Embedded FSM Language

Proposal Details

Authors	Patrick Lehmann
Date Proposed	2017-04-02
Last Updated	2017-04-02
Focus	simulation and synthesis
LCS	TBD

Summary

State machine are currently written in a 1-, 2- or 3 process pattern using regular VHDL process statements. It proposal presents a concurrent statement that describes a state machine with various extra features which are hard to implement in normal process based FSMs.

Current Situation

There are three accepted VHDL coding pattern to describe a state machine in VHDL:

1 process pattern
2 process pattern
3 process pattern

Requirement

The new statemachine statement shall support the following features:

Single clocked
Synchronous and asynchronous reset
Required default assignments (no latches)
Clear transitions on goto
Entry and exit handlers
Selectable encoding via attribute
Supports Mealy and Moore type
Supports init (FPGA) and reset state (both)

Planned features

Sub state machines
Virtual states
State machine to state machine communication

Use Cases

Add some use cases.

Implementation details

New reserved words:

statemachine
state
entry
goto

Code Examples

statemachine myFSM is
  configuration (
    Clock =>     clk,
    SyncReset => rst,
    -- ASyncReset => rst_a
    InitialState => IDLE,
    Reset state => IDLE
  );
  default (
    LeaveIdle => ‘0’,
    Working => ‘0’,
    ErrorRaised => ‘0’
  );
  state IDLE is
    goto WORKING;
    on exit to (all)
      LeaveIdle <= ‘1’;
    end on exit;
  end state;

  state WORKINK is
    Working <= ‘1’;
    goto ERROR when (err = ‘1’);
  end state;

  state ERROR is
    on entry from (WORKING)
      ErrorRaised <= ‘1’;
    end on entry;
  end state;
end statemachine;

Arguments FOR

Clean syntax
Lower complexity for big FSMs compared to process pattern
No pattern matching in VHDL tools to find FSMs

Arguments AGAINST

4 new reserved words

General Comments

Supporters

Patrick Lehmann - 2017-04-02
Add your signature here to indicate your support for the proposal

This topic: P1076 > WebHome > VHDLProposals > FSMLanguage
Topic revision: r2 - 2017-04-02 - 22:11:18 - PatrickLehmann