Transcript Digital Design

Basic Register
• Typically, we store multi-bit items
– e.g., storing a 4-bit binary number
• Register: multiple flip-flops sharing clock signal
– From this point, we’ll use registers for bit storage
• No need to think of latches or flip-flops
• But now you know what’s inside a register
I3
I2
I1
I0
4-bit register
D
D
Q
D
Q
I3 I2 I1 I0
reg(4)
D
Q
Q
clk
Q3 Q2 Q1 Q0
Q3
Q2
Q1
Q0
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Example Using Registers: Temperature Display
• Temperature history display
– Sensor outputs temperature as 5-bit binary number
– Timer pulses C every hour
– Record temperature on each pulse, display last three recorded values
Present
1 hour ago
2 hours ago
Display
Display
Display
a4 a3 a2 a1 a0 b4 b3 b2 b1 b0 c4 c3 c2 c1 c0
x4
x3
e
r
x2
tu
a
Temperature History Storage
r
x1
sensor
x0
empe
t
timer
C
(In practice, we would actually avoid connecting the timer output
C to a clock input, instead only connecting an oscillator output to a clock input.)
2
Example Using Registers: Temperature Display
• Use three 5-bit registers
a4 a3 a2 a1 a0
x4
x3
x2
x1
x0
C
I4
I3
I2
I1
I0
Q4
Q3
Q2
Q1
Q0
b4 b3 b2 b1 b0
I4
I3
I2
I1
I0
Ra
Q4
Q3
Q2
Q1
Q0
c4 c3 c2 c1 c0
I4
I3
I2
I1
I0
Q4
Q3
Q2
Q1
Q0
Rb
Rc
Temperature History Storage
x4...x0
15 18 20 21 21 22 24 24 24 25 25 26 26 26 27 27 27 27
C
Ra
0
18
21
24
25
26
27
Rb
0
0
18
21
24
25
26
Rc
0
0
0
18
21
24
25
3
Finite-State Machines (FSMs) and Controllers
• Want sequential circuit with
particular behavior over time
• Example: Laser timer
b Controller
x
laser
clk
– Push button: x=1 for 3 clock cycles
– How? Let’s try three flip-flops
• b=1 gets stored in first D flip-flop
• Then 2nd flip-flop on next cycle,
then 3rd flip-flop on next
• OR the three flip-flop outputs, so x
should be 1 for three cycles
patient
b
D
Q
D
Q
D
Q
clk
x
4
Need a Better Way to Design Sequential Circuits
• Trial and error is not a good design method
• Combinational circuit design process had two
important things
1. A formal way to describe desired circuit
behavior
• Boolean equation, or truth table
2. A well-defined process to convert that behavior
to a circuit
• We need those things for sequence circuit design
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Describing Behavior of Sequential Circuit: FSM
• Finite-State Machine (FSM)
– A way to describe desired
behavior of sequential circuit
• Akin to Boolean equations for
combinational behavior
– List states, and transitions
among states
Outputs: x
x=0
clk^
Off
x=1
On
clk^
• Example: Make x change
toggle (0 to 1, or 1 to 0) every
clock cycle
• Two states: “Off” (x=0), and
“On” (x=1)
• Transition from Off to On, or
On to Off, on rising clock edge
• Arrow with no starting state
points to initial state (when
circuit first starts)
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FSM Example: 0,1,1,1,repeat
• Want 0, 1, 1, 1, 0, 1, 1, 1, ...
