Re: Concat with a constant

On 5/22/2015 7:07 AM, Henry Gomersall wrote:
> Is there some neat way to concatenate a signal with a constant?
>

You can't simply concatenate with a constant that
is a Python `int` because it doesn't have a bit
length that MyHDL uses, the constant would need to
be an `intbv` or `bool`.

http://docs.myhdl.org/en/latest/manual/reference.html?highlight=concat#myhdl.concat

If we had initial values you could use constants
assigned outside the generators (see below).  The
best approach I can think of is to assign the constant
to a variable and then concat.

     # MYHDL
     def m_concat_const(x, y):
        # CONST1 will not work because initial values
        # is not enabled ...
        CONST1 = intbv(10, min=0, max=137)
        CONST2 = 5

        @always_comb
        def rtl():
            const2 = intbv(5, min=0, max=8)  # or intbv(CONST2 ...)?
            y.next = concat(x[4:0], CONST1, const2)
        return rtl


     # Converted VHDL
     M_CONCAT_CONST_RTL: process (x) is
         variable CONST1: unsigned(7 downto 0);
         variable const2: unsigned(2 downto 0);
     begin
         const2 := to_unsigned(5, 3);
         y <= resize(unsigned'(x & CONST1 & const2), 12);
     end process M_CONCAT_CONST_RTL;


If you want to concat outside of a generator (elaboration)
then you would want to review the thread @josyb
referenced.

You can use ConcatSignal where one of the signals is
simply assigned to the constant:

     Sig1 = ...
     Sig2 = ...
     Sig3 = ConcatSignal(Sig1, Sig2)

     @always_comb
     def assign():
         Sig2.next = CONSTANT

Hope that helps,
Chris


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