digital logic circuit
TRANSCRIPT
Digital Logic Circuits
By : Tamsil Shamsi
Half Adder
Logic Diagram
Truth Table
A half adder adds two one-bit binary numbers A and B. It has two outputs, S and C.
The simplest half-adder design, pictured on the right, incorporates an XOR gate for S and an AND gate for C. Half adders cannot be used compositely, given their incapacity for a carry-in bit.
Full AdderA full adder adds binary numbers and accounts for values carried in as well as out. A one-bit full adder adds three one-bit numbers, often written as A, B, and Cin; A and B are the operands, and Cin is a bit carried in.
A full adder can be constructed from two half adders by connecting A and B to the input of one half adder, connecting the sum from that to an input to the second adder, connecting Cin to the other input and OR the two carry outputs
Logic Diagram
Truth Table
SR Flip-Flop
Graphic Symbol
Truth Table
A SR flip-flop has three inputs, S (for set), R (for reset) and C (for clock). It has an output Q.
The undefined condition makes the SR flip-flop difficult to manage and therefore it is seldom used in practice.
D Flip-Flop
Graphic Symbol
Truth Table
The D flip-flop is a slight modification of the SR flip-flop by inserting an inverter between S and R and assigning the symbol D to the single input.
If D=1, the output goes to the state 1, and if D=0, the output of the flip flop goes to the 0 state.
JK Flip-Flop
Graphic Symbol
Truth Table
Inputs J and K behave like inputs S and R. When inputs J and K are both equal to 1, a clock transition switches the output of the flip-flop to their complement state.
T Flip-Flop
Truth Table
Graphic Symbol
The T flip-flop is obtained from a JK flip-flop when inputs J and K are connected to provide a single input designated by T.
The flip-flop thus has only two conditions.
Excitation Tables During the design of circuits, we need a table that lists the required input combinations for a given change of state. Such table is called a flip flop excitation table.
Sequential Circuits• A sequential circuit
is an interconnection of flip-flops and gates.
Example of a Sequential Circuit
Ax
Bx
Ax+Bx
A’x
x’
A+B (A+B).x
A=Ax+Bx, B=A’x
y=(A+B).x
State Table