L2.1 Lecture 2

L2.2: Designing standard logic circuits using VHDL. Multiplexers, demultiplexers [P2] Flat (single VHDL file) plans: structural plan A / behavioural plan B,

L2.3 [3/3]

1.7.2.2. Design examples (our list of design strategies)

Alternative ways to invent circuit architectures in VHDL:

Plan A) Structural (flat) - one VHDL file using equations

Plan B) Behavioural (flat) - one VHDL file using the truth table or algorithm

Plan C1) Structural (hierarchical) - describing a hierarchical structure in a single VHDL file (complicated/not recommended).

Plan C2) Structural (hierarchical) - several VHDL files using COMPONENTS, SIGNALS and logic equations. This modular approach is the best at CSD introductory level to design large circuits. Introduced in next P3. 

 

For example, MUX_8 device is developed in Lab2:

- Plan A. Multiplexer MUX_8 using equations. In this example equations are obtained using minilog tool.

- Plan B. Multiplexer MUX_8 using the truth table, algorithm or any other high-level interpretation.

1.7.2.3. MUX expansion circuits

Expanding circuits will be solved using plan C2. For instance, how to build a MUX_16 using MUX_4 components?

1.7.2.4. Commercial chips

74HCT151, 74HCT152, 74HCT153, etc. 

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1.7.3. De-multiplexers or data distributor

What is the application of a DeMUX? The idea is to send date from a single source to different destinations.

Fig. 1. Typical application oof data distributors.

1.7.3.1. DeMUX_4, DeMUX_8, etc.

What is the symbol and circuit's truth table of a DeMUX_16?

Fig. 1. DeMUX_8 symbol and truth table

1.7.3.2. Design examples using plan A and plan B

For instance DeMUX_8 project is proposed as assignments D1.6 and DeMUX_16 as D1.7.

 

1.7.3.3. DeMUX expansion circuits (plan C2)

For instance, how to invent a DeMUX_32 using small chips of the same kind?

- Draw the truth table of our DeMUX_32 symbol.

- Propose its internal architecture using DeMUX_8 components and other circuits if necessary.

 

1.7.3.4. Commercial chips

Chip 74HCT138 can be used as DeMUX_8 or as Dec_3_8. Infer how to connect it studying its datasheet.