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


Dual_MUX_4 plan B: Behavioural single-file VHDL (flat)


Type 74HCT153 chip

1. Specifications

Design a classic chip like the 74HCT153 using behavioral approach plan B.


Fig. 1. a) Case and pin naming and enumeration of a classic 74HCT153 chip. We will interpret it as shown in b). Vector data select S(1..0) may be represented in binary radix-2 or as an unsigned decinmal.

Symbol name is Dual_MUX_4 meaning that data input channels and output are dual, they have two-bit size Ch0 --> Ch0(1..0).

In other applications we may have other expanded devices like Octuple_MUX_8, meaning that Channel size is 8-bit for instance Ch0(7..0), and select channel is S(2..0) to be able to switch 8 data channels .

Enable is active-low (E_L) meaning that the chip operates when E_L = '0'.

b)symbol and truth table
truth table
- How many maxterns does Y(1) have?
- How many minterns does Y(0) have?

Draw the sketch of a timing diagram to demonstrate how the circuit works for several input stimulus.

Timing diagram
Fig. 2. Example timing diagram.


2. Planning

The idea now is the interpretation of the circuit's truth table as a flow chart, generating several possibilities. 

Version 1a) Version 3c)
Version 2b)
Fig. 3. Plan B schematics and flowcharts.  Several ways of interpreting the same truth table.

 Project and file locations (three plans have to be placed in three folders):





3. Development

Three VHDL translations of Fig. 3: plan B a) Dual_MUX_4.vhd; plan B b) Dual_MUX_4.vhd; plan B c) Dual_MUX_4.vhd

This is the entity description to CSD input and output naming.

Fig. 4. Entity definition that is the same whatever the plan.

Start an EDA synthesis project and examine results.

Several schematics are possible, depending on the flowcharts or truth table translations.  


Fig. 5a. RTL schematic of Dual_MUX_4 using Fig. 3a. This is the way in which the EDA synthesiser has translated the flowchart into an electronic circuit. Comparators, multiplexers and logic gates are used.



Fig. 5b. RTL schematic of Dual_MUX_4 using Fig. 3b.


Fig. 5c. RTL schematic of Dual_MUX_4 using Fig. 3c.

In the end, the same final circuit (technology view) will be placed and routed into the PLD accordingly to its technology (sPLD, CPLD  or FPGA) and manufacturer (Lattice, Intel, Xilinx, etc.).

Tech view
Fig. 6. Technology view.


4. Testing

In Fig. 7 is represented the testbench schematic to stimulate the unit under test (UUT) with input vectors. Generate the skeleton of the testbench file and add stimulus translating to VHDL input activity from the time diagram in Fig. 2. This is an example of a simple test bench Dual_MUX_4_tb.vhd where the inputs have been stimulated with logic values.

test bench fixture Fig. 7. Testbench fixture to verify that our circuit works as expected. 


Start the VHDL simulator project, Dual_MUX_4_functional_sim,  and run the test. Verify applying sufficient test vector that the device works as expected (verify how the information of each channel is selected). Print timing diagram screen and add comments on the signals to show how the device works.

wave result
Fig. 8. Waveform results from the logic analyser. 


5. Report



6. Prototyping