Designing User Interfaces Spring 1999

1
SE 746-NT Embedded Software Systems
Development Robert Oshana Lecture
14a For more information, please
contact NTU Tape Orders NTU Media
Services [phone]
oshana_at_airmail.net
tapeorders_at_ntu.edu
2
Finite state machines
3
Finite State Machines(FSMs)

• Widely used specification technique
• Used to specify system behavior in response to
  events
• Both output and next state can be determined
  solely on the basis of understanding the current
  state and the event that caused the transition
• H/W engineers have been using FSM for years

4
(No Transcript)
5
(No Transcript)
6
(No Transcript)
7
(No Transcript)
8
Same stimulus event
Different response
9
State Machines

• Models how a system responds differently to
  events over time

10
State Machines

• Models how a system responds differently to
  events over time

button switch
11
States
OFF
ON
12
States
Press
OFF
ON
Press
13
Finite State Machines

• State
• the memory of previous events
• Events
• inputs to a system
• Actions
• output in some form
• mechanical, electrical or software event

14
Finite State Machines

• State
• the memory of previous events
• Events
• inputs to a system
• Actions
• output in some form
• mechanical, electrical or software event

15
Coke Machine

• Events
• coin insertion
• Actions
• coke delivery
• coins returned
• State
• the memory of how much money has been deposited

16
FSM Diagram
Event-1
State 1
State 2
Action-k
17
FSM
Event-1
State 1
State 2
Action-k
Event-1
Not all state transitions have actions associated
with them
18
ScenarioQuarters Only, 75 cents

• Customer
• enter quarter
• Customer
• enter quarter
• Customer
• enter quarter
• Coke Machine
• deliver coke

19
Enumerate Events Actions
EventsE1 Deposit 25 cents Actions A1 Deliver
coke
20
Define States
Start State
E1
A
B
E1
E1/A1
C
EventsE1 Deposit 25 cents Actions A1 Deliver
coke
States A No money B 25 cents entered C 50
cents entered
21
State Transition Table
22
Coke Machine Scenario 2Coin Return

• Customer
• enter quarter
• Customer
• enter quarter
• Customer
• press coin return (CR)
• Coke Machine
• return coins

23
Enumerate Events Actions
EventsE1 Deposit 25 centsE2 Coin Return
(CR) Actions A1 Deliver coke A2 Return coins
24
Define States
Start State
E1
A
B
E1
E1/A1
E2/A2
C
EventsE1 Deposit 25 centsE2 Coin Return
(CR) Actions A1 Deliver coke A2 Return coins
States A No money B 25 cents entered C 50
cents entered
25
Define States
E2/A2
Start State
E1
A
B
E1
E1/A1
E2/A2
C
EventsE1 Deposit 25 centsE2 Coin Return
(CR) Actions A1 Deliver coke A2 Return coins
States A No money B 25 cents entered C 50
cents entered
26
Transition Table

• State E1 E2
• A B
• B C A/coin return
• C A/coke A/coin return

27
Transition Table
No BlanksAllowed !!

• State E1 E2
• A B
• B C A/coin return
• C A/coke A/coin return

28
Transition Table

• State E1 E2
• A B A
• B C A/coin return
• C A/coke A/coin return

29
Telephone System FSMLocal 4-digit exchange
Events d digit dialed h hang up Actions c1 con
nect to caller
30
Telephone System FSMLocal 4-digit exchange
d
d
d
d/c1
A
B
C
D
E
Events d digit dialed h hang up Actions c1 con
nect to caller
StatesA start B 1 digit dialed C 2 digits
dialed D 3 digits dialed E call in progress
31
Telephone System FSMLocal 4-digit exchange
h
d
d
d
d/c1
A
B
C
D
E
Events d digit dialed h hang up Actions c1 con
nect to caller
StatesA start B 1 digit dialed C 2 digits
dialed D 3 digits dialed E call in progress
32
Telephone System FSMLocal 4-digit exchange
h
d
d
d
d/c1
A
B
C
D
E
Events d digit dialed h hang up Actions c1 con
nect to caller
StatesA start B 1 digit dialed C 2 digits
dialed D 3 digits dialed E call in progress
33
From the article Embedded State Machine
Implementation

• Martin Gomez
• Embedded systems programming

34
Embedded state machines

• Many embedded software applications are natural
  candidates for mechanization as a state machine
• Program that must sequence a series of actions,
  or handle inputs differently depending on what
  mode its in
• Soft key interfaces to comm protocols

35
State machines

• An algorithm that can be in one of a small number
  of states
• Mealey
• Output a function of present state and output
• Moore
• Function only of state

36
Example state machine
input char / /no output
input char / /output and signal
start
find 2nd /
find 1st /
done
input char lt gt/ /no output
37
State machine approach

• Learn what the user wants
• Sketch the state transition diagram
• Code the skeleton of the state machine, without
  filling in the details of the transition actions
• Make sure the transitions work properly
• Flesh out the transition details
• Test

38
Another example

• Program that controls the retraction and
  extension of an airplanes landing gear
• Hardware
• Nose gear
• Left main gear
• Right main gear
• Hydraulically actuated

39
What does the user want?

• dont retract the landing gear if the airplane
  is on the ground
• Is it ok to retract the landing gear the instant
  the airplane leaves the ground?
• Use a white board or sequence enumeration!
• ltevent that caused transitiongt/ltoutput as a
  result of the transitiongt

40
First implementation only does what the user
asked for
gear lever up not on ground / pump on, dir
up
start
Gear down
Raising gear
gear lever down / pump on, dir down
41
Some things to keep in mind

• Computers are very fast compared to mechanical
  hardware you may have to wait
• Mechanical engineer may not know as much about
  computers!
• What happens if parts break?

