VHDL or verilog SR latch

0
votes

I'm trying to program my NEXYS2 board with a SR latch with NAND gates with an enable signal C. My inputs are (S, R, C) and outputs are (Q, Qbar). below is some code in VHDL that I tried but keep getting errors. also the schematics are below if that helps anyone to understand my question. if you know it in verilog that's okay too. Thank you in advance

process(S,R,C,Q,Qbar)
begin
    if (C = '1') then
        Q <= (R nand Qbar);
        Qbar <= (S nand Q);
    end if;
end process;

Schematic

2
vhdlverilog
Could you please add to the question the error messages you are seeing? - rick
if gives me a clock error but I'm not sure why because i'm trying to do this asynchronously without a clock by just using a control input - user3023323
The C input is probably being handled as "clock" by the synthesis tool, since C may be used as enable to the inferred latches in the implementation. The synthesis tools are not made to handle designs with loops like this, and may try to break the loop by inferring latches. I assume you design is only part of some kind of test, since design like this does not scale to large systems. - Morten Zilmer

2 Answers

2
votes

First off, your implementation of your schematic is incorrect: R should be ~S, and S should be ~R. try tracing the logic through with S or R equal to 1 when C is 1 - the outputs should set high.

Corrected Verilog (I've only got Altera & Verilog here):

module top(
           input wire S, R, C,
           output reg Q, Qbar);

   always @(S, R, C, Q, Qbar)
      if(C) begin
         Q    <= (~R & Q) | S;
         Qbar <= (~S & Q) | R;
      end
endmodule

This synthesises Ok on Altera, with 2 warnings along the lines of

Warning (10240): Verilog HDL Always Construct warning at test.v(5): inferring latch(es) for variable "Q", which holds its previous value in one or more paths through the always construct

and a warning from the timing analyser that it analysed two combinational loops as latches. The technology view looks plausible, but you have got a problem with this coding style, which could lead to issues.

The problem is that you explicitly code the only feedback paths, but you also leave the synthesiser to infer a latch. Your code includes if(C = '1'), so the synthesiser infers memory behaviour - a latch - when C is not 1. However, this is pointless, since you're also telling it explicitly where the latch is with your feedback paths. Don't do both; it's a mistake to assume that any synthesiser is smart enough to figure out what you really meant. Here's a version that leaves no doubt about what you want:

module top(
           input wire S, R, C,
           output wire Q, Qbar);

   wire   S2   = ~(C & S);
   wire   R2   = ~(C & R);
   assign Q    = ~(S2 & Qbar);
   assign Qbar = ~(R2 & Q);
endmodule

This instead gives two 'Found combinational loop of 2 nodes' warnings, as you'd expect. This also synthesises Ok, and the RTL/technology views look Ok, at first sight.

Standard disclaimer: timing analsysers are not good at timing designs with combinational loops. this will probably all just work if you're just playing around, but if this is a real design you'll need to think hard about your constraints, and whether the analyser has actually traced through your feedback path (it probably hasn't). You'll need to do a timing sim with sdf back-annotation to confirm that it actually works.

1
votes

Your schematic is correct.

The problem is in your VHDL code, because you didn't specify the circuit's functionalities completely (note that the output values for c='0' were not defined), which causes the inference of latches (only registered circuits can be designed with incomplete specifications because then the resulting signals are stored in D-type flip-flops anyway).

You can do the following (among other alternatives):

entity sr_latch is
  port ( 
    s, r, c: in bit;
    q, qbar: buffer bit);
end entity;

architecture sr_latch of sr_latch is 
begin 
  q <= (s nand c) nand qbar;
  qbar <= (r nand c) nand q;
end architecture