General Technical Discussion

[gerstn]

hello,

how can I write ABEL Code like this:

BR.clk = CK;    // BR is a node register CK is a input pin

BR.ar = RSN;  // RSN is a input pin

in VHDL.

Thanks for your help

[gszakacs]

You can find this in the language templates included in the ISE GUI.  What you're

describing in Abel is a flip-flop with an asynchronous clear input.  Normally you'd also

have an equation for the D input of your flip-flop.

-- Usage of Asynchronous resets may negatively impact FPGA resources
-- and timing. In general faster and smaller FPGA designs will
-- result from not using Asynchronous Resets. Please refer to
-- the Synthesis and Simulation Design Guide for more information.
process (<clock>, <reset>)
begin
   if <reset>='1' then  
      <output> <= '0';
   elsif (<clock>'event and <clock>='1') then
      <output> <= <input>;
   end if;
end process;

In the above code, <output> could be replaced with BR,

<input> would be whatever goes to BR.d, <reset> could be RSN, and

<clock> would be CK.  Note that this code generates an active

high reset input.  RSN seems to imply an active low signal, but your

Abel code doesn't seem to bear this out.  The language templates

also have standard templates for flip-flops with asynchronous

presets and clock enables in all combinations of rising and falling

edge and active high or low inputs.

[gerstn]

Hey thanks for your help!!

If I understand your description the following Abel Code:

when !BR6 then BR := BR +1
 else        BR := 0 ;
   BR.CLK = CK;
   BR.AR = RSN;
 UC := !UC;
   UC.CLK = BR6;
   UC.AR = RSN;

is the same as this in VHDL

if BR(6) ='0' then BR <= BR+1;

elsif BR<='0';

process (CK, RSN)
begin
   if RSN='1' then   
      BR <= '0';
   elsif (CK'event and CK ='1') then 
      BR <= BR;
   end if;
end process;

[gerstn]

sorry I forgot 

UC <= NOT UC;

process (BR(6), RSN)
begin
   if RSN='1' then   
      UC <= '0';
   elsif (BR(6)'event and BR(6) ='1') then 
      UC <= UC;
   end if;
end process;

thanks for your reply

[gszakacs · Accepted solution]

Not quite.  The D input part of the equation needs to be inside the process

block in order to happen at the clock edge.  In Abel you just use the := to

indicate at the clock edge.  In VHDL the assignment needs to take place

inside the elsif ...

Like:

process (CK, RSN)
begin
   if RSN='1' then   
      BR <= '0';
   elsif (CK'event and CK ='1') then

       if BR(6) ='0' then BR <= BR+1;

       else BR<="0000000"; -- in VHDL width of constant has to match width of BR

   end if;
end process;

And:

process (BR(6), RSN)
begin
   if RSN='1' then   
      UC <= '0';
   elsif (BR(6)'event and BR(6) ='1') then 
      UC <= not UC;
   end if;
end process;

[gerstn]

Ahh OK thanks a lot!!! Maybe I will have a question to my further work. Am I allowed to write you a pn if this happens ? :)