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Beginner
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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?

 

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7 Replies
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Valued Contributor II
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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.)

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Beginner
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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?

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Beginner
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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;

 

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Employee
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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

 

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Beginner
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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?

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Beginner
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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)

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Employee
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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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