No. 8-1 Chapter #8: Finite State Machine Design. No. 8-2 Motivation Counters: Sequential Circuits...

No. 8-1

Chapter #8: Finite State Machine Design

No. 8-2

Motivation

Counters: Sequential Circuits where State = Output

Generalizes to Finite State Machines: Outputs are Function of State (and Inputs) Next States are Functions of State and Inputs Used to implement circuits that control other circuits "Decision Making" logic

Application of Sequential Logic Design Techniques Word Problems Mapping into formal representations of FSM behavior Case Studies

No. 8-3

Chapter Overview

Concept of the State Machine

Partitioning into Datapath and Control

When Inputs are Sampled and Outputs Asserted

Basic Design Approach

Six Step Design Process

Alternative State Machine Representations

State Diagram, ASM Notation, VHDL, ABEL Description Language

Moore and Mealy Machines

Definitions, Implementation Examples

Word Problems

Case Studies

No. 8-5

Concept of the State MachineExample: Odd Parity Checker

Even [0]

Odd [1]

Reset

0

0

1 1

Assert output whenever input bit stream has odd # of 1's

StateDiagram

Present State Even Even Odd Odd

Input 0 1 0 1

Next State Even Odd Odd Even

Output 0 0 1 1

Symbolic State Transition Table

Output 0 0 1 1

Next State 0 1 1 0

Input 0 1 0 1

Present State 0 0 1 1

Encoded State Transition Table

No. 8-6

Concept of the State MachineExample: Odd Parity Checker

Next State/Output Functions

NS = PS xor PI; OUT = PS

D

R

Q

Q

Input

CLK PS/Output

\Reset

NS

D FF Implementation

T

R

Q

Q

Input

CLK

Output

\Reset

T FF Implementation

Timing Behavior: Input 1 0 0 1 1 0 1 0 1 1 1 0

Clk

Output

Input 1 0 0 1 1 0 1 0 1 1 1 0

1 1 0 1 0 0 1 1 0 1 1 1

No. 8-7

Concept of State Machine

Timing: When are inputs sampled, next state computed, outputs asserted?

State Time: Time between clocking events

Clocking event causes state/outputs to transition, based on inputs

For set-up/hold time considerations:

Inputs should be stable before clocking event

After propagation delay, Next State entered, Outputs are stable

NOTE: Asynchronous signals take effect immediately Synchronous signals take effect at the next clocking event

E.g., tri-state enable: effective immediately sync. counter clear: effective at next clock event

No. 8-8

Concept of State Machine

Example: Positive Edge Triggered Synchronous System

On rising edge, inputs sampled outputs, next state computed

After propagation delay, outputs and next state become stable

Immediate Outputs: affect datapath immediately could cause inputs from datapath to change

Delayed Outputs: take effect on next clock edge propagation delays must exceed hold timesOutputs

State T ime

Clock

Inputs

No. 8-9

Concept of the State MachineCommunicating State Machines

Fragment state diagramsInitial inputs/outputs: X = 0, Y = 0

One machine's output is another machine's input

CLK

FSM 1 X FSM 2

Y

A A B

C D D

FSM 1 FSM 2

X

Y

A [1]

B [0]

Y=0

Y=1

Y=0,1

Y=0

C [0]

D [1]

X=0

X=1

X=0

X=0

X=1

No. 8-10

Basic Design Approach

Six Step Process

1. Understand the statement of the Specification

2. Obtain an abstract specification of the FSM

3. Perform a state minimization

4. Perform state assignment (or encoding)

5. Choose FF types to implement FSM state register

6. Implement the FSM

1, 2 covered now; 3~6 covered later (Chap. 9);5 and 6 generalized from the counter design procedure

No. 8-11

Basic Design Approach

Example: Vending Machine FSM

General Machine Concept:deliver package of gum after 15 cents deposited

single coin slot for dimes (10 cents), nickels (5 cents)

no change

Block Diagram

Step 1. Understand the problem:

Vending Machine

FSM

N

D

Reset

Clk

OpenCoin

SensorGum

Release Mechanism

Draw a picture!

