VHDL code and unintended latches

Question

I am working on coding a Regsiter a1 with input signals b1,rst and wra1 the register a1 is initialized to a specific value at reset. a1 only changes its value to b1 when wra1 is 1 else it keeps the old value

process(clk,regrst) begin if(regrst='1')then a1 <= (others =>'0'); elsif(clk'event and clk='1') then if(wra1='1')then a1 <= b1; end if; end if; end process;

In this statement I have written the register process to update on the needed wra1 signal but I am confused on how to make it keep the old value of the output in case this write signal is 0 does this concept generate unintended latches to store the previous value? if so what can I add as a default register assignment in the begning of the process ? and should I include the else statement with

a1<=a1;

Thank you

Your code generates a register with asynchronous reset and write enable as intended. A else statement is not needed. This works because your sensitivity list has only asynchronous signals listed (the clock and reset signal). Unless there is a rising edge, there can't be any changes except for reset. So this is a true flip flop behavior, not a latch behavior :) — Paebbels
– Paebbels, Commented Dec 20, 2014 at 13:51

Philippe Aubertin · Accepted Answer · 2014-12-29 21:44:29Z

As Paebbels mentions in his comment, your process works exactly as you intend it.

You get a flip-flop, and not a latch, because of the condition on clk in your if statement, which specifies that a1 is assigned only when clk rises. clk'event means a change just happened on clk, so clk'event and clk='1' means "clk just changed and is now one", i.e. a rising edge.

A default assignment as the one you propose is not necessary because a signal keeps its previous value if one is not assigned explicitly. This simplifies sequential processes as the one you posted, but is also the source of unwanted latches in combinatorial processes.

The following is a latch:

process(wra1, b1) begin if(wra1='1')then a1 <= b1; end if; end process;

Note that 1) this process has no clock (i.e. no rising edge condition on a signal), just like a process describing combinatorial logic, but 2) no value is assigned to a1 when wra1 is not 1, which means it keep its current value until wra1 is 1 again, even if b1 changes. That's the semantic of a latch.

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