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**EET 1131 Unit 6 Exclusive-OR and Exclusive-NOR Gates**

Read Kleitz, Chapter 6. Do Unit 6 e-Lesson. Homework #6 and Lab #6 due next week. Quiz next week. Do Quiz #5. Fire up Quartus & Multisim. Difference in everyday English between inclusive & exclusive or: “Do you have any ice cream or cake in the kitchen?” vs. “You can have ice cream or cake: which do you want?”

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A X A X The XOR Gate B B The XOR gate produces a HIGH output only when the inputs are at opposite logic levels. The truth table is 0 0 0 1 1 0 1 1 1 -XOR gate has only two inputs. The XOR operation is written as X = AB + AB. Alternatively, it can be written with a circled plus sign between the variables as X = A + B.

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**The XOR Gate Example waveforms: A B X**

Notice that the XOR gate will produce a HIGH only when exactly one input is HIGH.

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Fixed Function Logic Some common gate configurations are shown.

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**The XNOR operation can be written as X = AB + AB or as X = A + B.**

The XNOR Gate B B The XNOR gate produces a HIGH output only when the inputs are at the same logic level. The truth table is 0 0 0 1 1 0 1 1 1 -XNOR gate has only two inputs. The XNOR operation can be written as X = AB + AB or as X = A + B.

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**The XNOR Gate Example waveforms: A B X**

Notice that the XNOR gate will produce a HIGH when both inputs are the same. This makes it useful for comparison functions.

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**Applications of XOR and XNOR Gates**

Three common applications: Comparators Controlled inverters Parity generation and checking

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**Convention for Multi-Bit Strings**

When dealing with multi-bit binary strings, we use subscripts to refer to the individual bits in the string. The least significant bit (LSB) always gets the smallest subscript, which may be either 1 or 0. Example: In a four-bit string A, the bits may be labeled either A4A3A2A1 or A3A2A1A0 8

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**Application #1: Comparator**

A comparator compares two string of bits to see whether they are equal to each other: Example: if string A = 0101 and string B = 0100, then A≠ B. Next slide shows how to build a 4-bit comparator from XNOR gates.

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**Comparator Circuit (Book’s Fig. 6-14)**

Look again at XNOR truth-table, particularly 1st and 4th rows.

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**Application #2: Controlled Inverter**

A controlled inverter takes an input string and, depending on the logic level on a control line, either Leaves the string unchanged or Inverts each bit in the string Next slide shows how to build an 8-bit controlled inverter from XOR gates.

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**Controlled Inverter (Book’s Fig. 6-15)**

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**Example Solution Application #3: Parity Checking**

Parity checking is a method of error detection for simple transmission errors involving one bit. A parity bit is an “extra” bit attached to a group of bits to force the total number of 1’s to be either even (even parity) or odd (odd parity). Example The ASCII character for “a” is and for “A” is What is the correct bit to append to make both of these have odd parity? Recall ASCII table on p. 19. Solution The ASCII “a” has an odd number of bits that are equal to 1; therefore the parity bit is 0. The ASCII “A” has an even number of bits that are equal to 1; therefore the parity bit is 1.

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Parity Generators To implement parity checking, we need circuitry on the sending end that generates the parity bit for each group of bits being sent. This circuitry is called a parity generator. Next slide shows how to build 4-bit even or odd parity generators.

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**Parity Generators (Book’s Fig. 6-9)**

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Parity Checkers On the receiving end, we need circuitry that checks the data bits and parity bit as they’re received to decide whether an error has occurred during transmission. This circuitry is called a parity checker. Next slide shows how to build a 4-bit-plus-parity even parity checker.

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**Parity Checker (Book’s Fig. 6-11)**

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**A Parity Generator/Checker Chip**

74280 Nine-bit Parity Generator/Checker Most chips we’ve studied have been SSI (small-scale integration) chips containing fewer than 10 gates that are not connected to each other. The is an MSI (medium-scale integration chip). Instead of containing a few disconnected gates, it contains about 45 gates connected internally on the chip to perform a specific function. -Follow links for pin-out and logic symbol. -Mention presence of and other macrofunctions in Quartus

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**Parity System (Book’s Fig. 6-13)**

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**Parity Generator/Checker Chip**

The can be used to generate a parity bit or to check an incoming data stream for even or odd parity. Checker: The can test codes with up to 9 bits. The even output will be HIGH if the data lines have even parity; otherwise it will be LOW. Likewise, the odd output will be HIGH if the data lines have odd parity; otherwise it will be LOW. Data inputs S Even S Odd Generator: To generate even parity, the parity bit is taken from the odd parity output. To generate odd parity, the output is taken from the even parity output. 74280

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**Printing from Our Oscilloscopes**

You can print the oscilloscope screen by pressing the PRINT button. There’s a delay of about 40 seconds before the page will print, so be patient. Only one oscilloscope can print at a time, or else the printer gets confused and prints hundreds of pages. Please shout “Printing!” before you press the PRINT button, and make sure that you don’t print while someone else is waiting for their page to print.

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