Digital Logic Design

1
Digital Logic Design

• Lecture 18
• University of Tehran

2
Outline

• Other Kinds of Flip Flops
• Dynamic Logic
• Sequential Circuit Design
• Designing a 110 Detector

3
Other Kinds of Flip Flops

• Last session we saw a D flip flop with a master
  slave structure, now lets see a SR flip flop
  with the same master slave structure as before.
  The same differences between the D latches and
  master slave D flip flops can be observed between
  the corresponding SR latches and SR flip flops,
  this difference being the isolation of data in
  flip flops.

4
Other Kinds of Flip Flops (continued)

• The problem we had with SR latches still exists
  in SR flip flops, that is we cant have activity
  on both S and R lines at the same time. When
  either of these 2 components is working properly
  the output lines should have complemented values,
  that is when line 1 is set to be high, line 2
  must be low and vice versa.
  

5
Other Kinds of Flip Flops (continued)

• The mentioned problem can be solved by adding the
  red feedback lines to the circuit, thus disabling
  the set line when the circuit is in its set state
  and disabling the reset line when the circuit is
  in a reset state. Doing this will not effect
  ordinary functionality of our circuit as it
  doesnt change our ability in changing states.
  

6
Other Kinds of Flip Flops (continued)

• This representation, shows an altered form of the
  SR flip flop called a JK Flip Flop and can be
  represented in the following forms
  
  

7
Other Kinds of Flip Flops (continued)

• Quote Structures that work on the clock pulse,
  such as JK flip flop are called pulse triggered.
  Structures that work with the rising or falling
  edge of the clock are called edge triggered,
  which work slightly faster than the pulse
  triggered structures but are less reliable.
• Note When we look at a pulse triggered and
  falling edge triggered on an oscilloscope
  alongside one another, they dont differ much on
  timing manners but their structures are
  completely different.

8
Other Kinds of Flip Flops (continued)

• In the shown JK flip flop, if we connect the J
  and K inputs to each other, we have a toggle flip
  flop that keeps its output when clocked on a low
  input and toggles the output when clocked on a
  high input.
  

9
Other Kinds of Flip Flops (continued)

• Flip flops can always be made from each other
  using some discrete logic, for instance consider
  the following example. Making a JK flip flop
  from a D flip flop
  

10
Dynamic Logic

• The basis of this sequential logic is of the
  following shape, instead of the cross coupled
  gates we have seen so far
• This structure which is called a half register
  stores the data inputted from the NMOS input as
  capacity charge in the inverter when the clock
  rises to 1, and will keep the data until the
  capacitors charge starts to fade.

11
Dynamic Logic (continued)

• Using the half register shown, we have the
  Dynamic D type master slave flip flop as shown in
  the figure
• When c is 1, data is stored in the first
  inverter structure and when it goes 0, data is
  stored in the second.

12
Dynamic Logic (continued)

• The main problem in these structures in that its
  data can quickly fade, and will need refreshing
  to keep its amount (that is when it is not
  clocked with new data for a while). This
  refreshing is done by feedbacking the output to
  the input as seen in red.
  

13
Dynamic Logic (continued)

• Quote The transistors controlled by signal E
  is driven by the pull up on the levels that come
  before this structure.
• Another point that must be taken to mind is that
  we use non-overlapping signals c1 and c2, instead
  of c and c for control signals to avoid any
  transparency that may occur otherwise. This
  structure is called a 2 phase non-overlapping
  clock structure.

14
Sequential Circuit Design

• We have seen basic discrete components in
  sequential design that are gates and flip flops.
  In sequential circuits, there are certain models
  that are usually followed. The model we will be
  using in which everything happening is
  synchronized to a clock that is implicitly part
  of our circuit is called the Huffman Model as
  seen in the following figure
  

15
Sequential Circuit Design (continued)

• Each sequential circuit that you will see has a
  combinational part and a register part where the
  register part feedbacks its output to the
  combinational part. What we need to be able to
  design such circuits with practical speed is a
  straight forward method.
• We saw that a flip flop (the main part in the
  register section of our model) has different
  representation, them being transition table,
  characteristic equation, excitation table, state
  diagram. Among these different representations,
  the state diagram is the nearest to our way of
  thought.

16
Sequential Circuit Design (continued)

• Using the state diagram as a basis to build on,
  from now on, the design flow will run from the
  problem description to state diagrams, a
  transition table and finally the circuit
  itself.
  

17
Designing a 110 Detector

• As our first design we will see a 110 detector
  that will give a high output whenever clocked
  with such a sequence on its input.
• Obviously such a circuit is sequential because
  its output relies on a series of certain inputs
  in its history.

18
Designing a 110 Detector (continued)

• Note In synchronous sequential circuits such as
  that of this problem, anything happening when the
  clock is in its active state.
• Note When doing our design of sequential
  circuits we consider the clock pulse to be
  implicit and no gating must be done on this
  signal.
• To do our circuit design of this problem we start
  from its state diagram. We may even not know how
  many states we need but we work our way through
  the different states.

19
Designing a 110 Detector (continued)

• This problem can be the same as looking for a
  special sequence of shops when walking along a
  road. Consider you are looking for 2 book shops
  followed by a grocery store. Now if we see a
  grocery store first we simply ignore it (meaning
  that we stay in our initial state), but seeing a
  book shop we will need to remember that we have
  passed the first shop we were looking for in that
  special sequence (meaning that we change to a new
  state).

20
Designing a 110 Detector (continued)

• The same method can be applied to our problem,
  where seeing a 0 is seeing a grocery store and a
  1 is a book shop. Thus as we saw above seeing a
  0 first must not change our state and so on.
  Applying this method to our problem gives the
  following state diagram
  

21
Designing a 110 Detector (continued)

• In our flow of design the only part that doesnt
  actually involve mechanical steps is the drawing
  of the problems state diagram. The rest that we
  see next session is just a series of straight
  forward steps that can be done by hand or using
  simulation tools.