state machine sync process

[Andrei Bejenari]

Hello,

In a Xilinx manual there is the above example of a state machine. My
question is why there is a separate process SYNC_PROC to change the
machine's state. Can't it be done in a single process and using a single
signal for the current state?

entity enum is
port (
CLOCK, RESET : in STD_LOGIC;
A, B, C, D, E : in BOOLEAN;
SINGLE, MULTI, CONTIG : out STD_LOGIC
);
end enum;

architecture BEHV of enum is
type STATE_TYPE is (S1, S2, S3, S4, S5, S6, S7);
signal CS, NS: STATE_TYPE;
begin
SYNC_PROC: process (CLOCK, RESET)
begin
if (RESET='1') then
CS <= S1;
elsif (CLOCK'event and CLOCK = '1') then
CS <= NS;
end if;
end process; --End SYNC_PROC
COMB_PROC: process (CS, A, B, C, D, E)
begin
case CS is
when S1 =>
MULTI <= '0';
CONTIG <= '0';
SINGLE <= '0';
..
..
..

Thanks,

4ndre1.

 

[Andrei Bejenari]

In a Xilinx manual there is the above example of a state machine. My

^^^^^^^
sorry, the example is below

entity enum is
port (
CLOCK, RESET : in STD_LOGIC;
A, B, C, D, E : in BOOLEAN;
SINGLE, MULTI, CONTIG : out STD_LOGIC
);
end enum;

architecture BEHV of enum is
type STATE_TYPE is (S1, S2, S3, S4, S5, S6, S7);
signal CS, NS: STATE_TYPE;
begin
SYNC_PROC: process (CLOCK, RESET)
begin
if (RESET='1') then
CS <= S1;
elsif (CLOCK'event and CLOCK = '1') then
CS <= NS;
end if;
end process; --End SYNC_PROC
COMB_PROC: process (CS, A, B, C, D, E)
begin
case CS is
when S1 =>
MULTI <= '0';
CONTIG <= '0';
SINGLE <= '0';
.
.
.

Thanks,

4ndre1.

 

[ALuPin]

Hi,

of course you can use one single clocked process.

The advantage of using two separated processes - one for the register
transfers and one for the combinational logic -
is that you can generate signals which are combinational. If you use
only one
clocked process you will generate registered output signals out of the
state machine.

One disadvantage of using two processes is that you have to take care
about the sensitivity list of the combinational process.

Hope this helps.

Rgds

 

[Neo]

No, it can be done both ways giving the same implementattion. But it
helps to have it seperate if the state determining logic is quite
elaborate.

 

[Mike Treseler]

No, it can be done both ways giving the same implementattion. But it
helps to have it seperate if the state determining logic is quite
elaborate.

An alternative is to collect that elaborate logic
into well-named procedures and functions in order
to unclutter your state case statement.
Put the declarations between the IS and BEGIN
of the clocked process. Doesn't cost a thing
in utilization and it makes it easier to
try out logic revisions.


-- Mike Treseler