Intro. Latches in Sequential Circuits

1. 1
DIGITAL LOGIC DESIGN
By
Muhammad Umar khattak
Bahria University Islamabad
SEQUENTIAL CIRCUITS

2. Sequential Circuits 2
Circuits
 Combinational
 The output variables are dependent only on the present input
values
 It uses only logic gates
 They are faster
 They are simple and easy to design.
 Example: Adder etc.
 Sequential
 The outputs depend on the present input as well as past
output values
 Slower than combinational circuit.
 It uses logic gates and storage elements
 Harder to design
 Example Counters etc.

3. Sequential Circuits 3
Block Diagram
 Memory elements can store binary information
 This information at any given time determines the state of the
circuit at that time
The sequential circuit receives binary information from external
inputs that together with the present state of the storage elements,
determine the binary value of the outputs.

4. Sequential Circuits 4

5. Sequential Circuits 5

6. Sequential Circuits 6
Sequential Circuit Types
 Synchronous
 Circuit output changes only at some discrete instants of
time.
 The circuits achieves synchronization by using a timing
signal called as the clock.
 A clock is used for synchronization
 Memory elements are affected only with the arrival of a
clock pulse
 If memory elements use clock pulses in their inputs,
the circuit is called
 Clocked sequential circuit (FlipFlops)
 The status of memory element is effected only at the active
edge of the clock, if input is changed.
 Slower because of clocking.

7. Sequential Circuits 7
Sequential Circuit Types
 Asynchronous
 Circuit output can change at any time (clock
less).
 Unclocked latch is used as a memory element.
 Faster because of no clock.
 Status of memory element is changed at any
time as soon as input is changed.

8. Sequential Circuits 8
Clock
 It emits a series of pulses with a
precise pulse width and precise
interval between consecutive pulses
 Timing interval between the
corresponding edges of two
consecutive pulses is known as the
clock cycle time, or period

9. 2017/12/19 PJF - 9
Sequential Circuits
Clock Signal
Different duty cycles
Clock generator: Periodic train of clock pulses

10. Latches

11. Sequential Circuits 11
Latches
 The basic storage elements is called a latch.
 It stores single bit of information i.e. 0 or 1.
 The most basic flip-flops
 They operate with signal levels
 The flip-flops are constructed from latches
 They are not useful for synchronous
sequential circuits
 They are useful for asynchronous sequential
circuits

12. Latches
 S-R (Set-Reset) latch
 Gated S-R latch
 Gated D latch

13. Latches
 S-R latch with NOR

14. Sequential Circuits 14
SR Latch with NOR

15. Sequential Circuits 15
SR Latch with NOR
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