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触发器
Flip-Flops数字系统设计2Spring
2015
ZDMC
–
Lec.
#7复习
时序电路的基本概念触发器本节内容
组合电路和Verilog语言测试数字系统设计3Spring
2015
ZDMC
–
Lec.
#7测试(补充)
测试是一个专门用来给电路的HDL模型施加的一个激励的HDL程序,目的是测试和观察其在激励下的响应。典型的测试模块没有输入和输出,加到设计模块用于模拟的输入信号在激励模块中定义为局部reg型数据。显示设计模块的测试输出在激励模块中定义为局部wire型数据。用局部的标识来例化测试的模块。测试模块的HDL格式:
module
test_module_name;
//Declare
local
reg
and
wire
identifiers.
//Instantiate
the
design
module
under
test.
//Specify
a
stopwatch,
using
$finish
to
terminate
the
simulation
//Generate
stimulus,
using
initial
and
always
statements.
//Display
the
output
response
(text
or
graphics
(or
both)).
endmodule数字系统设计4Spring
2015
ZDMC
–
Lec.
#7Sequential
Logic
时序逻辑
Sequential
Circuits
时序电路
Simple
circuits
with
feedback
Latches
(level
sensitive)
–
Storage
elements
that
operate
with
signal
levels
(rather
than
signal
transitions)
are
referred
to
as
latches.
Flip-flops
(edge
sensitive)
(abbreviated
FF)
–
A
flip-flop
is
a
binary
storage
device
capable
of
storing
one
bit
of
information.Timing
Methodologies定时
Cascading级联
flip-flops
for
proper
operation
Clock
skew时钟偏移数字系统设计5C3
value
C1comparatorequal
C2multiplexerreset
open/closedSpring
2015
ZDMC
–
Lec.
#7newmuxcontrolclock
equalcomb.
logic
stateSequential
Circuits
Circuits
with
Feedback
Outputs
=
f(inputs,
past
inputs,
past
outputs)
Basis
for
building
"memory"
into
logic
circuits
Door
combination
lock
is
an
example
of
a
sequential
circuit
–
State
is
memory
–
State
is
an
"output"
and
an
"input"
to
combinational
logic
–
Combination
storage
elements
are
also
memory数字系统设计6Spring
2015
ZDMC
–
Lec.
#7X1X2
•
•
•Xnswitching
networkZ1Z2
•
•
•ZnCircuits
with
Feedback
How
to
control
feedback?
What
stops
values
from
cycling
around
endlessly数字系统设计7Spring
2015
ZDMC
–
Lec.
#7"load""data""stored
value"
Simplest
Circuits
with
Feedback
Two
inverters
form
a
static
memory
cell
Will
hold
value
as
long
as
it
has
power
applied
"1"
"stored
value"
"0"
How
to
get
a
new
value
into
the
memory
cell?
Selectively
break
feedback
path
Load
new
value
into
cell
"remember"数字系统设计8Spring
2015
ZDMC
–
Lec.
#7RSQ
Q'RSQS'R'QQ
Q'S'R'
Memory
with
Cross-coupled
Gates
Cross-coupled
NOR
gates
Similar
to
inverter
pair,
with
capability
to
force
output
to
0
(reset=1)
or
1
(set=1)
Cross-coupled
NAND
gates
Similar
to
inverter
pair,
with
capability
to
force
output
to
0
(reset=0)
or
1
(set=0)数字系统设计Setting
the
Latch
(FF)
Pulsing
the
SET
input
to
the
0
state
when
(a)
Q
=
0
prior
to
SET
pulse;(b)
Q
=
1
prior
to
SET
pulse.
Note
that,
in
both
cases,
Q
ends
up
HIGH.9Spring
2015
ZDMC
–
Lec.
#7数字系统设计Resetting
the
Latch
(FF)
Pulsing
the
RESET
input
to
the
LOW
state
when
(a)
Q
=
0
prior
to
RESETpulse;
(b)
Q
=
1
prior
to
RESET
pulse.
In
each
case,
Q
ends
up
low.10Spring
2015
ZDMC
–
Lec.
#711Spring
2015
ZDMC
–
Lec.
#7ResetHoldSetSetResetRace
R
S
Q
\Q数字系统设计100Timing
BehaviorRSQ
Q'数字系统设计12Spring
2015
ZDMC
–
Lec.
