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Chapter04 - Chapter 4 Gates and Circuits Chapter The...

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Unformatted text preview: Chapter 4: Gates and Circuits Chapter The following Boolean operations are easy to incorporate The into circuitry and can form the building blocks of many more sophisticated operations… more The NOT Operation (i.e., what’s the opposite of the operand’s value?) NOT 1 = 0 NOT 0 = 1 NOT 10101001 = 01010110 NOT 00001111 = 11110000 The AND Operation (i.e., are both operands “true”?) 1 AND 1 1 1 AND 0 0 0 AND 1 0 0 AND 0 0 10101001 AND AND 10011100 10011100 10001000 00001111 AND AND 10110101 10110101 00000101 The OR Operation (i.e., is either operand “true”?) 1 OR 1 1 1 OR 0 1 0 OR 1 1 0 OR 0 0 10101001 OR 10011100 10111101 00001111 OR 10110101 10111111 Chapter 4 Gates and Gates Circuits Circuits Page 1 Page More Boolean Operators More The NAND Operation (“NOT AND”) 1 NAND 1 0 1 0 0 NAND 0 NAND 1 NAND 0 1 1 1 10101001 NAND 10011100 01110111 00001111 NAND 10110101 11111010 The NOR Operation (“NOT OR”) 1 NOR 1 0 1 NOR 0 0 0 NOR 1 0 0 NOR 0 1 10101001 NOR 10011100 01000010 00001111 NOR 10110101 01000000 The XOR Operation (“Exclusive OR”, i.e, either but not both is “true”) 1 XOR 1 0 1 XOR 0 1 0 XOR 1 1 0 XOR 0 0 10101001 XOR 10011100 00110101 00001111 XOR 10110101 10111010 Chapter 4 Gates and Gates Circuits Circuits Page 2 Page Transistors Transistors Transistors are relatively inexpensive mechanisms Transistors for implementing the Boolean operators. for In addition to the input connection (the base) In base Transistors are connected to both a power source and a voltage dissipating ground. and Essentially, when the input voltage is high, an electric Essentially, path is formed within the transistor that causes the power source to be drained to ground. source When the input voltage is low, the path is not created, When so the power source is not drained. so Chapter 4 Gates and Gates Circuits Circuits Page 3 Page Using Transistors to Create Logic A NOT gate is essentially NOT ates Gatesa transistor all by G implemented by itself. itself. A NAND gate uses a slightly more complex NAND setup in which both inputs would have to both be high to force the power source to be grounded. grounded. Use the output of a NAND gate as the input Use to a NOT gate to produce an AND gate, to A NOR gate grounds the power NOR source if either or both of the inputs are high. are Use the output of a NOR gate as the Use input to a NOT gate to produce an OR gate.. gate.. Chapter 4 Gates and Gates Circuits Circuits Page 4 Page How to Use Logic Gates for ANDs and ORs are all well and good, but how can they be ANDs Arithmetic Arithmetic used to produce binary arithmetic? used Let’s start with simple one-bit addition (with a “carry” bit Let’s just in case someone tries to add 1 + 1!). just 0 0 1 1 + + + + 0 1 0 1 = = = = Sum Carry Bit Bit 0 0 1 0 1 0 0 1 0 0 1 1 XOR XOR XOR XOR 0 1 0 1 = = = = Result 0 1 1 0 0 0 1 1 AND AND AND AND 0 1 0 1 = = = = Result 0 0 0 1 Notice that the sum bit always yields the same result as Notice the XOR operation, and the carry bit always yields the same result as the AND operation! same By combining the right circuitry, then, multiple-bit By addition can be implemented, as well as the other arithmetic operations. arithmetic Chapter 4 Gates and Gates Circuits Circuits Page 5 Page Memory Circuitry Memory With voltages constantly on the move, how can a piece of With circuitry be used to retain a piece of information? retain In the S-R latch, as long as the S and In R inputs remain at one, the value of the Q output will never change, i.e., the circuit serves as memory! the To set the stored value to one, merely set the S input to To set zero (for just an instant!) while leaving the R input at one. zero To set the stored value to zero, merely set the R input to To zero (for just an instant!) while leaving the S input at one. zero Question: What goes wrong if both Question: both inputs are set to zero simultaneously? simultaneously? Chapter 4 Gates and Gates Circuits Circuits Page 6 Page Integrated Circuits Integrated How does all of that elaborate circuitry get placed on the How tiny microchips used in modern computers? tiny A clean silicon clean wafer is oxidized to produce a thin layer of silicon dioxide, which is then coated with a radiation-sensitive film. film. The areas The covered by silicon dioxide remain positively charged. The silicon dioxide is removed is The wafer is The masked by lithography to expose it selectively to ultraviolet light, which causes the film layer to become dissolvable. dissolvable. The wafer is The oxidized again. oxidized Light-exposed Light-exposed areas are dissolved, exposing parts of the silicon dioxide layer, which are removed by an etching process. etching An opening is An etched down to the positively charged silicon using a reverse mask. reverse The remaining film is The removed in a liquid bath. The areas of silicon exposed by the etching process are negatively charged by exposure to either arsenic or phosphorus vapor at high temperatures temperatures Another oxidation Another cycle forms a thin layer of silicon dioxide on the positively charged region of the wafer. region Windows are Windows etched in the negatively charged silicon areas in preparation for metal deposits. metal Chapter 4 Gates and Gates Circuits Circuits Page 7 Page ...
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