digital logic designing preview explain.ppt
- 1. 1 Subject : EE227-Digital Logic Design Subject : EE227-Digital Logic Design Lecture ecture : #20 : #20 Date : 08 April, 2020 Wednesday Date : 08 April, 2020 Wednesday Instructor : Mr. Muhammad Kamran Instructor : Mr. Muhammad Kamran office : Exam Hall (M-117) office : Exam Hall (M-117) Email : muhammad.kamran@nu.edu.pk Email : muhammad.kamran@nu.edu.pk In the Name of Allah the Most Beneficent the Most Merciful
- 2. 2 Outline Flip-Flop with direct inputs Characteristic equations of D, SR and JK flip flops. Excitation table for D, DR and JK flip flop.
- 3. Flip Flops with direct inputs Flip Flops with direct inputs Special inputs for setting and resetting memory Special inputs for setting and resetting memory elements like flip-flops elements like flip-flops asynchronously . The special inputs that The special inputs that asynchronously set the set the flip-flop are called direct set input or preset. flip-flop are called direct set input or preset. 3
- 4. The special inputs that The special inputs that asynchronously Reset the set the flip-flop are called direct Reset input or clear. flip-flop are called direct Reset input or clear. These inputs change the state of the flip-flop These inputs change the state of the flip-flop independent of the clock input. independent of the clock input. 4
- 5. Why direct input? Why direct input? 5 Direct inputs are useful to bring the flip-flop in Direct inputs are useful to bring the flip-flop in reset state when power is turned on because we reset state when power is turned on because we don’t know what was previously stored in don’t know what was previously stored in memory. memory. Similarly we can bring flip-flop to set state Similarly we can bring flip-flop to set state without applying clock inputs. without applying clock inputs.
- 6. Positive edge triggered D FF Positive edge triggered D FF 6
- 7. The notations The notations C1 and and 1D shows control shows control dependency. dependency. An input labeled Cn, where n is any number, controls all the other inputs starting with the number n C1 controls input 1D. S and R have no 1 in front of them, and therefore they are not controlled by the clock at C1. 7
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- 10. The positioning of S and R at the top and bottom of the symbol rather than on the left edge implies that resulting output changes are not controlled by the clock C. 10
- 11. Find Characteristic equation of Find Characteristic equation of Flip-Flop Flip-Flop Characteristic table is used to find equations of Characteristic table is used to find equations of flip-flops. flip-flops. Characteristic table of a flip flop is designed by Characteristic table of a flip flop is designed by using truth table of that flip-flop. using truth table of that flip-flop. 11
- 12. D D Q(t+1) = D(t) Q(t+1) = D(t) SR SR Q(t+1) = S(t) + R’(t)Q(t) Q(t+1) = S(t) + R’(t)Q(t) JK JK Q(t+1) = J(t)Q’(t) + K’(t)Q(t) Q(t+1) = J(t)Q’(t) + K’(t)Q(t) 12
- 13. Excitation table of flip-flop Excitation table of flip-flop What will be the input value for a combination What will be the input value for a combination of present state and next state of a particular of present state and next state of a particular flip-flop flip-flop is described by excitation table. is described by excitation table. Excitation table of a flip-flop is designed using Excitation table of a flip-flop is designed using characteristic table of that particular flip-flop. characteristic table of that particular flip-flop. It is used to convert one flip-flop into another. It is used to convert one flip-flop into another. 13
- 14. Applications of Flip-Flops Applications of Flip-Flops Registers Registers Counters Counters Frequency Dividers Frequency Dividers 14
- 15. Register Register A memory block in CPU which is used to store a A memory block in CPU which is used to store a group of bits at the same time. group of bits at the same time. If we are able to store one bit, we can definitely If we are able to store one bit, we can definitely store more than one bit at a time. store more than one bit at a time. So we can say a register is a combination of flip So we can say a register is a combination of flip flops. flops. Which means if we want to store 4 bit data, we Which means if we want to store 4 bit data, we have to combine 4 flip flops in a single circuit to have to combine 4 flip flops in a single circuit to design 4 bit register. design 4 bit register. 15