Electronic – How to make a 7 to 3 priority encoder

encoder

I'm trying to make a 7 to 3 priority encoder for a circuit diagram for a class. The problem is that we have to take in a 7 bit number and output a 3 bit answer representing the maximum number of consecutive ones in the input.

Example:

0011100 = 011 (3)
1111111 = 111 (7)
0000000 = 000 (0)

The issue at the moment is that regardless of the input, it is always outputting 111 (7).

I have circuits for handling every possible combination of consecutive ones in the input, and am then piping that into a 7 to 3 priority encoder, but for some reason the encoder is not working the way it should. What am I doing wrong? Or is there a better way that I should be doing this?

Original Circuit diagram:

circuit

Attempt 2:

another try

Best Answer

  • on your diagram, did you confuse PE2 and PE3 outputs?
  • for 1111111, PE1..PE7 are all going to be 1, so the last diagram would be wrong.
  • NAND is a much easier function to play with for consecutive 1s.

here's how I would do it:

LEN1 = AND(OR(a,b,c,d,e,f,g),NOR(NAND(a,b),NAND(b,c),NAND(c,d),NAND(d,e),NAND(e,f),NAND(f,g)))
LEN2 = AND(OR(NAND(a,b),NAND(b,c),NAND(c,d),NAND(d,e),NAND(e,f),NAND(f,g)),
           NOR(NAND(a,b,c),NAND(b,c,d),NAND(c,d,e),NAND(d,e,f),NAND(e,f,g)))
LEN3 = AND(OR(NAND(a,b,c),NAND(b,c,d),NAND(c,d,e),NAND(d,e,f),NAND(e,f,g)),
           NOR(NAND(a,b,c,d),NAND(b,c,d,e),NAND(c,d,e,f),NAND(d,e,f,g)))
LEN4 = AND(OR(NAND(a,b,c,d),NAND(b,c,d,e),NAND(c,d,e,f),NAND(d,e,f,g)),
           NOR(NAND(a,b,c,d,e),NAND(b,c,d,e,f),NAND(c,d,e,f,g)))
LEN5 = AND(OR(NAND(a,b,c,d,e),NAND(b,c,d,e,f),NAND(c,d,e,f,g)),
           NOR(NAND(a,b,c,d,e,f),NAND(b,c,d,e,f,g))
LEN6 = AND(OR(NAND(a,b,c,d,e,f),NAND(b,c,d,e,f,g)),
           NOT(NAND(a,b,c,d,e,f,g)))
LEN7 = NOT(NAND(a,b,c,d,e,f,g))

ANS1 = OR(LEN1,LEN3,LEN5,LEN7)
ANS2 = OR(LEN2,LEN3,LEN6,LEN7)
ANS3 = OR(LEN4,LEN5,LEN6,LEN7)
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