SR Flip Flop Fundamentals:
Slide 2
NOR Gate SR Flip Flop.
Slide 3
SR Flip Flop.
Slide 4
SR Flip Flop with a positive edge clock:
Slide 5
SR Flip Flop with a negative edge clock:
Slide 6
Flip Flop waveform diagrams:
NOR Gate SR Flip Flop :
The cross-coupled NOR gates creates an SR Flip Flop. Flip Flops are the basic
elements used in computer memory. The S input is called Set. The R input is called
Reset.
R
01
01
00
1
10
0
11
0
01
0
S
11
0
00
1
00
1
Q
Q
Q settles at
logic 0
Q changes
Both
outputs
from
1 to 0.
Q changes
settle
to
0. 1.
Called
from 0 the
to
This
breaks
the
reset
!
Calledmode
the Set
definition.
This
mode!
condition is not
allowed
S
R
Output
0
0
No Change Q stays at 0
0
1
Reset: Q changes to 0
Set: Q changes to 1
1
0
1
1
Ambiguous : Not allowed!
Output Q and Q are by definition always opposite to each other. If Q=1 then Q =0.
Behaviour Table: Logic gates are defined by Truth Tables. Flip Flops are defined by behaviour tables.
Two different names for tables that do essentially the same job. To generate the behaviour table
you must assume an initial condition at output Q. This is necessary because the outputs are wired to
the inputs. This creates a feedback path that can only be analyzed when a starting point is assumed.
Start
Nextwith
S,R =
S,R1,1
0,1
1,0
= :0,0
The
: The
analysis
analysis
procedure
procedure
works
works
as follows:
as follows:
1-Place
1-Placethe
theinitial
initialconditions
conditionsatatoutput
outputQQon
onthe
thediagram.
diagram.Assume
AssumeQQ=0.
=1.
=0.
2-Place
2-Placethe
theinput
inputconditions
conditionsatatSSand
andR.R.
3-Analyze
3-Analyzethe
thetop
topNOR
NORgate
gateand
andrecord
recordQ.
Q.
4-Analyze
4-Analyzethe
thebottom
bottomNOR
NORgate
gateand
andrecord
recordQ.
Q.
5-Repeat
5-Repeatsteps
steps33and
and44until
untilQQand
andQQsettle.
settle.
Slide #2
Cross-coupled NOR gates create an SR Flip Flop. It is easy to remember the operation
of an SR flip flop using only the symbol without repeatedly analyzing the cross
coupled NOR gate system.
10
0
S
Q
00
1
R
Q
10
0
1 Assume
Output
Output
Q reset
does
reset
SETs
Q
SETs
not
starts
: :QQ
: =1.
change
:Q=1.
at
Q=0.
=0.
0.
01
1
0
Symbol
The S input is called SET the R input is called reset.
They are both active high. Active high means that S =1 sets the
flip flop (S =0 does not set). R =1 resets the flip flop (R =0 does not
reset).
SET means set output Q to “1”.
RESET means reset output Q to “0”.
When S =0 and R =0 then Q does not change. Q holds its logic
level (1 or 0). It is equivalent to not issuing either the set or the
reset command.
: :Hold
S=1
Hold
: Set
R=1
Mode
Mode
Mode
: :: :
reset Mode :
When S =1 and R =1 then Q is ambiguous. Both Q and Q outputs go to the same logic level which breaks the
definition of a flip flop. You can think of it this way … S =1 says SET and R =1 says reset. The flip flop does not
know whether the output should be Q =1 or Q =0. S=R=1 should never be used!
There is a second variety of SR flip flop that uses an active low S and R inputs. The internal system is cross coupled
NAND gates. Active low means that S =0 sets the flip flop (S =1 does not set). R =0 resets the flip flop (R =1 does
not reset).
0
1
S
Q
1
0
1
R
Q
S=0
::reset
Set Mode
R=0
: HOLD
Mode
Mode:: :
Slide #3
1 Output
0
Assume
Q does
SETs
Qnot
starts
: Qchange
=1.
at 0.
