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CLa_R
Novice
491 Views

VHDL multiple asynchronous signal handling

Hi everyone,

to realize an asynchronous project,

I would need to manage different signals and understand when they change state, both from 0 to 1 and from 1 to 0.

 

The change from these signals should change an output signal,

which drives another section of the circuit.

For this, I use:

process(signal_1) begin if something then signal_out <= '0'; end if; end;   process(signal_2) begin if something then signal_out <= '1'; end if; end;

In this case, I obtain an error:

Error (10028): Can't resolve multiple constant drivers for net "signal_out" .

For this reasons I try to insert all signals in one process, in this manner:

 

process(signal_1, signal_2) begin if rising_edge(signal_1) or falling_edge(signal_1) then if something then signal_out <= '0'; end if; end if;   if rising_edge(signal_2) or falling_edge(signal_2) then if something then signal_out <= '1'; end if; end if;     end;  

But, in this case, I have the error:

Error (10628): […] can't implement register for two clock edges combined with a binary operator.

 

What is the correct way to get what I want?

 

0 Kudos
7 Replies
Abe
Valued Contributor II
69 Views

Well, these are typically beginner mistakes in VHDL coding.

  1. A signal/variable cannot be updated in two different Processes.
  2. A signal/register/variable cannot be assigned values at both clock edges of two different clock signals.

process (signal_1, signal_2) begin if(signal_1) then signal_out <= 0; else signal_out <= 1; end if end process;

In the above code, there's an issue... signal_out gets assigned 0 when signal_1 =1 and irrespective of signal_2. Same goes for the else condition. To avoid, this we have to specify all logically exclusive conditions in the if-else loop.

 

process (signal_1, signal_2) begin if(signal_1 == '1' and signal_2 = '0') then signal_out <= 0; else if (signal_1 == '0' and signal_2 == '1') then --left out conditions are sig_1 = sig_2 == 1 , sig_1 = sig_2 == 0. signal_out <= 1; end if end process;

 

Normally, you assign signals on the rising edge or falling edge of a periodic signal like a clock and have the other signal in the sensitivity list of the process as the reset so that the Flops in the design can be reset to a known state on Power ON.

 

process( clock, reset) begin if(reset == '0') then signal_out <= '0'; else if (rising_edge/falling_edge clock) then signal_out <= '1' end if end process

This will make sure that the code infers flip-flops and not larches or other combinational logic that is unwanted (unless or course you are designing combinational circuits, in which case you would code it in a different way.)

CLa_R
Novice
69 Views

Thank you for your answer.

What I try to do is change signal_out when signal_1 or signal_2 change their state.

 

It is assumed that signal_1 and signal_2 CANNOT change at the same time.

 

In the first case, if signal_1 changes, then signal_out must be set to 0. In any case, both that signal_1 goes from 0 to 1, and that it passes from 1 to 0.

 

It's an asynchronous design and I'm not using any clock.

Can I do this with the code you showed me?

CLa_R
Novice
69 Views

Can I use this code? It is correct?

process (signal_1, signal_2) begin   if signal_1 = '0' or signal_1 = '1' then signal_out <= '0' end if;   if signal_2 = '0' or signal_2 = '1' then signal_out <= '1' end if;   end;

 

Vicky1
Employee
69 Views

Hi,

if possible please provide the project file, that will help us to better support.

if you don`t want to share it in community, you can use "My Messages" option to share with me. My_messages.JPG

Regards,

Vikas

 

CLa_R
Novice
69 Views

The basic problem is that I would like to implement the 2-phase handshake protocol.

 

For this reason, I need to understand when a signal changes state.

Is it possible to capture an event that indicates that the signal has changed its logic level?

CLa_R
Novice
69 Views

It is possible to make a edge detector in VHDL? I think that It can to resolve the problem.

 

The Edge detector is a circuit like this:

Edge_Detector.png

Can I make it in VHDL?

I try with:

 

process (myInput) begin If (not (not myInput)) xor myInput = '1' then --Edge detect! end if; end process;

 

 

but this code do not compile/work (Error (10476): VHDL error at Control.vhd(90): type of identifier "MyInput" does not agree with its usage as "boolean" type)

Vicky1
Employee
69 Views

Hi,

Could you check with .vhd file generated from .bdf file?

Open .bdf file( as designed in previous post) in Quartus & then,

"File" Menu->"Create/Update"->"Create HDL design file from current file(VHDL",

 

Regards,

Vikas

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