No. 8-12

Vending Machine Example

Tabulate typical input sequences:three nickelsnickel, dimedime, nickeltwo dimestwo nickels, dime

Draw state diagram:

Inputs: N, D, reset

Output: open

Step 2. Map into more suitable abstract representation

Reset

N

N

N

D

D

N D

[open]

[open] [open] [open]

S0

S1 S2

S3 S4 S5 S6

S8

[open]

S7

D

No. 8-13

Vending Machine Example

Step 3: State Minimization

Reset

N

N

N, D

[open]

15¢

0¢

5¢

10¢

D

D

reuse stateswheneverpossible

Symbolic State Table

Present State

0¢

5¢

10¢

15¢

D

0 0 1 1 0 0 1 1 0 0 1 1 X

N

0 1 0 1 0 1 0 1 0 1 0 1 X

Inputs Next State

0¢ 5¢ 10¢ X 5¢ 10¢ 15¢ X

10¢ 15¢ 15¢ X

15¢

Output Open

0 0 0 X 0 0 0 X 0 0 0 X 1

No. 8-14

Vending Machine Example

Step 4: State Encoding

Next State D 1 D 0

0 0 0 1 1 0 X X 0 1 1 0 1 1 X X 1 0 1 1 1 1 X X 1 1 1 1 1 1 X X

Present State Q 1 Q 0

0 0

0 1

1 0

1 1

D

0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1

N

0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1

Inputs Output Open

0 0 0 X 0 0 0 X 0 0 0 X 1 1 1 X

No. 8-15

Parity Checker Example

Step 5. Choose FFs for implementation

D FF easiest to use

D1 D0 Open

D1 = Q1 + D + Q0 N

D0 = N Q0 + Q0 N + Q1 N + Q1 D

OPEN = Q1 Q0

8 Gates

CLK

OPEN

CLK

Q 0

D

R

Q

Q

D

R

Q

Q

\ Q 1

\reset

\reset

\ Q 0

\ Q 0

Q 0

Q 0

Q 1

Q 1

Q 1

Q 1

D

D

N

N

N

\ N

D 1

D 0

No. 8-16

Parity Checker Example

Step 5. Choosing FF for Implementation

J-K FF

Remapped encoded state transition table

Next State D 1 D 0

0 0 0 1 1 0 X X 0 1 1 0 1 1 X X 1 0 1 1 1 1 X X 1 1 1 1 1 1 X X

Present State Q 1 Q 0

0 0

0 1

1 0

1 1

D

0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1

N

0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1

Inputs K 1

X X X X X X X X 0 0 0 X 0 0 0 X

K 0

X X X X 0 1 0 X X X X X 0 0 0 X

J 1

0 0 1 X 0 1 1 X X X X X X X X X

J 0

0 1 0 X X X X X 0 1 1 X X X X X

No. 8-17

Vending Machine Example

Implementation:

J1 = D + Q0 N

K1 = 0

J0 = Q0 N + Q1 D

K0 = Q1 N

7 Gates

OPEN Q 1

\ Q 0

N

Q 0 J

K R

Q

Q

J

K R

Q

Q

Q 0

\ Q 1

\ Q 1

\ Q 0

Q 1

\reset

D

D

N

N

CLK

CLK

No. 8-18

Alternative State Machine Representations

Why State Diagrams Are Not Enough

Not flexible enough for describing very complex finite state machines

Not suitable for gradual refinement of finite state machine

Do not obviously describe an algorithm: that is, well specified sequence of actions based on input data

algorithm = sequencing + data manipulation separation of control and data

Gradual shift towards program-like representations:

Algorithmic State Machine (ASM) Notation

Hardware Description Languages (e.g., VHDL)

No. 8-19

Alternative State Machine Representations

Algorithmic State Machine (ASM) Notation

Three Primitive Elements:

State Box

Decision Box

Output Box

State Machine in one state block per state time

Single Entry Point

Unambiguous Exit Path for each combination of inputs

Outputs asserted high (.H) or low (.L); Immediate (I) or delayed til next clock

State Entry Path

State Name

State Code

State Box

ASM Block

State Output List

Condition Box

Conditional Output List

Output Box

Exits to other ASM Blocks

Condition

* ***

T F

No. 8-20

Alternative State Machine Representations

ASM Notation

Condition Boxes:Ordering has no effect on final outcome

Equivalent ASM charts: A exits to B on (I0 & I1 = 1) else exit to C

A 010

I0

I1

T

T

F

F

B C

A 010

I0

T

T

F

F

B C

I1

No. 8-21

Alternative State Machine Representations

Example: Parity Checker

Nothing in output list implies Z not asserted

Z asserted in State Odd

Input X, Output Z

InputFTFT

PresentStateEvenEvenOddOdd

NextStateEvenOddOddEven

Output

AA

Symbolic State Table:

