Nesting Elseif, If, Else in VHDL

Question

In the code below, I want to pull out statements surrounding the the ** asterisks ** below.

stateS0 = 1' and o_done = '1' 

and

o_done = '1' and stateS0 = '0' 

into separate comparisons using if or elseif.

I am struggling at how to pull out and compare if I am in state S0 and o_done = 1 and I am in state 1 and o_done = 1 into seperate if, then, else, elsif statements.

 process begin
 wait until rising_edge(clk);
 if (reset = '1') then
 stuff;
 else
 **if( StateS0 = '1' AND o_done = '1' ) then**
 stuff;
 elsif( ...stuff... ) then
 stuff;
 end if;
 end if;
 end process;
 
 
 process begin
 wait until rising_edge(clk);
 if (reset = '1') then
 o_done <= '0';
 else
 if ( ...stuff ...) then
 o_done <= '1';
 **elsif ( o_done = '1' AND StateS0 = '0' ) then**
 o_done <= '1';
 else
 o_done <= '0';
 end if;
 end if;
 end process;

I was thinking something like:

if stateS0=0 then if
o_done='1' then...
if o_done='1' then if
stateS0='1' then...

but can you do...

process begin
 wait until rising_edge(clk);
 if (reset = '1') then
 stuff;
 else
 if( state0 = '1') then
 if o_done='1' then
 stuff;
 end if;
 end if;
 elsif( stuff ) then
 stuff;
 end if;
 end process;

is that legal code ?

Or is this legal ?

 process begin
 wait until rising_edge(clk);
 if (reset = '1') then
 o_done <= '0';
 else
 if ( stuff ) then
 o_done <= '1';
 elsif ( o_done = '1') then
 if state0 = '0' ) then
 o_done <= '1';
 else
 o_done <= '0';
 end if;
 end if;
 end process;

But I am not sure about if I am nesting these right. Can you nest an:

if
if
end if
end if

within after an elsif or else ?

Can you nest elseif in if, then else statements ? Or elseif only check 0 or 1 conditions and not combinatorial conditions like if then else can ?

Answer 1

Can you nest elseif in if, then else statements ?

You are certainly allowed to nest "if" statements inside each-other. This is perfectly legal code.

if boolean_expression_1 then
 if boolean_expression_1a then
 --some statements
 elsif boolean_expression_1b then
 --some statements
 elsif boolean_expression_1c then
 --some statements
 else
 --some statements
 end if;
 --some statements
elsif boolean_expression_2 then
 if boolean_expression_2a then
 --some statements
 elsif boolean_expression_2b then
 --some statements
 else
 --some statements
 end if;
else boolean_expression_3 then
 --some statements
end if;

Or elseif only check 0 or 1 conditions and not combinatorial conditions like if then else can ?

The types of expressions you can put after "elsif" are exactly the same as for "if".

As for your code. if the type of o_done is a character type that has '1' and '0' as possible values, and State0 and State1 are both booleans then the following should compile without problems.

 if ( State1 ) then
 o_done <= '1';
 elsif (o_done = '1' AND State0 ) then
 o_done <= '1';
 else
 o_done <= '0';
 end if;

This is also legal

if state=s0 then 
 if o_done='1' then
 --some statements here
 end if;
end if;
if o_done='1' then
 if stateS1='1' then
 --some statements here
 end if;
end if;