assigning data_out inside always block:

   module memory(
       output reg [7:0] data_out,
       input [7:0] address,
       input [7:0] data_in, 
       input write_enable,
       input clk
   );
       reg [7:0] memory [0:255];
   
       always @(posedge clk) begin
           if (write_enable) begin
               memory[address] <= data_in;
           end
           data_out <= memory[address];
       end
   
   endmodule
   

2. assigning data_out using assign statement:

   module memory(
       output [7:0] data_out,
       input [7:0] address,
       input [7:0] data_in, 
       input write_enable,
       input clk
   );
       reg [7:0] memory [0:255];
   
       always @(posedge clk) begin
           if (write_enable) begin
               memory[address] <= data_in;
           end
       end
   
       assign data_out = memory[address];
   
   endmodule
   

Any recommendations?

Answer 1

It depends on your requirements.

1. This registers your memory output. If you are synthesizing this to gates, you will have 8 more flip-flops than in case 2. That means you use a little more area. It also means your output will have less propagation delay relative to the clock than case 2. Furthermore, the output data will not be available until the next clock cycle.

2. Your output data will be available within the same clock cycle as it was written, albeit with longer propagation delay relative to the clock.

You need to decide which to use based on your requirements.

A third option is to use a generated RAM, which is a hard macro. This should have area, power and possibly timing advantages over both case 1 and 2.