comp541 state machines – ii: verilog descriptions

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COMP541

State Machines – II: Verilog Descriptions

Montek Singh

Sep 24, 2014

Today’s Topics Lab preview:

“Debouncing” a switch Verilog styles for FSM

Don’t forget to check synthesis output and console msgs.

State machine stylesMoore vs. Mealy

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Lab Preview: Buttons and Debouncing Mechanical switches “bounce”

vibrations cause them to go to 1 and 0 a number of timescalled “chatter”

hundreds of times!

We want to do 2 things:“Debounce”: Any ideas?Synchronize with clock

i.e., only need to look at it at the next +ve edge of clock

Think about (for Wed class):What does it mean to “press the

button”? Think carefully!!What if button is held down for a

long time? 3

Verilog coding styles for FSMs

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Verilog coding styles First: More on Verilog procedural/behavioral

statementsif-then-elsecase statement

Then: How to specify an FSMUsing two always blocks

Using a single always block??Using three always blocks

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Verilog procedural statements All variables/signals assigned in an always

statement must be declared as reg

Unfortunately, the language designers made things unnecessarily complicatednot every signal declared as reg is actually registered it is possible to declare reg X, even when X is the output of

a combinational function, and does not need a register!whattt???!!

They thought it would be awesome to let the Verilog compiler figure out if a function is combinational or sequential!a real pain sometimesyou will suffer through it later in the semester if not

careful!

Verilog procedural statements These statements are often convenient:

if / elsecase, casezmore convenient than “? : ” conditional expressions

especially when deeply nested

But: these must be used only inside always blocksagain, some genius decided that

Result:designers often do this:

declare a combinational output as reg Xso they can use if/else/case statements to assign to XHOPE the Verilog compiler will optimize away the

unintended latch/reg

Example: Comb. Logic using casemodule decto7seg(input [3:0] data, output reg [7:0] segments); // reg is optimized away always @(*) case (data) // abcdefgp 0: segments <= 8'b11111100; 1: segments <= 8'b01100000; 2: segments <= 8'b11011010; 3: segments <= 8'b11110010; 4: segments <= 8'b01100110; 5: segments <= 8'b10110110; 6: segments <= 8'b10111110; 7: segments <= 8'b11100000; 8: segments <= 8'b11111110; 9: segments <= 8'b11110110; default: segments <= 8'b0000001; // required endcaseendmodule

Note the *:it means that when any inputs used in the body of the always block change.

This include “data”

Beware the unintended latch! Very easy to unintentionally specify a latch/register in

Verilog! how does it arise?

you forgot to define output for some input combination in order for a case statement to imply combinational logic, all

possible input combinations must be described one of the most common mistakes!

one of the biggest sources of headache!you will do it a gazillion times

this is yet another result of the the hangover of software programmingforgetting everything in hardware runs in parallel, and time

is continuous

Solution good programming practice

remember to use a default statement with cases every if must have a matching else

check synthesizer output / console messages9

Beware the unintended latch! Example: multiplexer

out is output of combinational blockno latch/register is intended in this circuit recommended Verilog:

assign out = select? A : B;

But, an if statement (inside an always block) will incorrectly introduce a reg:

if (select) out <= A;if (!select) out <= B;

reg added to save old value if condition is false to avoid extra reg, cover all cases within one

statement:if (select) out <= A;else out <= B; 10

select

A

Bout

Combinational Logic using casez casez allows case patterns to use don’t cares

module priority_casez(input [3:0] a, output reg [3:0] y);

// reg will be optimized away

always @(*) casez(a) 4'b1???: y <= 4'b1000; // ? = don’t care 4'b01??: y <= 4'b0100; 4'b001?: y <= 4'b0010; 4'b0001: y <= 4'b0001; default: y <= 4'b0000; endcase

endmodule

Blocking vs. Nonblocking Assignments (review) <= is a “nonblocking assignment”

