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FSM Optimization

Finite State Machine (FSM) optimization is part of synthesis. You can write an FSM in either VHDL or Verilog, and your synthesis tool can re-code the states according to your specification. However sometimes your synthesis tool may appear to ignore your commands - this section looks at some reasons why this might happen.

The examples in this section use VHDL attributes to define the required coding - the same ideas can be used in Verilog, but using meta-comments (special comments that are interpreted by the synthesis tool).

Using Synopsys Synplify Pro® with FSMs

The default behaviour of Synplify is to "do its best" - finite state machine optimization if fully automatic.

According to the Synplify documentation, you can control state machine optimization in two ways, firstly be specifying the codes you want:

type StateType is (Idle, Start, Stop, Clear);
attribute syn_enum_encoding: string;
attribute syn_enum_encoding of StateType: type is "001 010 100 111";

and secondly by specifying the kind of coding:

attribute syn_encoding: string;
attribute syn_encoding of State: signal is "sequential,safe";

However for either of these to work, you must turn off FSM Compiler i.e. make sure the option FSM Compiler is not checked.

In a Tcl script, use the command:

set_options -symbolic_fsm_compiler 0
# or set_options -autosm  0

Using Xilinx XST with FSMs

XST allows control of finite state machines using attributes or from the GUI. However XST will ignore attributes (or meta-comments in Verilog) unless you set the FSM mode to "user". Here is an example of synthesis of a simple enumerated type and the results:

  type StateType is (Zero, Start, Running, Stop, Stopped, Reset);
  attribute enum_encoding : string;
  attribute enum_encoding of statetype : type is "001 010 100 110 111 101";

and here is the resulting encoding after synthesis with XST:

Analyzing FSM <FSM_0> for best encoding.
Optimizing FSM <State/FSM> on signal <State[1:3]> with sequential encoding.
---------------------
 State   | Encoding
---------------------
 zero    | 000
 start   | 001
 running | 010
 stop    | 101
 stopped | 011
 reset   | 100
---------------------

Hmmm... that doesn't look right. To get our attribute to have an effect, we need to set the encoding algorithm to User. To do this from the GUI:

  • Right clock on the XST-Synthesize process
  • Select Properties...
  • Highlight HDL Options
  • Set FSM Encoding Algorithm to User

To do the same using an attribute, use the code

  signal State, NextState: StateType;

  attribute fsm_encoding: string;
  attribute fsm_encoding of State : signal is "User";

Now after synthesis, the encoding is:

---------------------
 State   | Encoding
---------------------
 zero    | 001
 start   | 010
 running | 100
 stop    | 110
 stopped | 111
 reset   | 101
---------------------

as we intended.

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