Finite State Machine (FSM)

Nattha JindapetchDecember 2008

Agenda

Finite State Machine (FSM) Synthesis techniques Lab5

Finite State Machines Basically a FSM consists of

Control logic is used to decide the next state of the FSM sequential is used to store the current state of the FSM output logic 　 is a mixture of both comb. and seq. logic

Two types Moore machine

O = f(Q)Q’= f(Q,I)

Mealy machineO = f(Q,I)Q’= f(Q,I)

Output Logic

Control Logic

Register(Flip-Flops)

Inputs(I) Outputs(O)

Nextstate(Q')

Presentstate(Q)

(b) Mealy machine(a) Moore machine

Output Logic

Control Logic

Register(Flip-Flops)

Inputs(I)

Outputs(O)

Nextstate(Q')

Presentstate(Q)

Clock Clock

Example 1

State diagramState table

Example 1

State diagram

module jk_counter(count, clock);input clock;output reg [2:0] count;parameter [2:0] A = 3'b000,

B = 3'b100, C = 3'b111,D = 3'b010, E = 3'b011;

always @ (posedge clock)case (count)

A: count <= B;B: count <= C;C: count <= D;D: count <= E;E: count <= A;default: count <= A;

endcaseendmodule

Ex2 : Moore Machines

Check input steam If in = 1 two consecutive periods, then out=1

State DiagramState Diagram

0S0S““00””

1S1S““00””

2S2S““11

00

00

00

11 11 11

SymbolSymbol

0S0S““outout””

inin

The meaning of State Diagram

For example, at state S0 At state S0 the circuit output out = 0 At the rising-edge of clk, if in = 1, the

state will change to S1, otherwise S0

000S0S““00””

1S1S““00””

2S2S““11””

00

00

11 11 11

Ex2 : Moore Machines module Moore1 (clk, IN, OUT);

input clk, IN;output OUT;reg [1:0] State;parameter [1:0] s0=2’b00, s1=2’b01, s2=2’b11;

always @ (posedge clock)case (State)

s0: begin OUT <= 0; if (IN) State <= s1; else State <= s

0; end

s1: begin OUT <= 0; if (IN) State <= s2; else State <= s0;

ends2: begin

OUT <= 1; if (!IN) State <= s0; else State <= s2;

enddefault: begin

OUT <= 0; if (IN) State <= s1; else State <= s0;

endcaseendmodule

Ex3: Mealy Machines

State DiagramState Diagram

S0S0 S1S10/00/0

0/00/0

1/01/0 1/11/1

SymbolSymbol

S0S0

in/in/outout

Check input steam If in = 1 two consecutive periods, then out=1

For example, at state S1 At S1, the output OUT = IN (OUT=1 if

IN=1 and OUT=0 if IN=0) At the rising-edge of clk, if IN = 0 the

state will change to S0, otherwise S1

0/0/ 00

1/1/ 11S0S0 S1S10/00/0 1/01/0

The meaning of State Diagram

Ex3: Mealy Machines

module Mealy1 (clk, IN, OUT);input clk, IN;output OUT;reg State;parameter s0=1’b0, s1=1’b1;

always @ (posedge clock)case (State)

s0: begin OUT <= 0; if (IN) State <= s1; else State <= s0; end

s1: begin OUT <= IN; if (IN) State <= s2; else State <= s0;

enddefault: begin

OUT <= 0; if (IN) State <= s1; else State <= s0; end

endcaseendmodule

Ex4: Arbiter

First come first serve

Ex4_1: Arbiter Code Using A Function for Control Logic (1/3)

module fsm_using_function (clock, reset, req_0, req_1, gnt_0, gnt_1);

input clock,reset,req_0,req_1;output gnt_0,gnt_1;

wire clock,reset,req_0,req_1;

reg gnt_0,gnt_1;parameter SIZE = 3 ;parameter IDLE =3'b001,GNT0 = 3'b010,GNT1 =

3'b100;

reg [SIZE-1:0] state;wire [SIZE-1:0]

next_state;

assign next_state = fsm_function(state, req_0, req_1);

//----------Function for Control Logic----------function [SIZE-1:0] fsm_function; input [SIZE-1:0] state ; input req_0 ; input req_1 ; case (state) IDLE : if (req_0 == 1'b1) begin fsm_function = GNT0; end else if (req_1 == 1'b1)

begin fsm_function= GNT1; end else begin fsm_function = IDLE; end

Ex4_1: Arbiter Code Using A Function for Control Logic (2/3)

GNT0 : if (req_0 == 1'b1) begin

fsm_function = GNT0; end else begin fsm_function = IDLE; end GNT1 : if (req_1 == 1'b1)

begin fsm_function = GNT1; end else begin fsm_function = IDLE; end default : fsm_function = IDLE; endcaseendfunction

//----------Seq Logic-----------------------------always @ (posedge clock)begin : FSM_SEQ if (reset == 1'b1) begin state <= #1 IDLE; end else begin state <= #1 next_state; endend

Ex4_1: Arbiter Code Using A Function for Control Logic (3/3)

//----------Output Logic-------------

always @ (posedge clock)begin : OUTPUT_LOGICif (reset == 1'b1) begin gnt_0 <= #1 1'b0; gnt_1 <= #1 1'b0;endelse begin case (state) IDLE : begin gnt_0 <= #1 1'b0; gnt_1 <= #1 1'b0; end

