Jeff YiCS 147
Circuits
Combinatorial – Circuit that only relies on inputs.
Sequential - Circuit determined by input as well as the previous state of the circuit.
Types of sequential circuits Synchronous sequential circuit –
Uses a clock to determine when a events occur.
Asynchronous sequential circuit – Uses the previous circuit changes to determine when the next event occurs.
Feedback
Sequential circuits use the previous state as input by using feedback.
After passing through a flip-flop, the result is put back as input along with any other inputs.
Clock
A clock sends signals in high and low levels in intervals for sequential circuits.
Clocks have high and low levels as well as rising and falling edges.
Clock Examples
Flip-Flop
Flip-flops are a synchronous sequential circuit that is edge-triggered.
Latches are a synchronous sequential circuit that is level-triggered.
Types of Flip-flops
SR flip-flop uses 2 inputs: S (set) and R (reset).
JK flip-flop is a variation of SR where J is set and K is reset.
D flip-flop uses a data input of 1 or 0, where 1 sets the flip-flop and 0 resets it.
T flip-flop uses a toggle input of 1 to change states
SR Flip-flop
Has an S input to set and an R input to reset.
When S=R=0, then no change is made.
When S=1 and R=0, the state is changed to set.
When S=0 and R=1, the state is reset. However, when S=R=1, the output is
undefined.
SR Flip-flop Truth Table
SR Flip-flop (NOR)
SR Flip-flop (NAND)
JK Flip-flop
Variation of SR Flip-flop Named after Texas Instruments
engineer, Jack Kirby. J is the set input and K is the reset
input Combinations of J and K are the same
as the SR Flip-flop except for when both are 1.
In the case where J=K=1, the output simply toggles between set and reset.
JK Flip-flop
D Flip-flop
Uses data input to set S or R as 1. When D=1, S=1 and R=0. When D=0, R=1 and S=0. This prevents both S and R from both
being 1.
D Flip-flop
T Flip-flop
Uses a single toggle input for the JK Flip-flop.
When T=1, then whatever the state is, it switches to the opposite.
When T=0, nothing changes.
T Flip-flop
Questions?
References
Lecture Notes Anthony Des Reis, Assembly
Langugae And Computer Architecture Using C++ And Java, Thompson,2004.
Null and Lobur, The Essentials of Computer Organization and Architecture, Second Edition
“Flip-flops”. http://wearcam.org/ece385/lectureflipflops/flipflops/