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hello group,

I have written two small vhdl code and now I am asked the following
questions, please just point where I can find some reading about it to
answer them. (I am using xilinx ise)

regards,
amit


Different synthesis tools use different styles to synthesize the state
machines.
Whatstyle does XST use to synthesize the state machine that you
coded?
Is there any way to force the tool to use specific options for
synthesizing state machines?

Amit wrote:

> I have written two small vhdl code and now I am asked the following
> questions, please just point where I can find some reading about it to
> answer them. (I am using xilinx ise)

> Different synthesis tools use different styles to synthesize the state
> machines. What style does XST use to synthesize the state machine that you
> coded? Is there any way to force the tool to use specific options for
> synthesizing state machines?

It is unfortunate that your instructor would ask
such a poorly formed question.
A register that is declared as a type enumeration
has no explicit encoding. For example, a register
of the type

type TxState_t is (
IDLE,
START,
SEND,
STOP
);

is usually encoded using the binary values of two flops,
or onehot values of four flops. If the designer gives
no further hints, synthesis will use its default
encoding strategy. This strategy will vary by vendor,
target device, and tool version.

Last time I checked, brands A and X use binary
encoding up to enum length 4 and one-hot encoding
for larger enums. This is a synthesis optimization
and in 99.9% of cases the designer saves time
and gets adequate utilization by using the defaults.

Old school TTL and verilog designs did not have
the option of the enum abstraction, and many
academics remain fascinated by the notion
of an 'optimum' state encoding.

No worse than playing tetris, or working
crossword puzzles, I suppose.

-- Mike Treseler

[jeppe]

Well I'm an instructor and could have asked this question myself.

The correct answer will properly be:

If your implement a statemachine in a CPLD will the combinatorial logic compared with the number of F/Fs mean that binary and gray style gives the best implementation.

If how ever your coding for a FPGA will the number of F/F's be quite large and hence will a One hot coding give the best result. This specially true if you want to implement a statemachine with more then say 20 states.

BUT for a 5 states State Machine would it be hard to spot any diffence in the number af LUTs used.

If you right click at the Synthesize menu can you find a selection for the statemachine coding (See below)

Hope you found this useful
Jeppe

You can read Xilinx constraints guide. Here you will see that there is
an attribute called FSM_STYLE. By this you can force "XST" to use a
defined way of encoding. Or you can do it through synthesis options in
"XST".

I think this helps you.

Good luck,

--enes