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Lab 08: SR Flip Flop Fundamentals:
Slide 2
Slide 3
NOR Gate SR Flip Flop.
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:
Output Q and Q are by definition always opposite to each other. If Q=1 then Q =0.
S R Output
0 0
0 1
1 0
1 1
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.
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.
Lab 08: NOR Gate SR Flip Flop :
Q
Q
R
S
1
0
Start with S,R = 0,0 : The analysis procedure works as follows:
1-Place the initial conditions at output Q on the diagram. Assume Q =0. 2-Place the input conditions at S and R.3-Analyze the top NOR gate and record Q.4-Analyze the bottom NOR gate and record Q.5-Repeat steps 3 and 4 until Q and Q settle.
0
0
1
0
Q settles at logic 0
No Change Q stays at 0
Next S,R = 0,1 : The analysis procedure works as follows:
1-Place the initial conditions at output Q on the diagram. Assume Q =1. 2-Place the input conditions at S and R.3-Analyze the top NOR gate and record Q.4-Analyze the bottom NOR gate and record Q.5-Repeat steps 3 and 4 until Q and Q settle.
0
1
0
1
00
0 1
1
Reset: Q changes to 0
Q changes from 1 to 0.Called the reset mode!
Next S,R = 1,0 : The analysis procedure works as follows:
1-Place the initial conditions at output Q on the diagram. Assume Q =0. 2-Place the input conditions at S and R.3-Analyze the top NOR gate and record Q.4-Analyze the bottom NOR gate and record Q.5-Repeat steps 3 and 4 until Q and Q settle.
0
11
0
1
0 0
01
1 Set: Q changes to 1
Q changes from 0 to 1.Called the Set mode!
Next S,R = 1,1 : The analysis procedure works as follows:
1-Place the initial conditions at output Q on the diagram. Assume Q =0. 2-Place the input conditions at S and R.3-Analyze the top NOR gate and record Q.4-Analyze the bottom NOR gate and record Q.5-Repeat steps 3 and 4 until Q and Q settle.
1
01
1
1
0 0
0
Both outputs settle to 0. This breaks the definition. This condition is not allowed
Ambiguous : Not allowed!
Slide #2
1
0 Assume Q starts at 0.
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.
Lab 08: SR Flip Flop :
S
R
Q
Q
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”.
0
1
S=1 : Set Mode :
0
1 Output SETs : Q =1.
1
0
R=1 : reset Mode :
1
0 reset : Q =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.
0
1
S=1 : Set Mode :
0
1 Output SETs : Q =1.
0
0
: Hold Mode :
0
1 Q does not change
1
0
R=1 : reset Mode :
1
0 reset : Q =0.
0
0
: Hold Mode :
1
0 Q does not change
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).
S
R
Q
Q 1
0 Assume Q starts at 0.
1
0
S=0 : Set Mode :
0
1 Output SETs : Q =1.
0
1
R=0 :reset Mode :
1
0 Output SETs : Q =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.
1
1
: HOLD Mode :
1
0 Q does not change
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!
Slide #3
1
0 Assume Q starts at 0.
0
1 Output SETs : Q =1.
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.
Lab 08: SR Flip Flop with a Positive Edge Triggered Clock Input :
S
R
Q
Q
>Clk
SR Flip Flop with edge triggered clock
An edge triggered clock is identified with “>Clk” on the symbol.
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”.
0
1
S=1 : Set Mode :
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.
Inside the SR Flip Flop with Positive Edge Triggered Clock:
The clock signal is applied to the input.
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.
>Clk
S
R
S
R
Q
Q
1
3
21/0
1/0
1/0
1/0
0
0
0
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.
Lab 08: SR Flip Flop with a Negative Edge Triggered Clock Input :
1
0 Assume Q starts at 0.
0
1 Output SETs : Q =1.
SR Flip Flop with edge triggered clock
An edge triggered clock is identified with “o|>Clk” on the symbol.
0
1
S=1 : Set Mode :
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.
S
R
Q
Q
>Clk
Here is a summary of the flip flop devices
S
R
Q
Q
S
R
Q
Q
Non-Clocked SR
S
R
Q
Q
>Clk
S
R
Q
Q
>Clk
Edge Triggered
S and R control the response at Q continuously.
S and R control the response at Q only when Clk is making a transition. On the edge of the clock signal.
Slide #5
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.
Note Pack 5 : Flip Flop Waveform Diagrams :
Set
ResetClock
S
R
Q
Q
>Clk
The initial condition Q =0 is marked as a dot on the output waveform diagram.
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.
Until the clock changes from 1 to 0 it is NOT asserted. Thus Q holds at 0.
Analyze the waveform and draw Q.
On this negative edge S=R=0: No Change Mode. Thus Q holds at 0. No analysis is required until next negative edge.
On this negative edge S=1 and R=0: SET Mode. Thus Q sets to 1. No analysis is required until the next negative edge.
Slide #6