– Each value for one clock cycle
• Can describe as FSM
Outputs: x
x=0
Off
clk^
x=1
clk^
On1
– Four states
– Transition on rising clock
edge to next state
x=1
On2
clk^
x=1
On3
clk^
clk
State Off On1On2On3 Off On1On2 On3 Off
Outputs:
x
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Extend FSM to Three-Cycles High Laser Timer
• Four states
• Wait in “Off” state while b is
0 (b’)
• When b is 1 (and rising
clock edge), transition to
On1
Inputs: b; Outputs: x
x=0
Off
clk^
b’*clk^
b*clk^
x=1 clk^
x=1
On1
On2
clk^
x=1
On3
– Sets x=1
– On next two clock edges,
transition to On2, then On3,
which also set x=1
• So x=1 for three cycles after
button pressed
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FSM Simplification: Rising Clock Edges Implicit
• Showing rising clock on every
transition: cluttered
Inputs: b; Outputs: x
x=0
Off
– Make implicit -- assume every
edge has rising clock, even if not
shown
– What if we wanted a transition
without a rising edge
• We don’t consider such
asynchronous FSMs -- less
common, and advanced topic
• Only consider synchronous
FSMs -- rising edge on every
transition
clk^
b’ *clk^
b*clk ^
x=1 clk^
x=1
On1
On2
clk^
x=1
On3
Inputs: x; Outputs: b
x=0
Off
b’
b
x=1
x=1
x=1
On1
On2
On3
Note: Transition with no associated condition thus
transistions to next state on next clock cycle
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FSM Definition
• FSM consists of
– Set of states
Inputs: x; Outputs: b
x=0
• Ex: {Off, On1, On2, On3}
– Set of inputs, set of outputs
• Ex: Inputs: {x}, Outputs: {b}
– Initial state
Off
b’
b
x=1
x=1
x=1
On1
On2
On3
• Ex: “Off”
– Set of transitions
• Describes next states
• Ex: Has 5 transitions
– Set of actions
• Sets outputs while in states
• Ex: x=0, x=1, x=1, and x=1
We often draw FSM graphically,
known as state diagram
Can also use table (state table), or
textual languages
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FSM Example: Secure Car Key
• Many new car keys include
tiny computer chip
– When car starts, car’s
computer (under engine hood)
requests identifier from key
– Key transmits identifier
Inputs: a; Outputs: r
• If not, computer shuts off car
• FSM
– Wait until computer requests
ID (a=1)
– Transmit ID (in this case,
1101)
Wait
r=0
a
a’
K1
K2
K3
K4
r=1
r=1
r=0
r=1
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FSM Example: Secure Car Key (cont.)
Inputs: a; Outputs: r
• Nice feature of FSM
– Can evaluate output behavior
for different input sequence
– Timing diagrams show states
and output values for different
input waveforms
Wait
r=0
a
a’
K1
K2
K3
K4
r=1
r=1
r=0
r=1
Q: Determine states and r value for
given input waveform:
clk
clk
Inputs
a
Inputs
a
State
Outputs
r
Wait Wait
K1
K2
K3
K4 Wait Wait
State
Wait Wait
K1
K2
K3
K4 Wait K1
Output
r
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FSM Example: Code Detector
• Unlock door (u=1) only when
buttons pressed in sequence:
s
Start
u
r
g
b
a
Red
Green
Blue
– start, then red, blue, green, red
• Input from each button: s, r, g, b
– Also, output a indicates that
some colored button pressed
Inputs: s,r,g,b,a;
Outputs: u
• FSM
– Wait for start (s=1) in “Wait”
– Once started (“Start”)
• If see red, go to “Red1”
• Then, if see blue, go to “Blue”
• Then, if see green, go to
“Green”
• Then, if see red, go to “Red2”
– In that state, open the door
(u=1)
• Wrong button at any step, return
to “Wait”, without opening door
Wait
u=0 s
Start
u=0
Door
lock
Code
detector
s’
ar’
ab’
ag’
ar’
a’
ar
Red1
u=0
ab
a’
Blue
u=0
ag
a’
Green
u=0
ar
a’
Red2
u=1
Q: Can you trick this FSM to open the door,
without knowing the code?
A: Yes, hold all buttons simultaneously
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Improve FSM for Code Detector
Wait
u=0 s
Start
u=0
s’
ar’ ab’ ag’
ar’
a’
ar
Red1
u=0
•
•
Inputs: s,r,g,b,a;
Outputs: u
ab
a’
Blue
u=0
ag
a’
Green
u=0
ar
a’
Red2
u=1
New transition conditions detect if wrong button pressed, returns to “Wait”
FSM provides formal, concrete means to accurately define desired behavior
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Standard Controller Architecture
Inputs: x; Outputs: b
x=0
– Use standard architecture
Off
– 2-bit state register, can represent four
states
– Input b, output x
I
O
Combinational
logic
S
clk
x=1
x=1
On1
On2
On3
x
b
Combinational n1
logic
n0
s1
s0
clk
m
m-bit
state register
x=1
FSM
outputs
FSM
inputs
– Known as controller
b
FSM
inputs
• State register -- to store the present
state
• Combinational logic -- to compute
outputs, and next state
• For laser timer FSM
b’
outpu
FSM
FSM
outputs
• How implement FSM as sequential
circuit?
State register
m
N
General version
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