Digital compass (cont’d)

• Detecting a change in compass direction

  • 4 bits change from 0001 to 0011 to 0010 to 0110 to 0100 …
  • always alternating between one bit on and two bits on

• Parity tree can detect difference between one and two bits being asserted

  • XOR tree of four bits (one TTL SSI package)
  • output must change at least once for every change in orientation

N

E

S

W

e.g., NE -> E -> SE

1100 -> 0100 -> 01100 -> 1 -> 0

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