Sequential circuits
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Sequential circuits consist of combinational logic with feedback provided by storage elements like latches and flip-flops. There are two types of sequential circuits: asynchronous which depend on input signal order and are difficult to design, and synchronous which are defined by discrete clock pulses and are easier to design. Common storage elements include SR latches, D latches, and edge-triggered master-slave flip-flops which are used to build synchronous sequential circuits like RAM, ROM and other digital devices that can store a single bit.
Sequential circuits
- 2. Sequential Circuits • Consist of a combinational circuit to which storage elements are connected to form a feedback path • State: –the state of the memory devices now, also called current state • Next states and outputs are functions of inputs and present states of storage elements
- 3. Two Types of Sequential Circuits • Asynchronous sequential circuit • Depends upon the input signals at any instant of time and their change order • May have better performance but hard to design • Synchronous sequential circuit • Defined from the knowledge of its signals at discrete instants of time • Much easier to design (preferred design style) • Synchronized by a periodic train of clock pulses
- 5. Memory elements • Latch -—a level-sensitive memory element • SR latches • D latches • Flip-Flop —- an edge-triggered memory element • Master-slave flip-flop • Edge-triggered flip-flop • RAM and ROM —a mass memory element
- 6. Latches • A latch is binary storage element • Can store a 0 or 1 • The most basic memory • Easy to build • Built with gates (NORs, NANDs, NOT)
- 7. Latches • SR LatchR S Q Q S R Q0 Q Q’ 0 0 0 0 1 0 0 0 1 Q = Q0 Initial Value
- 8. Latches • SR LatchR S Q Q S R Q0 Q Q’ 0 0 0 0 1 0 0 1 1 0 0 0 1 0 Q = Q0 Q = Q0
- 9. Latches • SR LatchR S Q Q S R Q0 Q Q’ 0 0 0 0 1 0 0 1 1 0 0 1 0 0 0 1 1 0 1 Q = 0 Q = Q0
- 10. Latches • SR LatchR S Q Q S R Q0 Q Q’ 0 0 0 0 1 0 0 1 1 0 0 1 0 0 1 0 1 11 0 1 0 0 1 Q = 0 Q = Q0 Q = 0
- 11. Latches • SR LatchR S Q Q S R Q0 Q Q’ 0 0 0 0 1 0 0 1 1 0 0 1 0 0 1 0 1 1 0 1 1 0 0 0 1 0 1 1 0 Q = 0 Q = Q0 Q = 1
- 12. Latches • SR LatchR S Q Q S R Q0 Q Q’ 0 0 0 0 1 0 0 1 1 0 0 1 0 0 1 0 1 1 0 1 1 0 0 1 0 1 0 1 1 0 0 1 1 0 Q = 0 Q = Q0 Q = 1 Q = 1
- 13. Latches • SR LatchR S Q Q S R Q0 Q Q’ 0 0 0 0 1 0 0 1 1 0 0 1 0 0 1 0 1 1 0 1 1 0 0 1 0 1 0 1 1 0 1 1 0 0 1 1 1 0 0 Q = 0 Q = Q0 Q = 1 Q = Q’ 0
- 14. Latches • SR LatchR S Q Q S R Q0 Q Q’ 0 0 0 0 1 0 0 1 1 0 0 1 0 0 1 0 1 1 0 1 1 0 0 1 0 1 0 1 1 0 1 1 0 0 0 1 1 1 1 0 1 1 0 0 Q = 0 Q = Q0 Q = 1 Q = Q’ 0 Q = Q’
- 15. SR Latch R S Q Q S R Q 0 0 Q0 0 1 0 1 0 1 1 1 Q=Q’=0 No change Reset Set Invalid S R Q Q S R Q 0 0 Q=Q’=1 0 1 1 1 0 0 1 1 Q0 Invalid Set Reset No change
- 16. FlipFlops(Controlled Latches) • Digital Devices consist of logic gates can preserve 1 bit (1 or 0) • Types: •S-R FlipFlop •D FlipFlop •J-K FlipFlop •T Flipflop
- 17. S-R FlipFlops • SR Latch with Control Input C S R Q 0 x x Q0 1 0 0 Q0 1 0 1 0 1 1 0 1 1 1 1 Q=Q’ No change No change Reset Set Invalid S R Q Q S R C S R Q Q S R C
- 18. D FlipFlop D Q QCLK I/P O/P CLR
- 19. J-K Flip-Flops D = JQ’ + K’Q J Q QK
- 20. T Flip-Flops D = TQ’ + T’Q = T Q J Q QK T D Q Q T D = JQ’ + K’Q T Q Q
- 21. THANK U
Editor's Notes
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