Occurs simultaneously with others

= is a “blocking assignment” Occurs in the order it appears in the file

// nonblocking assignmentsmodule syncgood(input clk, input d, output reg q); reg n1; always @(posedge clk) begin n1 <= d; // nonblocking q <= n1; // nonblocking endendmodule

// blocking assignmentsmodule syncbad(input clk, input d, output reg q); reg n1; always @(posedge clk) begin n1 = d; // blocking q = n1; // blocking endendmodule

Cheat Sheet for comb. vs seq. logic Sequential logic:

Use always @(posedge clk)Use nonblocking assignments (<=)Do not make assignments to the same signal in more

than one always block!

e.g.:always @ (posedge clk) q <= d; // nonblocking

Cheat Sheet for comb. vs seq. logic Combinational logic:

Use continuous assignments (assign …) whenever readable

assign y = a & b;

OR

Use always @ (*)All variables must be assigned in every situation!

must have a default case in case statementmust have a closing else in an if statement

do not make assignments to the same signal in more than one always or assign statement

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Revisit the sequence recognizer(from last lecture)

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Let’s encode states using localparammodule seq_rec (input CLK, input RESET, input X,output reg Z);

reg [1:0] state;

localparam A = 2'b00, B = 2'b01, C = 2'b10, D = 2'b11;

Notice that we have assigned codes to the states.localparam is more appropriate here than parameter because these constants should be invisible/inaccessible to parent module.

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Next State logic

reg [1:0] next_state; // optimized awayalways @(*)

begincase (state) A: if (X == 1)

next_state <= B; else

next_state <= A; B: if(X) next_state <= C; else next_state <=

A; C: if(X) next_state <= C; else next_state <=

D; D: if(X) next_state <= B; else next_state <=

A; default: next_state <= A;endcase

end The last 3 cases do same thing.Just sparse syntax.

Register for storing state Register with reset

synchronous reset (Lab 5)reset occurs only at clock transition

always @(posedge CLK)if (RESET == 1) state <= A;else state <= next_state;

prefer synchronous reset

asynchronous resetreset occurs whenever RESET goes high

always @(posedge CLK or posedge RESET)if (RESET == 1) state <= A;else state <= next_state;

use asynchronous reset only if you really need it!

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Notice that state only gets updatedon posedge of clock (or on reset)

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Output// Z declared as: output reg Z// reg is optimized away

always @(*)case(state)

A: Z <= 0;B: Z <= 0;C: Z <= 0;D: Z <= X ? 1 : 0;

default: Z <= 0;endcase

Comment on Code Could shorten it somewhat

Don’t need three always clausesAlthough it’s clearer to have combinational code be

separateDon’t need next_state, for example

Can just set state on clock

Template helps synthesizerCheck to see whether your state machines were

recognized

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Verilog: specifying FSM using 2 blocks Let us divide FSM into two modules

one stores and update stateanother produces outputs

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Verilog: specifying FSM using 2 blocksreg [1:0] state; reg outp;…

always @(posedge clk) case (state)

s1: if (x1 == 1'b1) state <= s2; else state <= s3; s2: state <= s4; s3: state <= s4; s4: state <= s1; endcase //default not required for seq logic!

always @(*)

case (state) s1: outp <= 1'b1; s2: outp <= 1'b1; s3: outp <= 1'b0; s4: outp <= 1'b0; default: outp <= 1'b0; //default required for comb logic! endcase

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Synthesis (see console output)Synthesizing Unit <v_fsm_2>. Related source file is "v_fsm_2.v". Found finite state machine <FSM_0> for signal <state>. ----------------------------------------------------------------------- | States | 4 | | Transitions | 5 | | Inputs | 1 | | Outputs | 1 | | Clock | clk (rising_edge) | | Reset | reset (positive) | | Reset type | asynchronous | | Reset State | 00 | | Power Up State | 00 | | Encoding | automatic | | Implementation | LUT | ----------------------------------------------------------------------- Summary:

inferred 1 Finite State Machine(s).Unit <v_fsm_2> synthesized.