0
1
When S =1 and R =1 then Q does not change. Q holds its logic
level (1 or 0). It is equivalent to not issuing either the set or the
reset command.
When S =0 and R =0 then Q is ambiguous. The flip flop does
not know whether the output should be Q =1 or Q =0. S=R=0
should never be used!
SR Flip Flop with a Positive Edge Triggered Clock Input :
A Positive EDGE triggered flip flop has a new input called clock. The clock requires a
transition from 0 to 1 in order that S and R controls output Q. Holding a constant logic 1
or a constant logic 0 at the clock input does not allow SR to change output Q.
An edge triggered clock is identified with “>Clk” on the symbol.
1
0
S
Q
>Clk
R
Q
0 Assume
1
Output SETs
Q
starts
: Q =1.
at 0.
1
0
A transition from 0 to 1 at “>Clk” is required in order for the flip flop
to respond to S and R. This is called a” Positive Edge”.
Watch the animation to see how you would set the flip flop.
Holding “>Clk” at logic 1 will not result in S and R controlling Q. Only the
0 to 1 transition at “>Clk“ causes the output Q to change.
SR Flip Flop with edge
triggered clock
S=1 : Set Mode :
Inside the SR Flip Flop with Positive Edge Triggered Clock:
The clock signal is applied to the input.
1/0 S
>Clk
1
1/0 R
0
2
1/0
0 S
Q
3
1/0
0 R
Q
The NOT gate delays the signal because it has
a propagation delay. Propagation delay is the
reaction time of the inverter. Let’s use 3 to 10
nanoSec.
3 to 10 nanoSec
delay.
During the 3 to 10 nanoSec interval, AND gate #1 outputs a 1. AND gates #2 and #3 transfer the logic levels to
internal SR and Q responds.
Slide #4
After the 3 to 10 nanoSec interval AND gate #1 outputs a 0. AND gates #2 and #3 transfer the logic 0 to
internal SR and Q holds(S=R=0 is Hold mode). To re-clock the flip flop you need another positive edge.
Clock must return to 0 and re-change back to 1.
SR Flip Flop with a Negative Edge Triggered Clock Input :
A negative edge triggered flip flop requires a transition from 1 to 0 at at the clock input
in order for the flip flop to respond to S and R. This is called a” Negative Edge”. It is the
opposite of a positive edge triggered flip flop.
An edge triggered clock is identified with “o|>Clk” on the symbol.
1
0
S
Q
>Clk
R
Q
0 Assume
1
Output SETs
Q
starts
: Q =1.
at 0.
1
0
Watch the animation to see how you would set the flip flop.
Holding “o|>Clk” at logic 0 will not result in S and R controlling Q. Only
the 1 to 0 transition at “o|>Clk“ causes the output Q to change.
SR Flip Flop with edge
triggered clock
S=1 : Set Mode :
Here is a summary of the flip flop devices
S
R
Q
Q
S
R
Q
S
Q
>Clk
R
Q
Non-Clocked SR
S and R control the response
at Q continuously.
Slide #5
Q
S
Q
>Clk
R
Q
Edge Triggered
S and R control the response at Q only
when Clk is making a transition.
On the edge of the clock signal.
Note Pack 5 : Flip Flop Waveform Diagrams :
To draw waveforms for flip flops you need to begin with an initial condition at Q, mark the
area where the clock input is asserted and then draw the output response. Let’s use an
initial condition of Q =0.
The initial condition Q =0 is
marked as a dot on the output
waveform diagram.
Set
The flip flop has a negative edge
triggered clock. The clock is
asserted when Clk makes a
transition from 1 to 0. The
asserted zone is marked off in
yellow.
Reset
Clock
Analyze the waveform and draw
Q.
S
Q
>Clk
R
Q
Untilthis
On
thenegative
clock changes
edge S=1
S=R=0:
fromand
1 to
No
R=0:
0Change
it SET
is NOT
Mode.
Mode.
asserted.
Thus Q sets
holdstoat1.0.No
Noanalysis
analysisisisrequired
requireduntil
untilthe
next
next
negative edge.
Slide #6