Sequential logic circuits

Sequential logic circuits Outline Sequential Circuit Models Asynchronous Synchronous Latches Flip-Flops CS 3402--Digital Logic Sequential logic circuits

Sequential logic circuits The main characteristic of combinational logic circuits is that their output values depend on their present input values. Sequential logic circuits differ from combinational logic circuits because they contain memory elements so that their output values depend on both present and past input values CS 3402--Digital Logic Sequential logic circuits

Sequential logic circuits Sequential circuits can be Asynchronous or synchronous. Asynchronous sequential circuits change their states and output values whenever a change in input values occurs. Synchronous sequential circuits change their states and output values at fixed points of time, i.e. clock signals. CS 3402--Digital Logic Sequential logic circuits

Sequential Circuit Models Universal model CS 3402--Digital Logic Sequential logic circuits

Sequential logic circuits Combinational circuit model Mealy machine model Moore machine model CS 3402--Digital Logic Sequential logic circuits

Sequential logic circuits Memory Devices Latches A latch is a memory element whose excitation signals control the state of the device. A latch has two stages set and reset. Set stage sets the output to 1. Reset stage set the output to 0. Flip-flops A flip-flop is a memory device that has clock signals control the state of the device. CS 3402--Digital Logic Sequential logic circuits

Sequential logic circuits Latch Flip-flop CS 3402--Digital Logic Sequential logic circuits

Sequential logic circuits Inverter Chains Ring oscillator CS 3402--Digital Logic Sequential logic circuits

Sequential logic circuits Latches RS Latch The RS latch is the basic memory element consists of two cross-coupled NOR gates. It has two input signals, S set signal and R reset signal. It also has two outputs Q and Q'; and two states, a set state when Q = 1 and a reset state when Q = 0 (Q' = 1) CS 3402--Digital Logic Sequential logic circuits

Sequential logic circuits 1 S R Q hold 1 0 reset 1 set unstable CS 3402--Digital Logic Sequential logic circuits

Sequential logic circuits 1 S R Q unstable 1 0 reset 1 set hold CS 3402--Digital Logic Sequential logic circuits

Sequential logic circuits Flip-Flops A flip-flop is a level-sensitive latch with a clock input. RS flip-flop Q+ = S +R'Q CS 3402--Digital Logic Sequential logic circuits

Sequential logic circuits T (Toggle) flip-flop D Q(t) Q(t+1) Hold 1 Toggle Q+ = TQ' + T'Q CS 3402--Digital Logic Sequential logic circuits

Master Slave Flip-Flops A master slave flip-flop consists of two latches and an inverter. Master-slave RS flip-flop CS 3402--Digital Logic Sequential logic circuits

Master-Slave JK Flip-Flops CS 3402--Digital Logic Sequential logic circuits