As far as I know, there are 4 types of FSM. Mealy, Moore, Medvedev, and registered output. In registered output, you can have up to 4 processes (you can find an example here). In Medvedev, there can be 1 or 2. In others, there can be 1 or 2 processes for the state-machine, and 1 process for output that can be merged with the combinational process.
Suppose this FSM:
One-Process FSM VHDL code for that would be:
FSM_FF: process (CLK, RESET)
begin
if RESET='1' then
STATE <= START;
elsif CLK'event and CLK='1' then
case STATE is
when START =>
if X=GO_MID then
STATE <= MIDDLE;
end if;
when MIDDLE =>
if X=GO_STOP then
STATE <= STOP;
end if;
when STOP =>
if X=GO_START then
STATE <= START;
end if;
when others => STATE <= START;
end case;
end if;
end process FSM_FF;
Two-Process FSM VHDL code:
FSM_LOGIC: process( STATE, X)
begin
case STATE is
when START =>
if X=GO_MID then
NEXT_STATE <= MIDDLE;
end if ;
when MIDDLE =>
...
when others => NEXT_STATE <= START;
end case;
end process FSM_LOGIC;
---------------------------------------------
FSM_FF: process(CLK, RESET)
begin
if RESET='1' then
STATE <= START;
elsif CLK'event and CLK='1' then
STATE <= NEXT_STATE;
end if;
end process FSM_FF;
WHICH ONE SHOULD YOU USE?
According to Jensen,
It depends on a number of things. Do you need a Moore machine or a
Mealy machine? Is the downstream logic that received the output signal
synchronous or combinatorial? It also depends on which style of coding
you prefer.
Prof. Saheb Zamani compared the 1-Process and 2-Process from three aspects (slide 86):
- Structure and Readability: From the hardware perspective, combinational and sequential elements are two different things, so a
separated design is closer to hardware. On the other side, the
graphical FSM (without output equations)
resembles more a 1 process than a 2 process description.
- Simulation: Error detection easier with two state processes due to access to
intermediate signals.
- Synthesis: 2 state processes can lead to smaller generic netlist and therefore
to better synthesis results (depends on synthesizer but in general,
it is closer to hardware)
P.S. I couldn't find the original sources for these links, so I just added the sources that I used. But this one is more comprehensive and includes some examples as well.
