Chapter 3 - Digital logic structures. This chapter presents the following content: Electronic ones and zeros, transistors, CMOS transistors, inverter gate, NOR gate, NOR gate - Operation, OR gate, NAND & AND Gates, logic gates & symbols, De Morgan’s Law,.and other contents. | Chapter 3 Digital Logic Structures Transistors Logic gates & Boolean logic Combinational logic Storage Elements Memory Transistors An electronic switch like light switch or faucet switches between insulator and conductor switch is closed (conducts) between A & B depending on the voltage on G (gate) Open switch: no current flows Closed switch: current flows 3 - A B G A B G N Closed if gate is “on” Open if gate is “off” P Open if gate is “on” Closed if gate is “off” A B G CMOS Transistors CMOS = Complementary Metal-Oxide Semiconductor Standard type for digital applications Two types: N (negative) and P (positive) N and P transistors operate in inverse modes 3 - v 0 v Out In Inverter Gate When the input is on (In = high voltage), the P-type transistor is open and the N-type is closed, so the output is off (Out = low voltage). Vice-versa: when the Input is off (In = low voltage), the output is connected to the high voltage. P N 3 - NOR Gate C A B v 0 v 0 v P N P N 3 - NOR Gate - Operation v 0 v 0 v P N P 0 v 0 v v v 0 v 0 v N P N v v 0 v N 0 v 0 v P N P N v v 0 v 0 v P 3 - OR Gate D A B C = a NOR gate followed by an inverter 3 - NAND & AND Gates A B C D 3 - Logic Gates & Symbols Note: gates can have more than two inputs. Inverter (=NOT) inverts 0 to 1 and 1 to 0 AND gate c is 1 only if both a and b are 1 a b c OR gate c is 1 if either a or b is 1 a b c NAND gate c is 0 only if both a and b are 1 a b c NOR gate c is 0 if either a or b are 1 a b c 3 - De Morgan’s Law A and B = not((not A) or (not B)) A or B = not((not A) and (not B)) Completeness AND, OR and NOT are sufficient to represent any truth table. Actually: AND & NOT, or OR & NOT are separately sufficient. So: all we need are NOR or NAND gates. = = 3 - Representation of Logic Functions A logic function can be represented as a truth table a logic expression a logic circuit Example a b c d f 0 0 0 0 0 0 0 0 1 0 0 0 1 0 1 0 0 1 1 1 0 1 0 0 0 0 | Chapter 3 Digital Logic Structures Transistors Logic gates & Boolean logic Combinational logic Storage Elements Memory Transistors An electronic switch like light switch or faucet switches between insulator and conductor switch is closed (conducts) between A & B depending on the voltage on G (gate) Open switch: no current flows Closed switch: current flows 3 - A B G A B G N Closed if gate is “on” Open if gate is “off” P Open if gate is “on” Closed if gate is “off” A B G CMOS Transistors CMOS = Complementary Metal-Oxide Semiconductor Standard type for digital applications Two types: N (negative) and P (positive) N and P transistors operate in inverse modes 3 - v 0 v Out In Inverter Gate When the input is on (In = high voltage), the P-type transistor is open and the N-type is closed, so the output is off (Out = low voltage). Vice-versa: when the Input is off (In = low voltage), the output is connected to the high voltage. P N 3 - NOR Gate C A B v 0 v 0 v P N P N
