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 let’s 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 can’t 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 doesn’t 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 don’t 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 capacitor’s 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 doesn’t actually involve mechanical steps is the drawing of the problem’s 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.