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Incorrect: Putting all in one always Using one always block

generally incorrect! (But may work for Moore FSMs)ends up with unintended registers for outputs!

always@(posedge clk)case (state)s1: if (x1 == 1'b1) begin state <= s2; outp <= 1'b1; end else begin state <= s3; outp <= 1'b0; ends2: begin

state <= s4; outp <= 1'b1; ends3: begin

state <= s4; outp <= 1'b0; ends4: begin

state <= s1; outp <= 1'b0; endendcase

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Synthesis OutputSynthesizing Unit <v_fsm_1>. Related source file is "v_fsm_1.v". Found finite state machine <FSM_0> for signal <state>. ----------------------------------------------------------------------- | States | 4 | | Transitions | 5 | | Inputs | 1 | | Outputs | 4 | | Clock | clk (rising_edge) | | Reset | reset (positive) | | Reset type | asynchronous | | Reset State | 00 | | Power Up State | 00 | | Encoding | automatic | | Implementation | LUT | ----------------------------------------------------------------------- Found 1-bit register for signal <outp>. Summary:

inferred 1 Finite State Machine(s).inferred 1 D-type flip-flop(s).

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Textbook Uses 3 always Blocks

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Three always Blocks reg [1:0] state; reg [1:0] next_state; … … … always @(*) // Process 1 case (state) s1: if (x1==1'b1) next_state <= s2; else next_state <= s3; s2: next_state <= s4; s3: next_state <= s4; s4: next_state <= s1; default: next_state <= s1; endcase

always @(posedge clk) // Process 2

if (RESET == 1) state <= s1;else state <= next_state;

reg outp;………always @(*) // Process 3 case (state) s1: outp <= 1'b1; s2: outp <= 1'b1; s3: outp <= 1'b0; s4: outp <= 1'b0; default: outp <= 1'b0; endcase

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Synthesis (again, no unintended latch)Synthesizing Unit <v_fsm_3>. Related source file is "v_fsm_3.v". Found finite state machine <FSM_0> for signal <state>. ----------------------------------------------------------------------- | States | 4 | | Transitions | 5 | | Inputs | 1 | | Outputs | 1 | | Clock | clk (rising_edge) | | Reset | reset (positive) | | Reset type | asynchronous | | Reset State | 00 | | Power Up State | 00 | | Encoding | automatic | | Implementation | LUT | ----------------------------------------------------------------------- Summary:

inferred 1 Finite State Machine(s).Unit <v_fsm_3> synthesized.

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My Preference The one with 3 always blocks

Easy to visualize the state transitionsFor really simple state machines:

2 always blocks is okay tooNever put everything into 1 always block!

Follow my template fsm_3blocktemplate.v (posted on the website)

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Moore vs. Mealy FSMs? So, is there a practical difference?

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CLKM Nk knext

statelogic

outputlogic

Moore FSM

CLKM Nk knext

statelogic

outputlogic

inputs

inputs

outputs

outputsstate

statenextstate

nextstate

Mealy FSM

Moore vs. Mealy Recognizer

reset

Moore FSM

S00

S10

S20

S30

S41

0

1 1 0 1

1

01 00

reset

S0 S1 S2 S3

0/0

1/0 1/0 0/01/1

0/01/0

0/0

Mealy FSM

Mealy FSM: arcs indicate input/output

Moore and Mealy Timing Diagram

Mealy Machine

Moore Machine

CLK

Reset

A

S

Y

S

Y

Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5 Cycle 6 Cycle 7 Cycle 8 Cycle 9 Cycle 10

S0 S3?? S1 S2 S4 S4S2 S3 S0

1 1 0 1 1 0 1 01

S2

S0 S3?? S1 S2 S1 S1S2 S3 S0S2

Moore vs. Mealy FSM Schematic Moore Mealy

S2

S1

S0

S'2

S'1

S'0

Y

CLK

Reset

A

S2

S1

S0

S'1

S'0

CLK

Reset

S1

S0

A

Y

S0S1

Next VGA Displays

timing generationuses 2D xy-counter

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