Subject: RE: FSM Enhancement Goals and Thoughts
From: Michael McNamara (mac@verisity.com)
Date: Tue Dec 11 2001 - 17:50:13 PST
Clifford E. Cummings writes:
> Hi, All
> PROPOSALS
>
> enum - works great as is.
>
> If we do want a separate state type, I suggest:
>
> enum_state - to at least get away from the commonly used identifier name of
> "state."
>
> NEW! Add an enumeration type that would benefit onehot logic. This would be
> a true enhancement over both Verilog and VHDL. Add either:
>
> enum_onehot
> enum_state_onehot
Also useful in hardware design in general (not necessarily for FSMs)
for fault tolerant systems are gray codes -> where a change in a
single bit always gets you to a known state; and avoids the
transistion from state 3'b100 -> 3'b010 inadvertantly appearing to be
appearing to be decoded as a walk through something like:
3'b100 -> 3'b000 -> 3'b010
Instead of adding yet another keyword (enum_gray), I'd propose we use
our attribute:
enum my_state [
IDLE, READ1, READ2, WRITE1, WRITE2, WRITE3
] (* enum_encoding={onehot|onecold|gray|increment|decrement}*) ;
That why we allow more encodings than we can today imagine, and we
don't take away anymore namespace.
>
> Waveform viewers would now treat the onehot enumeration as an index as
> opposed to an actual encoding.
>
> Probably would also require:
>
> case_onehot(state)
> IDLE: ... ->> S0; // treated the same as:
> // case(1'b1)
> // state[IDLE]: ... next[S0] = 1'b1;
>
> Using something like the above syntax would make the transition from
> binary to one hot as simple as changing "enum" to "enum_onehot" and "case"
> to "case_onehot." And the waveform viewer will now look at which index bits
> are hot to determine which statename to display.
>
> Note: state machines are not the only hardware that use onehot codes. Wide
> fast multiplexers are often coded using onehot selects wired to tristate
> drivers that drive a common bus. Control logic is often coded as either
> encoded or onehot, for the latter each control bit does not have to first
> be decoded.
>
> Just some additional thoughts on goals and techniques.
>
> Regards - Cliff
>
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