Newspaper Vending Machine Controller Design Specification Consider a newspaper vending machine like those found at many street corners. Newspapers cost 35 cents, and the machine accepts nickels, dimes and quarters. When enough money has been deposited, the machine unlocks a door which allows the buyer to retrieve a newspaper, and returns change if necessary. You will design the control logic for this vending machine as a finite-state machine using D flip-flops. The coin slot mechanism detects which coin has been deposited, and produces two signals A and B that represent the value of the coin as shown in the table below. Invalid coins are pennies, foreign currency, etc., and are immediately returned to the buyer. Coin A B No coin deposited, or invalid coin 0 0 Nickel 0 1 Dime 1 0 Quarter 1 1 When a valid coin is inserted, the values of A and B are asserted as shown above, and are reset to zero after the next rising edge of the clock. This prevents the signals A and B from being asserted for more than one clock cycle when a single coin is inserted. Your FSM will take A and B as inputs, and produce outputs V and C. A rising edge on V (vend) will cause the door lock to open, and the lock will remain open until the buyer closes the door. It is not necessary to assert V for more than one clock cycle in order to do this. The output C is normally 1. If C is 0 on the rising edge of the clock, then a nickel will be returned as change. If the buyer deposits more than 35 cents, then your FSM must return the difference in nickels. You may assume that there is an inexhaustible supply of nickels available for change. 1. Draw a state transition diagram for your design. 2. Choose a suitable state assignment. 3. Determine the state transition table. Include the excitations for JK flip-flops. 4. Determine the state and output logic. 5. Draw the circuit schematic.
Consider a newspaper vending machine like those found at many street corners. Newspapers cost 35 cents, and the machine accepts nickels, dimes and quarters. When enough money has been deposited, the machine unlocks a door which allows the buyer to retrieve a newspaper, and returns change if necessary. You will design the control logic for this vending machine as a finite-state machine using D flip-flops. The coin slot mechanism detects which coin has been deposited, and produces two signals A and B that represent the value of the coin as shown in the table below. Invalid coins are pennies, foreign currency, etc., and are immediately returned to the buyer.
Coin A BNo coin deposited, or invalid coin 0 0 Nickel 0 1 Dime 1 0 Quarter 1 1
When a valid coin is inserted, the values of A and B are asserted as shown above, and are reset to zero after the next rising edge of the clock. This prevents the signals A and B from being asserted for more than one clock cycle when a single coin is inserted. Your FSM will take A and B as inputs, and produce outputs V and C. A rising edge on V (vend) will cause the door lock to open, and the lock will remain open until the buyer closes the door. It is not necessary to assert V for more than one clock cycle in order to do this. The output C is normally 1. If C is 0 on the rising edge of the clock, then a nickel will be returned as change. If the buyer deposits more than 35 cents, then your FSM must return the difference in nickels. You may assume that there is an inexhaustible supply of nickels available for change. 1. Draw a state transition diagram for your design. 2. Choose a suitable state assignment. 3. Determine the state transition table. Include the excitations for JK flip-flops. 4. Determine the state and output logic. 5. Draw the circuit schematic.
Stat
e Tr
ansi
tion
Dia
gram
1.
N
icke
l Dep
osits
.
• N
ote
that
stat
es a
re la
bele
d w
ith th
e ou
tput
val
ues V
C, a
nd tr
ansi
tions
are
labe
led
with
the
inpu
t val
ues A
B.
•
Als
o no
te th
at fo
r sta
tes R
eset
, 5c,
…, 3
0c, F
SM re
mai
ns in
that
stat
e w
hen
AB
=00,
but
this
is n
ot sh
own
in th
e di
agra
m.
R
eset
01
5c 01
10c
01
15c
01
01
20c
01
01
25c
01
01 01
01
30c
0101
Ope
n11
01
XX
2.
Mak
ing
Cha
nge.
•
The
buye
r may
dep
osit
as m
uch
as 5
5 ce
nts t
otal
, so
the
FSM
mus
t be
able
to re
turn
up
to 2
0 ce
nts c
hang
e.
• O
nce
the
tota
l dep
osite
d ex
ceed
s 35
cent
s, ch
ange
is re
turn
ed n
o m
atte
r wha
t the
inpu
ts a
re.
R
eset
01
5c 01
10c
01
15c
01
01
20c
01
01
25c
01
01 01
01
30c
0101
Ope
n11
01
40c
00
XX
45c
00
XX
50c
00
55c
00
XX
XX
XX
3.
Dim
e D
epos
its.
R
eset
01
5c 01
10c
01
15c
01
0110
20c
01
01
25c
01
01 01
01
30c
0101
Ope
n11
01
40c
00
XX
45c
00
XX
50c
00
55c
00
XX
XX
10
10
10
1010
10
XX
Res
et01
5c 01
10c
01
15c
01
0110
20c
01
01
25c
01
01 01
01
30c
0101
Ope
n11
01
40c
00
XX
45c
00
XX
50c
00
55c
00
XX
XX
10
10
10
1010
10
11
1111
11
11
1111
XX
4.
Qua
rter D
epos
its
State Transition Table
Present Next State Outputs State AB=00 AB=01 AB=10 AB=11 VC Reset Reset 5c 10c 25c 01
State and Output Logic for the One-Hot State Assignment The state logic expressions can be found without K-maps. For example, note that from state 000010000000 (Open) the next state is always 000000000001 (Reset), for any input values. The only other way to reach state 000000000001 is from the same state when the inputs are AB = 00.
= +D Q Q A70 0 B
The state logic for the other flip-flops expressions can be found in similar fashion.
= +
= + +
= + +
= + +
= + +
= + +
D Q AB Q AB1 1 0D Q AB Q AB Q AB2 2 1 0D Q AB Q AB Q AB3 3 2 1D Q AB Q AB Q AB4 4 3 2D Q AB Q AB Q AB+Q AB5 5 4 3 0D Q AB Q AB Q AB+Q AB5 46 6 1
Note the pattern in the expressions for D1 through D6. Each could be implemented by a 4:1 MUX with A and B as the select inputs. The state logic for D7 is more complicated because there are seven different present state and input combinations that lead to state 000010000000 as the next state.
= + + +D Q Q AB Q AB Q A7 58 6 2 B Similarly,
= + +
= +
=
D Q Q AB Q A8 9 6 3D Q Q AB49 10
D Q AB10 6
B
7
The output logic can also be determined by inspection. The output V = 1 if and only if the FSM is in state 000010000000 (Open). The output C = 1 if the FSM is in any of the states 000000000001 (Reset) through 000010000000 (Open). Therefore,
+ + +
=
= + + + +
V Q7C Q Q Q Q Q Q Q Q4 50 1 2 3 6
The added cost of the extra flip-flops for this one-hot state assignment is clearly worth it, given the reduction in complexity of the state logic.
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