#7S00R01Qhold011011unstableState
Behavior
of
R-S
latch
Truth
table
of
R-S
latch
behaviorQ
Q'
0
1Q
Q'
1
0Q
Q'0
0Q
Q'1
1数字系统设计13Spring
2015
ZDMC
–
Lec.
#7Theoretical
R-S
Latch
Behavior
State
Diagram
States:
possible
values
Transitions:
changesbased
on
inputsQ
Q'
0
1Q
Q'
1
0Q
Q'
0
0Q
Q'
1
1SR=00SR=11SR=00SR=10SR=01SR=00SR=10SR=00SR=01SR=11SR=11SR=10SR=01SR=01SR=10SR=11possible
oscillationbetween
states
00
and
11数字系统设计14Observed
R-S
Latch
Behavior
SR=00SR=00Q
Q'
0
1Q
Q'
1
0Very
difficult
to
observe
R-S
latch
in
the
1-1
state
One
of
R
or
S
usually
changes
firstAmbiguously
returns
to
state
0-1
or
1-0
A
so-called
"race
condition"
Or
non-deterministic
transition
SR=10SR=01SR=00SR=10SR=00SR=01SR=01SR=10
SR=11
Q
Q'
0
0SR=11
SR=11
Spring
2015
ZDMC
–
Lec.
#700X110X1数字系统设计15Spring
2015
ZDMC
–
Lec.
#7RQQ'Q(t+∆)RSQ(t)SSRQ(t)Q(t+∆)000001010011110001010011111101XXholdresetsetnot
allowedcharacteristic
equation
Q(t+∆)
=
S
+
R’
Q(t)R-S
Latch
Analysis
Break
feedback
pathQ(t)RS16Spring
2015
ZDMC
–
Lec.
#7
QQ'
R'enable'
S'R
SGated
R-S
Latch
Control
when
R
andS
inputs
matter
Otherwise,
the
slightest
glitch
on
R
or
S
while
enable
is
low
could
causechange
in
valuestoredSetReset
S'
R'
enable'
Q
Q'数字系统设计100数字系统设计17
periodSpring
2015
ZDMC
–
Lec.
#7Clocks
Used
to
keep
time
Wait
long
enough
for
inputs
(R'
and
S')
to
settle
Then
allow
to
have
effect
on
value
storedClocks
are
regular
periodic
signals
Period
(time
between
ticks)
Duty-cycle
(time
clock
is
high
between
ticks
-
expressed
as
%
of
period)
duty
cycle
(in
this
case,
50%)Q'数字系统设计18
clockSpring
2015
ZDMC
–
Lec.
#7R'
and
S'Clocks
(cont’d)
Controlling
an
R-S
latch
with
a
clock
Can't
let
R
and
S
change
while
clock
is
active
(allowing
R
and
S
to
pass)
Only
have
half
of
clock
period
for
signal
changes
to
propagate
Signals
must
be
stable
for
the
other
half
of
clock
period
Qstable
c
hanging
stable
c
hanging
stable
R'clock'
S'R
S19Spring
2015
ZDMC
–
Lec.
#7
clock数字系统设计RSQ'
QRSQ'
QRSCascading
Latches
Connect
output
of
one
latch
to
input
of
anotherHow
to
stop
changes
from
racing
through
chain?
Need
to
control
flow
of
data
from
one
latch
to
the
next
Advance
from
one
latch
per
clock
period
Worry
about
logic
between
latches
(arrows)
that
is
too
fast20Spring
2015
ZDMC
–
Lec.
#7Master-Slave
Structure
Break
flow
by
alternating
clocks
(like
an
air-lock)
Use
positive
clock
to
latch
inputs
into
one
R-S
latch
Use
negative
clock
to
change
outputs
with
another
R-S
latchView
pair
as
one
basic
unit
master-slave
flip-flop
twice
as
much
logic
output
changes
a
few
gate
delays
after
the
falling
edge
of
clock
but
does
not
affect
any
cascaded
flip-flopsmaster
stageslave
stageP'P
CLK数字系统设计RSQ'
QRSQ'
QRSRRQ'21Set
1scatch
S
R
CLK
P
P'
Q
Q'数字系统设计Reset
Master
Outputs
Slave
OutputsSpring
2015
ZDMC
–
Lec.
#7The
1s
Catching
Problem
In
first
R-S
stage
of
master-slave
FF
0-1-0
glitch
on
R
or
S
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