42
A better implementation
pilots lever rising edge squat_switch
AIR/start timer, lights RED
timer 2 sec / pump on, dir UP
Waiting For takeoff
start
Raising gear
Gear down
squat_switch GND or pilots lever down / restart
timer
pilots lever down / dir DOWN
All 3 down switches made/ pump OFF, lights GREEN
pilots lever up / dir UP
Gear up
Lowering gear
pilots lever down / lights RED, pump on, dir
DOWN
All 3 down switches made/ pump off, extinguish
lights
43
Implementation
typedef enum GEAR DOWN 0, WTG_FOR_TKOFF,
RAISINGGEAR, GEAR_UP, LOWERING_GEAR
State_type / this table contains a pointer to
the function to call in each state / void (
state_table )( ) GearDown, WtgForTakeoff,
RaisingGear, GearUp, LoweringGear State_Type
curr_state main( ) InitializLdgGearSM( )
/ the heart of the state machine is this
one loop. The function corresponding to the
current state is called once per iteration
/ while( 1 ) state_tablecurr_state
( ) DecrementTimer( ) / do other
functions, not related to this state machine /

44
Implementation
void InitializeLdgGearSM( ) curr_state
GEAR_DOWN timer 0.0 / stop all
the hardware, turn off the lights, etc / void
GearDown( ) / raise the gear upon
command, but not if the airplane is on the ground
/ if ((gear_lever UP)
(prev_gear_lever DOWN) (squat_switch
UP) timer 2.0
curr_state WTG_FOR_TKOFF
prev_gear_lever gear_lever / store for edge
detection / void RaisingGear( ) / once
all 3 legs are up, go to the GEAR_UP state /
if ((nosegear_is_up MADE) (leftgear_is_up
MADE) (rtgear_is_up MADE))
curr_state GEAR_UP
45
Implementation
/ if the pilot changes his mind, start
lowering the gear / if (gear_lever DOWN)
curr_state LOWERING_GEAR
void GearUp( ) / if the pilot moves
the lever to DOWN, lower the gear / if
(gear_lever DOWN) curr_state
LOWERING_GEAR void WtgForTakeoff( )
/ once weve been airborne for 2 seconds, start
raising the gear / if (timer lt 0.0)
curr_state RAISING_GEAR / if we
touch own again, or if the pilot changes his
mind, start over /
46
Implementation
if ((squat_switch DOWN)
(gear_lever DOWN))
timer 2.0 curr_state
GEAR_DOWN / dont want to
require that pilot toggle the lever again this
was just a bounce /
prev_gear_lever DOWN Void
LoweringGear( ) if (gear_lever UP)
curr_state RAISING_GEAR if
((nosegear_is_down MADE) (leftgear_is_down
MADE) (rtgear_is_down MADE))
curr_state GEAR_DOWN
47
RaisingGear() function
void RaisingGear( ) / once all 3 legs
are up, go to the GEAR_UP state / if
((nosegear_is_up MADE) (leftgear_is_up
MADE) (rtgear_is_up MADE))
pump_motor OFF gear_lights
EXTINGUISH curr_state GEAR_UP
/ if the pilot changes his mind,
start lowering the gear / if (gear_lever
DOWN) pump_direction DOWN
curr_state GEAR_LOWERING
48
Testing

• Test plan almost writes itself
• Avoid hidden states
• Number of transitions is measure of systems
  complexity
• Use print statement to output current state
• Be careful when connecting to actual hardware

49
Quote

• By relieving the brain of all unnecessary work,
  a good notation sets it free to concentrate on
  more advanced problems, and, in effect, increases
  the mental power of the race
• Alfred North Whitehead (English mathematician)

50
Complexity

• Simplicity is a basic tenet of all good
  engineering
• Can we build skyscrapers with Lego blocks
• Were asked to build increasingly complex systems
• Modern jet has over 6 million parts

51
Essential acceleration/deceleration mechanism
cruising
too fast
too slow
speed ok
speed ok
acceleration
deceleration
52
Adding an off state
off
off
off
off
on
cruising
too fast
too slow
speed ok
speed ok
acceleration
deceleration
53
Dealing withadditionalrequirements
diagnosing
off
ok
off
off
off
off
on
suspended
suspend
resume
cruising
too fast
too slow
speed ok
suspend
speed ok
acceleration
deceleration
54
SE 746-NT Embedded Software Systems
Development Robert Oshana End of
Lecture For more information, please
contact NTU Tape Orders NTU Media
Services [phone]
oshana_at_airmail.net
tapeorders_at_ntu.edu