A Nx2^N decoder circuit has N control signals and 2^N output signals:

The (one) output which is equal to 1 will be used to activate a circuit (usually a memory circuit)

Example 1:

The (one) output which is equal to 1 will be used to activate a circuit (usually a memory circuit)

Example 2:

The (one) output which is equal to 1 will be used to activate a circuit (usually a memory circuit)

Functionality of the 2x4 decoder:

We start with the input (control) signals and output signals:

Make all combinations of the control signals:

This AND gate will make o0=1 if and only if c1=0, c0=0:

This AND gate will make o1=1 if and only if c1=0, c0=1:

This AND gate will make o2=1 if and only if c1=1, c0=0:

This AND gate will make o3=1 if and only if c1=1, c0=1:

Sample operation:

DEMO: /home/cs355001/demo/circuits/decoder

Notice that the decoder will always have ONE output equal to 1:

We sometimes, we need to option of no activation (i.e.: all outputs = 0)

Decoder with the option of no activation:

When Activate = 0, all outputs = 0
When Activate = 1, exactly 1 output = 1 (others = 0)