Input0101

PresentState

0011

NextState

0110

Output0011

Encoded State Table:

Trace paths to derivestate transition tables

Trace paths to derivestate transition tables

Even

Odd

H. Z

X

X

0

1

F

T

T F

No. 8-22

Alternative State Machine Representations

ASM Chart for Vending Machine

0¢

15¢

H.Open

D

Reset

00

1 1

T

T

F

N F

T

F

10¢

D

10

T

N F

T

F

5¢

D

01

T N

F T

F

0¢

No. 8-23

Moore and Mealy Machine Design ProcedureDefinitions

Moore Machine

Outputs are functionsolely of the current

state

Outputs change synchronously with

state changes

Mealy Machine

Outputs depend onstate AND inputs

Input change causesan immediate output

change

Asynchronous outputs

State Register Clock

State Feedback

Combinational Logic for

Outputs and Next State

X Inputs

i Z Outputs

k

Clock

state feedback

Combinational Logic for

Next State (Flip-flop Inputs)

State Register

Comb. Logic for Outputs

Z Outputs

k

X Inputs

i

No. 8-24

Moore and Mealy MachinesState Diagram Equivalents

Outputs are associated with State

Outputs are associated with Transitions

Reset/0

N/0

N/0

N+D/1

15¢

0¢

5¢

10¢

D/0

D/1

(N D + Reset)/0

Reset/0

Reset/1

N D/0

N D/0

MooreMachine Reset

N

N

N+D

[1]

15¢

0¢

5¢

10¢

D

[0]

[0]

[0]

D

N D + Reset

Reset

Reset

N D

N D

MealyMachine

No. 8-25

Moore and Mealy MachinesStates vs. Transitions

Mealy Machine typically has fewer states than Moore Machine for same output sequence

EquivalentASM Charts

Asserts its output whenever at least two 1’s in sequence

Same I/O behavior

Different # of states

1

1

0

1

2

0

0

[0]

[0]

[1]

1/0

0

1

0/0

0/0

1/1

1

0

S0

S1

IN

IN

S2

IN

10

01

00

H.OUT

S0

S1

IN

IN

1

0

H.OUT

F F

F F

T

T

T

T

T

F

No. 8-26

Moore and Mealy MachinesTiming Behavior of Moore Machines

Reverse engineer the following: why Moore ???

Input XOutput ZState A, B (= Z)

Two Techniques for Reverse Engineering:

Ad Hoc: Try input combinations to derive transition table

Formal: Derive next state/output functions by analyzing the circuit

JCK R

Q

Q

FFa

JCK R

Q

Q

FFb

X

X

X

X

\Reset

\Reset

A

Z

\A

\A\B

\B

Clk

No. 8-27

Moore and Mealy Machines

Ad Hoc Reverse Engineering

Behavior in response to input sequence 1 0 1 0 1 0:

Partially DerivedState Transition

Table

A 0

0

1

1

B 0 1 0 1

X 0 1 0 1 01 0 1

A+ ? 1 0 ? 1 0 1 1

B+ ? 1 0 ? 0 1 1 0

Z 0 0 1 1 0 0 1 1

X = 1 AB = 00

X = 0 AB = 1 1

X = 1 AB = 1 1

X = 0 AB = 10

X = 1 AB = 10

X = 0 AB = 01

X = 0 AB = 00

Reset AB = 00

100

X

Clk

A

Z

\Reset

No. 8-28

Moore and Mealy MachinesFormal Reverse Engineering

Derive transition table from next state and output combinational functions presented to the flipflops!

