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Enable input

Also called: enable, enable line, enable signal, EN input, decoder enable

A control input that switches a block on or off. When an enable is inactive, a decoder holds all of its outputs at 0, whatever its other inputs say.

An enable input is a master on/off switch for a circuit block. On a decoder, the enable E is ANDed into every output:

  • Y0 = , Y1 = , Y2 = , Y3 =

With E = 1, the decoder works normally and exactly one output is 1. With E = 0, every AND gate has a 0 input, so all outputs are 0, no matter what A and B are.

That small addition unlocks two tricks:

  • DEMUX: feed a data signal into E. The selected output now copies the data, and the decoder is a demultiplexer.
  • Expansion: drive the enables of several small decoders from other address bits, so only one of them is active. That's how decoder expansion builds a 4-to-16 decoder from 2-to-4s.

Many real chips make the enable active low, so the block is on when the pin is 0. The same idea appears all over the course: a memory's chip select, a register's load enable and a counter's count enable are all enables.

EABY0Y1Y2Y3

Worked examples

Example

Enable off, enable on

The 2-to-4 decoder above has inputs A = 0, B = 1.

  1. 1.

    E = 0: Y1 = = 0 · 1 · 1 = 0. The other outputs are 0 too. Nothing is active.

  2. 2.

    E = 1: Y1 = 1 · 1 · 1 = 1, and Y0, Y2, Y3 are 0. Normal decoding.

  3. 3.

    So A and B choose which output; E decides whether any output is on.

Example

Using E as a data input

Keep A = 1, B = 0 fixed and let E carry a data stream 1, 0, 1, 1.

  1. 1.

    Only Y2 = can be 1, and it equals E.

  2. 2.

    Y2 follows the stream: 1, 0, 1, 1. Y0, Y1, Y3 stay 0.

  3. 3.

    That is a demultiplexer routing the data to output 2.

Common mistakes

  • Thinking E = 0 still leaves one output at 1. With the enable off, every output is 0.

  • Treating E as one of the code bits. It doesn't change which output is chosen; it only allows or blocks it.

  • Ignoring the bar on an active-low enable. A pin labelled with a bar (or a bubble) enables the block when it is 0.

Practice Enable input

Interactive questions with instant feedback and a worked solution for every wrong answer.

Combinational Logic lesson full course

Latches and Flip-Flops lesson full course

Counters lesson full course

Practice exams

Related terms

Learn it step by step

Enable input is taught in Combinational Logic and Memory.

Combinational Logic →Memory →

All terms