Electronic – Logic to decrement by one

digital-logic

I am looking for some way using discrete logic (TTL) to decrement an 8-bit value by one.

Basically I want to present it with an 8 bit bus with a binary number on it, and have it give an output of the input minus one on its output:

eg:

...I3 I2 I1 I0 | ...Q3 Q2 Q1 Q0
   0  0  0  0  |    1  1  1  1
   0  0  0  1  |    0  0  0  0
   0  0  1  0  |    0  0  0  1
   0  0  1  1  |    0  0  1  0
....
   1  1  1  1  |    1  1  1  0

I have been googling and drawn a blank for a chip that can do it, and I have been racking my brains of the best way to do it using just normal gates.

Does anyone either know of a chip for this, or have some ideas on how I could construct it from discrete gates?

Best Answer

Have you considered adders? You can subtract with an adder.

You just need to add -1 to the input value. A pair of 7483 4-bit full adders should handle your 8-bit input. Apply the input X to the 'A' inputs, and apply the binary representation of -1 to the 'B' inputs. Fortunately, the 2's complement representation of -1 is all ones; this means that if you just tie the 'B' inputs all high, you're done. What comes out should be X-1.

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