Decoder and encoder design in VHDL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main function of a decoder in digital circuits?

A decoder takes a binary input and activates exactly one output line corresponding to that input, converting coded inputs into a one-hot output.

beginner

What does an encoder do in digital logic?

An encoder takes multiple input lines and converts the active input into a binary code on the output lines, essentially the reverse of a decoder.

intermediate

In VHDL, which statement is commonly used to implement a decoder?

The 'with-select' or 'case' statement is commonly used to implement a decoder by selecting output lines based on input values.

intermediate

How do you represent a 3-to-8 decoder in VHDL?

A 3-to-8 decoder has 3 input bits and 8 output lines. The output line corresponding to the binary value of the input is set to '1', others to '0'. This can be coded using a 'with-select' or 'case' statement.

advanced

What is a priority encoder and how does it differ from a simple encoder?

A priority encoder outputs the binary code of the highest priority active input when multiple inputs are active, unlike a simple encoder which assumes only one input is active.

What output does a 2-to-4 decoder produce when input is "10" (binary)?

A All outputs are low

B Output line 1 is high, others low

C Output line 3 is high, others low

D Output line 2 is high, others low

For input '10' (binary 2), the decoder activates output line 2 only.

Which VHDL construct is best for implementing an encoder?

A For loop

B Process with if-elsif statements

C Wait statement

D Generate statement

A process block with if-elsif statements is commonly used to check inputs and assign the encoded output.

What is the output of a priority encoder if inputs 3 and 5 are both active?

A Binary code of input 5

B Binary code of input 3

C All outputs high

D No output

The priority encoder outputs the code of the highest priority active input, here input 5.

How many outputs does a 4-to-16 decoder have?

A 4

B 8

C 16

D 32

A 4-to-16 decoder has 16 output lines, one for each possible 4-bit input combination.

Which statement best describes an encoder?

A Converts one-hot input to binary output

B Converts binary input to one-hot output

C Amplifies input signals

D Stores data temporarily

An encoder converts one-hot input signals into a binary code output.

Explain how a 3-to-8 decoder works and how you would implement it in VHDL.

Think about how each input combination activates a unique output.

Describe the difference between a simple encoder and a priority encoder.

Consider what happens when more than one input is active.