VHDL FSM description subset

This FSM subset is neither accepted by the logic simulator asimut, nor the formal prover proof.

This VHDL subset is defined to enable classical MOORE and MEALY synchronous finite state machine description as well as stack FSM description (see man syf for further information about this kind of FSM).

A FSM description is made of two and only two processes.

Connectors and signals can only be of IN, OUT, and two user defined enumerated types. Vectors of IN and OUT types are also allowed.

FSM's states and stack control signals must be declared as enumerated type.

For the scan-path, three more signals are required:

• scan_test: IN BIT
• scan_in: IN BIT
• scan_out: OUT BIT

For a ROM implementation, the Vdd and Vss signals must be explicitely declared as:

• rom_vdd : IN BIT
• rom_vss : IN BIT

These signals, declared in the interface, are not used or assigned in the FSM description.

The '-P' option of syf(1) allows scan-path implementation.

Pragmas

Three pragmas are used, their generic names are:

• CLOCK : External clock signal name.
• CURRENT_STATE : Current State name.
• NEXT_STATE : Next State name.

Ten other pragmas are optional.

1. Three pragmas are required only for scan-path implementation.
   • SCAN_TEST : Enable test mode (scan-path).
   • SCAN_IN : scan-path input.
   • SCAN_OUT : scan-path output.
2. Five others are used only in a STACK FSM.
   • RETURN_STATE : Return State name.
   • CONTROL : Stack Control signal name.
   • POP : POP operation on the stack.
   • PUSH : PUSH operation on the stack.
   • NOP : NOP operation on the stack.
3. The last ones for ROM implementation
   • ROM_VDD : Name of the Vdd signal of the ROM.
   • ROM_VSS :Name of the Vss signal of the ROM.

Two different processes are used: The first process, called state process, allows to describe state transition and outputs generation. It is not controlled by the clock. The second process is controlled by the clock and describes the state register and stack registers modifications.

State process sensitivity list contains inputs and CURRENT_STATE, it means that the state process is activated when the CURRENT_STATE or an input signal changes. A CASE statement is used to describe, for each state, the next state and outputs.

The second process sensitivity list contains the clock signal, so this process is enabled whenever clock changes.

Both Level sensitive latches, and falling edge triggered flip flops can be used for state registers and stack implementation.

EXAMPLES

ENTITY FSM_Ex IS

PORT (
   ck    : IN BIT;
   reset : IN BIT;
   t_mode: IN BIT;
   s_in  : IN BIT;
   i     : IN BIT;
   s_out : OUT BIT;
   o     : OUT BIT
);
End FSM_Ex;

ARCHITECTURE auto OF FSM_Ex IS

TYPE STATE_TYPE IS ( S0, S1, S2, S3, S4, S5 );
TYPE CONTROL IS ( PUSH, POP, NOP );

-- pragma CLOCK ck
-- pragma CURRENT_STATE CURRENT_STATE
-- pragma NEXT_STATE NEXT_STATE
-- pragma RETURN_STATE RETURN_STATE
-- pragma CONTROL CTRL
-- pragma PUSH PUSH
-- pragma POP POP
-- pragma NOP NOP
-- pragma SCAN_TEST t_mode
-- pragma SCAN_IN s_in
-- pragma SCAN_OUT s_out

SIGNAL CURRENT_STATE, NEXT_STATE, RETURN_STATE : STATE_TYPE;
SIGNAL CTRL : CONTROL;
SIGNAL STACK_0, STACK_1 : STATE_TYPE ;

BEGIN

PROCESS ( CURRENT_STATE, I, reset )
  BEGIN
    IF ( reset ) THEN
      NEXT_STATE <= S0;
      o <= '0' ;
    ELSE
      CASE CURRENT_STATE IS
        WHEN S0 =>
                      NEXT_STATE <= S1;
                      RETURN_STATE <= S5;
                      CTRL <= PUSH;
                      o <= '0';

        WHEN S1 =>
                      IF ( I = '1' ) THEN
                        NEXT_STATE <= S2;
                        CTRL <= NOP;
                      ELSE
                        NEXT_STATE <= S3;
                        CTRL <= NOP;
                      END IF;
                      o <= '0';

        WHEN S2 =>
                      NEXT_STATE <= S4;
                      CTRL <= NOP;
                      o <= '0';

        WHEN S3 =>
                      NEXT_STATE <= S4;
                      CTRL <= NOP;
                      o <= '0';

        WHEN S4 =>
                      NEXT_STATE <= STACK_0;
                      CTRL <= POP;
                      o <= '1';

        WHEN S5 =>
                      IF ( I = '1' ) THEN
                        NEXT_STATE <= S1;
                        RETURN_STATE <= S0 ;
                        CTRL <= PUSH;
                     ELSE
                        NEXT_STATE <= S5;
                        CTRL <= NOP;
                     END IF ;
                     o <= '0';

       WHEN others =>
                      ASSERT ( '1' )
                      REPORT "Illegal state";

      END CASE;
    END IF ;
END PROCESS;

PROCESS ( ck )
  BEGIN
    IF ( ck = '0' AND NOT ck'STABLE ) THEN
      CURRENT_STATE <= NEXT_STATE;
      CASE CTRL IS

        WHEN POP =>
                      STACK_0 <= STACK_1;

        WHEN PUSH =>
                      STACK_1 <= STACK_0;
                      STACK_0 <= RETURN_STATE;

        WHEN NOP =>
                      NULL;

      END CASE;
    END IF;
END PROCESS;

END auto;

BUG REPORT

Laboratoire d'Informatique de Paris 6 Université Pierre et Marie Curie - CNRS