Implement a Function using 3×8 decoder (74LS138)

decoderdigital-logic

I need to implement the function below using 3×8 decoder (74LS138) and a minimum number of gates but I did not see 74LS138 before.

F = (A.B)'

I implement the function using a normal 3×8 decoder but I think it is not the best way to do that and I also need to use 74LS138.

My Solution:

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Do you have any idea about the solution?

Thanks in advance.

Best Answer

The function can implemented using only the 74LS138. Look at the data sheet (for example at http://www.alldatasheet.com/datasheet-pdf/pdf/51039/FAIRCHILD/74LS138.html , truth table and logic diagram on page 2). The pins are G1, G2A, G2B, A, B, C, Y0, ... Y7. Functionality it can be described as:

Y[7:0] = (!G1 || G2A || G2B) ? 8'hFF : ~(1'b1 << {C,B,A})

There are may possible solutions. Pick a Y pin and choose an input configuration that matches your function. For example: A and B pins will connect to the corresponding names from your function. Set G1 high, G2A low, and G2B low. If C is set to low, then F will be Y3. If C is set to high, then F will be Y7.

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