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Magnitude Comparator Dr. Ahmed Telba
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Integrated Circuits An integrated circuit is a piece (also called a chip) of silicon on which multiple gates or transistors have been embedded These silicon pieces are mounted on a plastic or ceramic package with pins along the edges that can be soldered onto circuit boards or inserted into appropriate sockets
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Integrated Circuits SSI, MSI, LSI: They perform small tasks such as addition of few bits. small memories, small processors VLSI Tasks: - Large memory - Complex microprocessors, CPUs
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An SSI chip contains independent NAND gates
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Examples of Combinational Circuits
a) Decoders b) Encoders c) Multiplexers d) Demultiplexers E) Magnitude Comparator
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Decoder Accepts a value and decodes it Consists of:
Output corresponds to value of n inputs Consists of: Inputs (n) Outputs (2n , numbered from 0 2n - 1) Selectors / Enable (active high or active low)
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The truth table of 2-to-4 Decoder
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2-to-4 Decoder
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2-to-4 Decoder
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The truth table of 3-to-8 Decoder
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3-to-8 Decoder
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3-to-8 Decoder with Enable
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2-to-4 Decoder: NAND implementation
Decoder is enabled when E=0 and an output is active if it is 0
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2-4 Decoder with 2-input and Enable
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Decoder Expansion Decoder expansion
Combine two or more small decoders with enable inputs to form a larger decoder 3-to-8-line decoder constructed from two 2-to-4-line decoders The MSB is connected to the enable inputs if A2=0, upper is enabled; if A2=1, lower is enabled.
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Decoder Expansion
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Combining two 2-4 decoders to form one 3-8 decoder using enable switch
The highest bit is used for the enables
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Combinational Circuit Design with Decoders
Combinational circuit implementation with decoders A decoder provide 2n minterms of n input variables Since any Boolean function can be expressed as a sum of minterms, one can use a decoder and external OR gates to implement any combinational function.
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Combinational Circuit Design with Decoders
Example Realize F (X,Y,Z) = Σ (1, 4, 7) with a decoder:
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1 bit Digital comparator
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4-bit Magnitude Comparator
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BCD to 7-Segment Display Decoder
As we saw in the previous tutorial, a Digital Decoder IC, is a device which converts one digital format into another and one of the most commonly used device for doing this is called the Binary Coded Decimal (BCD) to 7-Segment Display Decoder. 7-segment LED (Light Emitting Diode) or LCD (Liquid Crystal Display) type displays, provide a very convenient way of displaying information or digital data in the form of numbers, letters or even alpha-numerical characters. Typically 7-segment displays consist of seven individual coloured LED’s (called the segments), within one single display package. In order to produce the required numbers or HEX characters from 0 to 9 and A to F respectively, on the display the correct combination of LED segments need to be illuminated and BCD to 7-segment Display Decoders such as the 74LS47 do just that. A standard 7-segment LED display generally has 8 input connections, one for each LED segment and one that acts as a common terminal or connection for all the internal display segments. Some single displays have also have an additional input pin to display a decimal point in their lower right or left hand corner. In electronics there are two important types of 7-segment LED digital display. 1. The Common Cathode Display (CCD) – In the common cathode display, all the cathode connections of the LED’s are joined together to logic “0″ or ground. The individual segments are illuminated by application of a “HIGH”, logic “1″ signal to the individual Anode terminals. 2. The Common Anode Display (CAD) – In the common anode display, all the anode connections of the LED’s are joined together to logic “1″ and the individual segments are illuminated by connecting the individual Cathode terminals to a “LOW”, logic “0″ signal.
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BCD to 7-Segment Display Decoder
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74LS47 for common-anode LED types
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Truth Table for 7 Segment display
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4-to-2 Bit Binary Encoder
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Priority Encoder The Priority Encoder solves the problems mentioned above by allocating a priority level to each input. The priority encoders output corresponds to the currently active input which has the highest priority. So when an input with a higher priority is present, all other inputs with a lower priority will be ignored. The priority encoder comes in many different forms with an example of an 8-input priority encoder along with its truth table shown below.
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3-8 Binary Decoder with Enable
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2-to-4 Line NAND Binary Decoder Logic Diagram
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De multiplexer
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Magnitude comparator
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