A casex statement cannot be directly translated to a VHDL case
statement as VHDL does not treat the don't-care bit as special.
The solution here is to generate an if statement from the casex
which compares only the non-don't-care bit positions.
The exponentiation operator in VHDL is not defined for numeric_std
types. We can get around this by converting the operands to integers,
performing the operation, then converting the result back to the
original type. This will work OK in simulation but certainly will not
synthesise unless the operands are constant.
However, even this does not work quite correctly. The Integer type in
VHDL is signed and usually only 32 bits, therefore any result larger
than this will overflow and raise an exception. I can't see a way
around this at the moment.
Previously this was handled by creating an internal
signal that was connected to the output and could also
be read inside the entity. The correct solution is to
make the output `buffer' rather than `out'. However, this
does not work in the case when an output is connected to
an output of a child entity, and that values is read
in the parent. In this case *both* the outputs of the child
and the parent need to be made `buffer'.
Combinatorial UDPs will be implemented with a `with ... select'
statemetnt. However the input to this must be "locally static".
This patch joins the inputs into a vector which can be used as
the select expression.
This patch adds a forward declaration for every user funciton.
This fixes VHDL compile problems if a function calls another
before it has been declared.
Whilst adding `wait until ...' at the end of every
process is a valid translation of the input, it is not
actually synthesisable in at least one commercial
synthesiser (XST). According to the XST manual the
correct template is to use `wait until ...' at the
start of sequential processes and `wait on ...'
(equivalent to `wait until ...' with 'Event on all
the signals) at the end of combinatorial processes.
This patch implements that.
Initial processes set a magic flag in the code generator
which allows it to push constant assignments into the
VHDL signal initialisation and omit the assignment.
However, it should only do this if the signal has not
already been read (otherwise the previous read would
not get the undefined value as expected)
If output P of A is connected to output Q of B (and A is
instantiated inside B) then VHDL does not allow B to read
the value of Q (also P), but Verilog does. To get around
this the output Q is mapped to P_Sig which is then connected
to P, this allows B to read the value of P/Q via P_Sig.
Nick Gasson
Larry Doolittle