UPC EETAC Bachelor's Degree in Telecommunications Systems and in Network Engineering EEL


Comp_1bit plan B: behavioural single-file VHDL


Design of a 1-bit expandable comparator (Plan B)

1. Specifications

Design a 1-bit expandable comparator Comb_1bit using a single file based on a behavioural interpretation of the circuit's truth table, plan B.

 It has the symbol and truth table represented in Fig. 1. The truth table has been adapted from the standard 4-bit commercial comparator chip 74LS85, the datasheet of which can be used to figure out how to chain comparator components to obtain larger circuits of the same kind.

Truth table

Fig. 1. Symbol and truth table of a 1-bit comparator with extra signals for expandability.

In Fig. 2 there is a simple sketch that represents some values of the truth table in time. The timing diagram wll be used in section 4th for testing the circuit using a VHDL testbench. All the 32 combinations can be applied sequentially in time to complete the test.

Timing diagram
Fig. 2. Timing diagram where all the inputs are applied sequentially and hence, the output is predicted accordingly to the truth table.


2. Planning

The VHDL file will be named Comp_1bit.vhd and the project name and location will be: Comp_1bit_prj,  "L:\CSD\P3\Comp_1bit_B\(files...) "

Plan B schematic to be translated to VHDL.

Fig. 3. Schematic where the truth table is used as an artefact along with some buffers to interface ports. 


3. Developing the project using EDA tools

Plan B behavioural file from schematic above Comp_1bit.vhd

Run a EDA project to synthesise the circuit. Fig. 3 shows an example of RTL schematic.

Fig. 3. RTL view. The EDA  synthesiser has interpreted the circuit in this way.

Print and examine the technology view as well. Compare it with other realisations accordingly to plan A or plan C2. Is there any difference to highlight?

4. Testing and validating the design

 An example test bench Comp_1bit_tb.vhd. Run the VHDL simulation EDA tool to obtain and discuss the timing diagram.  Remember that from the testing point of view, even if you have designed the Adder_1bit using several plans, you can use all the time the same testbench.


Fig. 4. Example testbench showing results.


5. Report

Project report starting with the template sheets of paper, scanned figures, file listings, docx , pptx, or any other resources.


6. Prototyping

Use training boards and perform laboratory measurements to verify how the circuit works.