74154 4 line to 16 line decoder

  • Thread starter ravenprp
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  • #1
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Hi.. my HW question reads:

Design a circuitboard based on a 74154 4 line to 16 line decoder that will output a HIGH henever the 4 bit binary is greater than 12 and output a low when it is less than 12 or equal to 12.

http://pdf1.alldatasheet.com/datasheet-pdf/view/7826/NSC/74154.html

I have the IC drawn on my paper, and what I have done is for the inputs from 13-15, I have it going to an NAND gate.

so if one of those inputs is an active LOW input, then once gates 13, 14, 15 go through the NAND with the values 0, 1, 1, (respectively), it will output a "1" or "HIGH"... Now, for gates 0 - 12... I'm using a NOR gate.

Now, I connect the NAND gate and the NOR gate into a XOR gate.


Brief recap: gates 13-15 go into a NAND. That output goes into one input for an XOR. Then gates 0-12 go into a NOR. Then that output goes into one input for the same XOR as above.
 
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Answers and Replies

  • #2
berkeman
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I'm not sure that I understand your solution. The outputs of the 4:16 demux represent the count of the input lines, right? So if you want to output a 0 when the input count is 12 or more, what can you put on lines 12-15 to output a 0 if any input is a 1, and output a 0 otherwise?
 

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