GNT0 : begin gnt_0 <= #1 1'b1; gnt_1 <= #1 1'b0; end GNT1 : begin gnt_0 <= #1 1'b0; gnt_1 <= #1 1'b1; end default : begin gnt_0 <= #1 1'b0; gnt_1 <= #1 1'b0; end endcaseendend // End Of Block OUTPUT_LOGIC

endmodule // End of Module arbiter

Ex4_2: Arbiter Code Using Two Always Blocks (1/2)

module fsm_using_always (clock, reset, req_0, req_1, gnt_0, gnt_1);

input clock,reset,req_0,req_1;output gnt_0,gnt_1;wire clock,reset,req_0,req_1;reg gnt_0,gnt_1;

parameter SIZE = 3 ;parameter IDLE =

3'b001,GNT0 = 3'b010,GNT1 = 3'b100 ;

reg [SIZE-1:0] state; reg [SIZE-1:0]

next_state;

always @ (state or req_0 or req_1)begin : FSM_CONTROL next_state = 3'b000; case (state) IDLE : if (req_0 == 1'b1) next_state = GNT0; else if (req_1 == 1'b1) next_state= GNT1; else next_state = IDLE; GNT0 : if (req_0 == 1'b1) next_state = GNT0; else next_state = IDLE; GNT1 : if (req_1 == 1'b1) next_state = GNT1; else next_state = IDLE; default : next_state = IDLE; endcaseend

Ex4_2: Arbiter Code Using Two Always Blocks (2/2)

//----------Seq Logic-always @ (posedge clock)begin : FSM_SEQ if (reset == 1'b1)

state <= #1 IDLE; else state <= #1 next_state;end//----------Output

Logic---------------always @ (posedge clock)begin : OUTPUT_LOGIC if (reset == 1'b1) begin gnt_0 <= #1 1'b0; gnt_1 <= #1 1'b0; end

else begin case (state) IDLE : begin gnt_0 <= #1 1'b0; gnt_1 <= #1 1'b0; end GNT0 : begin gnt_0 <= #1 1'b1; gnt_1 <= #1 1'b0; end GNT1 : begin gnt_0 <= #1 1'b0; gnt_1 <= #1 1'b1; end default : begin gnt_0 <= #1

1'b0; gnt_1 <= #1 1'b0; end endcase endend // End Of Block OUTPUT_LOGICendmodule // End of Module arbiter

always @ (posedge clock)begin : FSMif (reset == 1'b1) begin state <= #1 IDLE; gnt_0 <= 0; gnt_1 <= 0;end else case (state) IDLE : if (req_0 == 1'b1) begin state <= #1 GNT0; gnt_0 <= 1; end else if (req_1 == 1'b1)

begin gnt_1 <= 1; state <= #1 GNT1; end else state <= #1 IDLE;

module fsm_using_always (clock, reset, req_0, req_1, gnt_0, gnt_1);

input clock,reset,req_0,req_1;output gnt_0,gnt_1;wire clock,reset,req_0,req_1;reg gnt_0,gnt_1;

parameter SIZE = 3 ;parameter IDLE =

3'b001,GNT0 = 3'b010,GNT1 = 3'b100 ;

reg [SIZE-1:0] state; reg [SIZE-1:0]

next_state;

Ex4_3: Arbiter Code Using Single Always For Sequential, Control Logic And Output Logic (1/2)

GNT0 : if (req_0 == 1'b1) begin state <= #1 GNT0; end else begin gnt_0 <= 0; state <= #1 IDLE; end GNT1 : if (req_1 == 1'b1) begin state <= #1 GNT1; end else begin gnt_1 <= 0; state <= #1 IDLE; end default : state <= #1 IDLE;endcaseendendmodule // End of Module arbiter

Ex4_3: Arbiter Code Using Single Always For Sequential, Control Logic And Output Logic (2/2)

Synthesis Techniques

Using Hierarchy leads to greater design readability, reuse, and debug

Hierarchical Design

Top-Level

Control Datapath Memory

FSM1 FSM2 ROM RAMALU Counters Regs

Benefits of Using Hierarchy Design readability

Easier to understand the design functionality and data flow

Easier to debug Easy to reuse parts of a design

Coding Tips Synchronous reset—better system control

but depend on the circuit behavior

Asynchronous Reset

always @(posedge CLOCK or posedge RESET)if (RESET)

Q = 0;else

Q = D_IN;

Asynchronous Reset

always @(posedge CLOCK or posedge RESET)if (RESET)

Q = 0;else

Q = D_IN;

Synchronous Reset

always @(posedge CLOCK)if (RESET)

Q = 0;else

Q = D_IN;

Synchronous Reset

always @(posedge CLOCK)if (RESET)

Q = 0;else

Q = D_IN;

Coding Tips Order and group arithmetic and

logical functions and operators

A <= (B + C) + (D + E);is better than

A <= B + C + D + E;

Basic Performance Tips Simple coding yields better performance Avoid high-level loop constructs

Synthesis tools may not produce optimal results Avoid nested if-then-else statements

Most tools implement these in parallel; however, multiple nested if-then-else statements can result in priority encoded logic

Use case statements for large decoding Rather than if-then-else

State Machine Encoding

Use enumerated types to define state vectors

Most synthesis tools have commands to extract and re-encode state machines described in this way

Use one-hot encoding for high-performance state machines

Uses more registers, but simplifies next-state logic

Experiment to discover how your synthesis tool chooses the default encoding scheme

Lab5: FSM for display scan Four 7-segment Digits share the same data bus

Lab5: FSM for display scan

State3D=0111sel=11

State0D=1110sel=00

State1D=1101sel=01State2

D=1011sel=10

Reset