Introduction to Digital Systems By Dr. John Abraham UT-Panam

Digital systems n classified as – combinational n contain no memory n example will be a binary adder (input, output only) – sequential n requires memory to remember the present state to go to the next state n counter is an example n Computers are clocked sequential systems

Digital Integrated Circuits n Diode – is an electronic switch – Positive terminal p-type semiconductor (anode) – Neg n-type semiconductor material(cathod) – When v=0.6 is applied the switch closes – Anode Cathode

Transistors – Also an electronic switch – Transistor also acts like a logic device called an inverter - low input gives high output and high input gives a low output. – Transistors can therefore amplify signals – Two types of transistors, NPN AND PNP n NPN is used in digital circuits n transistor is a three-terminal device, base, emitter and collector. n Transistor is current controlled switch

Transistors – adequate current at the base will close the switch allowing a current to flow from the collector to the emitter. – Transistor has three modes of operation: cutoff, saturation, and active – when no current flows it is in the cutoff mode – When the base current flows such that the voltage across the base and the emitter is at least 0.6 V the switch closes. – If the base current increases there will be a constant voltage across the collector and the emitter. This is the On mode. – The active mode is between Off and On mode

transistor n Base collector emitter NPN TRANSISTOR

Light Emitting Diodes – Driven by low voltage and low current – LED will generate light when its cathode is sufficiently negative with respect to its anode. – A seven-segment LED display can be used to display decimal numbers from 0 to 9.

Transistor Transistor Logic-TTL n TTLs evolved from diodes and transistors – Power supplied required is +5V – TTLs outputs n open-collector, totem-pole, and tristate

MOS Transistors n Metal-Oxide Semiconductor – occupies less space and consume less power – used in highly integrated circuuits – MOS transistor operates as a voltage controlled resistance switch. – If resistance is very high switch is off. – If the resistance is very low it is on. – Two types - nMOS and pMOS

Complementary MOS (CMOS) n CMOS is fabricated by combining nMOS and pMOS together. – CMOS dissipates low power and offers short propagation delays. – CMOS provides high circuit density – high noise immunity

Integrated Circuits (Ics) n Device level design utilizes transistors to design circuits called gates. – One or more gates on a single silicon chip is an integrated circuit – small-scale integration (SSI) – medium scale integration (MSI) – Large-scale integration (LSI) – Very large scale integration (VLSI)

Basic logic operations – Not operation. Symbol is + n not gate is an INVERTER n a transistor acts an inverter – OR operation Or operation Or gate may be implemented using 2 diodes or transistors

AND – Variables A and B must be true to get a true output. Symbols are. And ^ And gate can be constructed with 2 diodes or transistors

Other operations n NOR operation – invert the result of an or operation n NAND – invert the output of an AND operation n XOR – produces a one if inputs are different, zero if inputs are the same. n XNOR – produces a one if inputs are the same.

Combinational logic design – A combinational circuit is designed using logic gates in which application of inputs generates the outputs at any time – examples are: adders, subtractors, decoders, encoders, multiplexers and demultiplexers. – The results are based on the truth table

Sequential Logic Design – Designed using logic gates and memory elements known as flip-flops. n A flip flop is a one bit memory. (a latch). n Output will depend upon present inputs and previous states stored in memory. n To distinguish different states a clock is used. n There are synchronous and asynchronous sequential circuits. Synchronous are regulated by a clock. In asynchronous, completion of one task automatically starts another, no clock is needed.

RAM n RAM is of two types – static ram - stores each bit in a flip-flop – dynamic ram - stores each bit as a charge in a capacitor. n Capacitors can store charges only for few ms. n So refreshing is needed.