Video summary

Encoder in Digital Electronics | Working, Application and Logic Circuit of Encoder

Main summary

Key takeaways

Educational

Main ideas / lessons

  • Binary encoder definition: A binary encoder is a combinational circuit that converts one-hot input information (where only one input is high) into a compact binary code at the output.
  • Encoder vs. decoder: The encoder operation is described as the reverse of a decoder:
    • Decoder: N inputs → up to (2^N) outputs; for a given input, one output goes high.
    • Encoder: up to (2^N) inputs → N outputs; for a given one-hot input, the encoded N-bit output corresponds to which input is high.
  • Core assumption for “normal” (non-priority) encoder: At any time, at most one input is high.
  • Logic circuit construction: Output bits are derived using OR gate expressions from the truth table (e.g., each output bit is the OR of the inputs that correspond to output bit = 1).

Methodology / key design steps (as presented)

A) Building a 4-to-2 encoder (example)

  • Given: Inputs (D0, D1, D2, D3) and outputs (A, B) (so (N=2), inputs = (2^N = 4)).
  • Truth table mapping (one input high → encoded output):
    • If (D0 = 1) → (AB = 00)
    • If (D1 = 1) → (AB = 01)
    • If (D2 = 1) → (AB = 10)
    • If (D3 = 1) → (AB = 11)
  • Derive Boolean expressions from the truth table:
    • (A = D2 + D3) (A is high when (D2) or (D3) is high)
    • (B = D1 + D3) (B is high when (D1) or (D3) is high)
  • Implement using OR gates:
    • Compute (A) using OR gates for inputs ((D2, D3))
    • Compute (B) using OR gates for inputs ((D1, D3))

B) Building an 8-to-3 encoder (example)

  • Given: Inputs (D0) to (D7) and outputs (A, B, C) (so (N=3), inputs = 8).
  • Truth table concept (one input high → 3-bit code):
    • (D0 \rightarrow 000)
    • (D5 \rightarrow 101)
    • (D7 \rightarrow 111)
    • (Other input-to-code correspondences follow the same pattern.)
  • Derive Boolean expressions (which inputs make each output bit = 1):
    • (A = D4 + D5 + D6 + D7)
    • (B = D2 + D3 + D6 + D7)
    • (C = D1 + D3 + D5 + D7)
  • Implement using OR gates for each output bit expression.

C) Practical application use-case (key switches → fewer I/O pins)

  • Scenario: Connect many input devices (e.g., 8 switches) to a controller/processor.
  • Each switch represents an action/symbol:
    • Pressed: the corresponding input pin is Vcc (1)
    • Not pressed: pin is 0
  • Without encoder: would require 8 I/O pins.
  • With an 8-to-3 encoder:
    • Feed the 8 switch states into the encoder
    • Send the encoder’s 3-bit output to the controller
    • Controller decodes which switch is active
  • Benefit: reduces required controller I/O from 8 pins to 3 pins.

D) Using encoders for number conversions

  • 8-to-3 encoder can support octal-to-binary conversion:
    • Each input corresponds to an octal digit
    • Output gives the equivalent binary code
  • Decimal to BCD encoder:
    • Given: 10 inputs (decimal digits 0–9) and 4 outputs (BCD digits)
    • Each active input produces the 4-bit BCD code for that decimal digit
    • Outputs are expressed as OR-based Boolean functions from the truth table, e.g.:
      • (A = D8 + D9)
      • (B = D4 + D5 + D6 + D7)
      • (C = D2 + D3 + D6 + D7)
      • (D = D1 + D3 + D5 + D7 + D9)
    • Implement each output bit using OR gates.

E) Limitations of a normal encoder and how a priority encoder fixes them

  1. Normal encoder limitation #1: “all inputs 0” ambiguity

    • If all inputs are 0, the normal encoded outputs can become the same as a valid one-input case (example discussed: output becomes 000 both when (D0=1) and when all inputs are 0, due to lack of connection in the OR expressions for that case).
    • Fix: Add a validity output (V) (extra output) that indicates whether any input is high.
      • If all inputs are 0 → (V=0)
      • If one or more inputs is high → (V=1)

  2. Normal encoder limitation #2: multiple inputs high causes invalid code

    • If more than one input is high, OR-based logic can produce a code that does not correspond to any single input.
    • Fix: Use a priority encoder
      • When multiple inputs are high, the input with the higher subscript (higher priority) determines the output.
    • Also include the validity output (V):
      • (V=0) when all inputs are 0
      • (V=1) when one or more inputs are high

Speakers / sources featured

  • Channel/Source: ALL ABOUT ELECTRONICS (speaker implied by narration of the channel)
  • No other specific individuals or external sources are explicitly named in the subtitles.

Original video