Ja = XJb = X

Ka = X BKb = X xor A

Z = B

FF excitation equations for J-K flipflop: Q+ = J \Q +\K Q

A+ = Ja A + Ka A = X A + (X + B) AB+ = Jb B + Kb B = X B + (X A + X A) B

Next State K-Maps:

State 00, Input 0 State 00State 01, Input 1 State 11

No. 8-29

Moore and Mealy MachinesComplete ASM Chart for the Mystery Moore Machine

Note: All Outputs Associated With State BoxesNo Separate Output Boxes, Intrinsic in Moore Machines

S 0 00

X

H. Z

S 1 01

X

S 3 1 1

S 2 10

H. Z

X

X

0 1

1 0

0

1

0 1

No. 8-30

Moore and Mealy MachinesReverse Engineering a Mealy Machine

Input X, Output Z, State A, B

State register consists of D FF and J-K FF

D

C R

Q

Q

J C K R

Q

Q

X

X

X

Clk

A

A

A

B

B

B

Z

\Reset \Reset

\ A

\ A

\ X

\ X

\ X \ B

DA

DA

No. 8-31

Moore and Mealy MachineAd Hoc Method

Signal Trace of Input Sequence 101011:

Note glitchesin Z!

Outputs valid atfollowing falling

clock edge

Partially completedstate transition tablebased on the signal

trace

A 0

0

1

1

B 0 1 0 1

X 0 1 0 1 0 1 0 1

A+ 0 0 ? 1 ? 0 1 ?

B+ 1 0 ? 1 ? 1 0 ?

Z 0 0 ? 0 ? 1 1 ?

Reset AB =00

Z =0

X =1 AB =00

Z =0

X =0 AB =00

Z =0

X =1 AB =01

Z =0

X =1 AB =10

Z =1

X =0 AB =1 1

Z =1

X =1 AB =01

Z =0

X

Clk

A

B

Z

\Reset

100

No. 8-32

Moore and Mealy MachinesFormal Method

A+ = B (A + X) = A B + B X

B+ = Jb B + Kb B = (A xor X) B + X B

= A B X + A B X + B X

Z = A X + B XMissing Transitions and Outputs:

State 01, Input 0 -> State 00, Output 1State 10, Input 0 -> State 00, Output 0State 11, Input 1 -> State 11, Output 1A+

B+

Z

No. 8-33

Moore and Mealy MachinesASM Chart for Mystery Mealy Machine

S0 = 00, S1 = 01, S2 = 10, S3 = 11

NOTE: Some Outputs in Output Boxes as well as State BoxesThis is intrinsic in Mealy Machine implementation

S 0 00

X

H. Z S 1 01

S 2 10

X

H. Z

H. Z

S 3 1 1

X X

0

1

0

0 1

1

0 1

No. 8-34

Moore and Mealy MachinesSynchronous Mealy Machine

latched state AND outputs

avoids glitchy outputs!

State Register Clock

Clock

Combinational Logic for

Outputs and Next State

state feedback

X Inputs

i Z Outputs

k

No. 8-35

Finite State Machine Word ProblemsMapping English Language Description to Formal Specifications

Four Case Studies:

Finite String Pattern Recognizer

Complex Counter with Decision Making

Traffic Light Controller

Digital Combination Lock

We will use state diagrams and ASM Charts

No. 8-36

Finite State Machine Word ProblemsFinite String Pattern Recognizer

A finite string recognizer has one input (X) and one output (Z).The output is asserted whenever the input sequence …010…has been observed, as long as the sequence 100 has never beenseen.

Step 1. Understanding the problem statement

Sample input/output behavior:

X: 00101010010Z: 00010101000

X: 11011010010Z: 00000001000

No. 8-37

Finite State Machine Word Problems

Finite String Recognizer

Step 2. Draw State Diagrams/ASM Charts for the strings that must be recognized. I.e., 010 and 100.

Moore State DiagramReset signal places FSM in S0

Outputs 1 Loops in State

No. 8-38

Finite State Machine Word ProblemsFinite String Recognizer

Exit conditions from state S3: if next input is 0 then have …0100 (state S6) if next input is 1 then have …0101 (state S2)

No. 8-39

Finite State Machine Word ProblemsFinite String Recognizer

Exit conditions from S1: recognizes strings of form …0 (no 1 seen) loop back to S1 if input is 0

Exit conditions from S4: recognizes strings of form …1 (no 0 seen) loop back to S4 if input is 1

No. 8-40

Finite State Machine Word ProblemsFinite String Recognizer

S2, S5 with incomplete transitions

S2 = …01; If next input is 1, then string could be prefix of (01)1(00) S4 handles just this case!

