Re: [myhdl-list] myhdl.AlwaysCombError: signal (startmpy) used as inout in always_comb function argument

[Josy B. <jos...@gm...>]

Christopher Felton <chris.felton <at> gmail.com> writes:

> 
> Why have a single comb vs. two comb?

Because they target the same outputs. I'll augment the simplified 
example a bit:

def contrivedtwostatemachineexample( Clk, Reset,  StartP, Status, ... )
    states_first = enum( "IDLE" , "ONE" , "TWO")
    smfp, smfn = [Signal( states_first.IDLE ) for _ in range( 2 )]
    states_second = enum( "IDLE" , "BUSY")
    smfp, smfn = [Signal( states_second .IDLE ) for _ in range( 2 )]
    startsecond, donesecond = [Signal(bool(0)) _ for _ in range(2)}

     <at> always_comb
    def smcomb():
        startsecond.next = 0
        donesecond.next = 0
        Status.next = 0

        if smfp == states_first.IDLE:
           Status.next = 1
           if StartP:
               smfn.next = states_first.ONE
               startsecond.next = 1
           else:
               smfn.next = states_first.IDLE

        elif smfp == states_first.ONE:
           Status.next = 2
           if donesecond:
               smfn.next = states_first.TWO
               startsecond.next = 1
           else:
              smfn.next = states_first.ONE

        elif smfp == states_first.TWO:
           Status.next = 3
           if donesecond:
               smfn.next = states_first.IDLE
           else:
              smfn.next = states_first.TWO

       if smsp == states_second.IDLE:
           Status.next = 10
           if startsecond:
               smsn.next = states_second.BUSY
           else:
               smsn.next = states_second.IDLE

       elif smsp == states_second.BUSY:
           Status.next = 11
           donesecond.next = 1
           # must catch an immediate start command
           if startsecond:
               smsn.next = states_second.BUSY
           else:
               smsn.next = states_second.IDLE

     <at> always_seq(Clk.posedge, reset=Reset)
    def smprocess():
        smfp.next = smfn
        smsp.next = smsn

   return smcomb, smprocess

If we split it over two processes, VHDL will complain about multiple 
drivers ...

> 
>      def smcombfp():
>      ...
> 
>      def smcombsn():
>      ...
> 
> I believe the inout comb detection is a mechanism to
> protect against unwanted comb feedback.  If you have
> a complicated  <at> always_comb it probably make sense to
> split it out rather than remove the comb feedback
> detection.

What is **unwanted comb feedback**? Is this in the Verilog LRM? Or is 
the comb feedback in the code to protect us from ourselves? If it is not 
against Verilog, the **comb feedback detection** should, IMHO, be 
reworked. If Verilog has a problem with this ... you tell me.

Splitting my actual code in two processes would add clumsy workarounds, 
making the code not any prettier.

Regards,

Josy