S5 = …10; If next input is 1, then string could be prefix of (10)1(0) S2 handles just this case!

Final State Diagram

No. 8-41

Finite State Machine Word ProblemsFinite String Recognizer

Review of Process:

Write down sample inputs and outputs to understand specification

Write down sequences of states and transitions for the sequences to be recognized

Add missing transitions; reuse states as much as possible

Verify I/O behavior of your state diagram to insure it functions like the specification

No. 8-42

Finite State Machine Word ProblemsComplex Counter

A sync. 3 bit counter has a mode control M. When M = 0, the countercounts up in the binary sequence. When M = 1, the counter advancesthrough the Gray code sequence.

Binary: 000, 001, 010, 011, 100, 101, 110, 111Gray: 000, 001, 011, 010, 110, 111, 101, 100

Valid I/O behavior:

Mode Input M0011100

Current State000001010110111101110

Next State (Z2 Z1 Z0)001010110111101110111

No. 8-43

Finite State Machine Word ProblemsComplex Counter

One state for each output combinationAdd appropriate arcs for the mode control

S 0 000

S 1 001 H. Z 0

M

S 3 01 1 H. Z 1 H. Z 0

H. Z 1

S 2 010

M M

S 6 1 10 H. Z 2 H. Z 1

H. Z 2 H. Z 1 H. Z 0

S 4 100

M

H. Z 2

H. Z 2 H. Z 0

M

S 5 101

M

S 7 1 1 1

0 1

0 1

0

1

0

1 0

1

0 1

No. 8-44

Finite State Machine Word ProblemsTraffic Light Controller

A busy highway is intersected by a little used farmroad. DetectorsC sense the presence of cars waiting on the farmroad. With no caron farmroad, light remain green in highway direction. If vehicle on farmroad, highway lights go from Green to Yellow to Red, allowing the farmroad lights to become green. These stay green only as long as a farmroad car is detected but never longer than a set interval. When these are met, farm lights transition from Green to Yellow to Red, allowing highway to return to green. Even if farmroad vehicles are waiting, highway gets at least a set interval as green.

Assume you have an interval timer that generates a short time pulse(TS) and a long time pulse (TL) in response to a set (ST) signal. TSis to be used for timing yellow lights and TL for green lights.

No. 8-45

Finite State Machine Word ProblemsTraffic Light Controller

Picture of Highway/Farmroad Intersection:

Highway

Highway

Farmroad

Farmroad

HL

HL

FL

FL

C

C

No. 8-46

Finite State Machine Word ProblemsTraffic Light Controller

Tabulation of Inputs and Outputs:

Input SignalresetCTSTL

Output SignalHG, HY, HRFG, FY, FRST

Descriptionplace FSM in initial statedetect vehicle on farmroadshort time interval expiredlong time interval expired

Descriptionassert green/yellow/red highway lightsassert green/yellow/red farmroad lightsstart timing a short or long interval

Tabulation of Unique States: Some light configuration imply others

StateS0S1S2S3

DescriptionHighway green (farmroad red)Highway yellow (farmroad red)Farmroad green (highway red)Farmroad yellow (highway red)

No. 8-47

Finite State Machine Word ProblemsTraffic Light Controller

Refinement of ASM Chart:

Start with basic sequencing and outputs:

S 0 S 3

S 1 S 2

H.HG H.FR

H.HR H.FY

H.HR H.FG

H.HY H.FR

No. 8-48

Finite State Machine Word ProblemsTraffic Light Controller

Determine Exit Conditions for S0: Car waiting and Long Time Interval Expired- C · TL

Equivalent ASM Chart Fragments

S 0 S 0

H.HG H.FR

H.HG H.FR

TL TL •  C

H.ST C

H.ST S 1

H.HY H.FR

S 1

H.HY H.FR

0

0

1

1

0

1

C · TL

No. 8-49

Finite State Machine Word ProblemsTraffic Light Controller

S1 to S2 Transition: Set ST on exit from S0 Stay in S1 until TS asserted Similar situation for S3 to S4 transition

S 1

H.HY H.FR

TS

H.ST

S 2

H.HR H.FG

0 1

No. 8-50

Finite State Machine Word ProblemsTraffic Light Controller

S2 Exit Condition: no car waiting OR long time interval expired

Complete ASM Chart for Traffic Light Controller

S 0 S 3

H.HG H.FR H.ST

H.HR H.FY

TS TL • C

H.ST H.ST

S 1 S 2

H.HY H.FR

H.ST H.HR H.FG

TS TL + C

0 0 1

1

1 0

1

0

No. 8-51

Finite State Machine Word ProblemsTraffic Light Controller

Compare with state diagram:

Advantages of ASM Charts:

Concentrates on paths and conditions for exiting a state

Exit conditions built up incrementally, later combined into single Boolean condition for exit

Easier to understand the design as an algorithm

S0: HG

S1: HY

S2: FG

S3: FY

Reset

TL + C

S0TL•C/ST

TS

S1 S3

S2

TS/ST

TS/ST

TL + C/ST

TS

TL • C

No. 8-52

Finite State Machine Word ProblemsDigital Combination Lock

"3 bit serial lock controls entry to locked room. Inputs are RESET,ENTER, 2 position switch for bit of key data. Locks generates anUNLOCK signal when key matches internal combination. ERRORlight illuminated if key does not match combination. Sequence is:(1) Press RESET, (2) enter key bit, (3) Press ENTER, (4) repeat (2) &(3) two more times."

Problem specification is incomplete:

how do you set the internal combination?

exactly when is the ERROR light asserted?

Make reasonable assumptions:

hardwired into next state logic vs. stored in internal register

assert as soon as error is detected vs. wait until full combination has been entered

Our design: registered combination plus error after full combination

No. 8-53

Finite State Machine Word ProblemsDigital Combination Lock

Understanding the problem: draw a block diagram

InternalCombination

Operator Data

Inputs:ResetEnterKey-InL0, L1, L2

Outputs:UnlockError

UNLOCK

ERROR

RESET

ENTER

KEY -IN

L 0

L 1

L 2

Combination Lock FSM

No. 8-54

Finite State Machine Word ProblemsDigital Combination Lock

Enumeration of states:

what sequences lead to opening the door? error conditions on a second pass

START state plus three key COMParison states

START entered on RESET

Exit START when ENTER is pressed

Continue on if Key-In matches L0

ST ART

Reset

Enter

COMP0

KI = L 0

N

Y

1

0

0

1

No. 8-55

Finite State Machine Word ProblemsDigital Combination Lock

Path to unlock:

Wait for Enter Key press

Compare Key-IN

COMP0

N KI = L 0

Y IDLE0

Enter

COMP1

N

Y

KI = L 1

IDLE1

COMP2

Enter

N KI = L 2

Y DONE

H.Unlock

Reset

ST ART

1

0

0

1

0

1

No. 8-56

Finite State Machine Word ProblemsDigital Combination Lock

Now consider error paths

Should follow a similar sequence as UNLOCK path, except asserting ERROR at the end:

COMP0 error exits to IDLE0'

COMP1 error exits to IDLE1'

COMP2 error exits to ERROR3

IDLE0'

Enter

ERROR1

IDLE1'

Enter

ERROR2

ERROR3

H.Error

Reset

ST ART

0 0 0

1 1 1

No. 8-57

Finite State Machine Word ProblemsDigital Combination Lock

Equivalent State Diagram

Reset

Reset + Enter

Reset • Enter

Start

Comp0

KI = L0 KI ° L0

Enter

Enter

Enter

Enter

Idle0 Idle0'

Comp1 Error1

KI ° L1KI = L1Enter

Enter

EnterEnter

Idle1 Idle1'

Comp2 Error2

KI ° L2KI = L2

Done [Unlock]

Error3 [Error]

Reset

Reset Reset

Reset

StartStart

No. 8-58

Chapter Review

Basic Timing Behavior an FSM

when are inputs sampled, next state/outputs transition and stabilize

Moore and Mealy (Async and Sync) machine organizations outputs = F(state) vs. outputs = F(state, inputs)

First Two Steps of the Six Step Procedure for FSM Design

understanding the problem

abstract representation of the FSM

Abstract Representations of an FSM

ASM Charts

Word Problems

understand I/O behavior; draw diagrams

enumerate states for the "goal"; expand with error conditions

reuse states whenever possible