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

Chapter 2 problems

      - D1.9 -

4-bit (nibble) shifter operator

B

Products

D1.8

D1.10

1. Specifications

In digital systems it is often necessary to have circuits that can shift the bits of a vector by one or more bit positions to the left or right.

Design a circuit that can shift a 4-bit vector W(3..0), also called nibble, one bit position to the right when a control signal SR = '1', and one bit position to the left when SL = '1'. When the control signals are not active SR = '0', SL = '0', the output is a copy of the vector W. We don't care about the output values if by any chance both control signals are activated SR = '1', SL = '1'; assuming that this condition will never occur. The symbol and operations of this device are represented in Fig. 1.

Example shifting operations

Fig.1. Symbol of the Shifter_4bit.

Complete the circuit's truth table sketched in Fig. 2.

Truth table idea

Fig.1. Truth table idea.

Draw an example of timing diagram to be used later as stimulus in the VHDL testbench when verifying the synthesised circuit. Consider Min_Pulse = 2.45 ms.

The same project B3.9 is proposed in Chapter 3 for learning the basics of μC software organisation and basic digital I/O.

 

CPLD or FPGA target chip options:

 Target option #1.: MAX II

 Target option #2.: MAX 10

 Target option #3.: Cyclone IV

 

2. Planning

Plan A structural circuit based on logic gates and equations in a single VHDL file. We have many strategies, for instance:

option #1: use Minilog to obtain PoS.

Project location:

 C:\CSD\P2\Shifter_4bit_PoS\(files)

How many FPGA resources (logic cells, logic elements, etc.) are used? What is the percentage of the target chip used in this design?

 

option #2: use Minilog to obtain SoP.

Project location:

 C:\CSD\P2\Shifter_4bit_SoP\(files)

How many FPGA resources (logic cells, logic elements, etc.) are used? What is the percentage of the target chip used in this design?

 

Plan B behavioural approach writing the truth table or the high-level description or algorithm in VHDL in a single file (flat).

Project location:

 C:\CSD\P2\Shifter_4bit_B\(files)

 

How many FPGA resources (logic cells, logic elements, etc.) are used? What is the percentage of the target chip used in this design?

 

Chapter 2 problems

      - D2.3 -

Barrel p-shifter n-bit

A

Products

D1.8

D1.10

1. Specifications

Let us propose the design of an n-bit configurable barrel shifter capable of shifting p-bit to the right or the left. 

The idea is the following reference: Ercegovac, M., Lang, T., Moreno, J. H. Introduction to digital systems, John Wiley & Sons, 1999. Chapter 9, slides from 58 to 62.

Generic symbol for the barrel shifter

Fig.1. Symbol of the Barrel_p_shifter_nbit.

For example, let us propose a 4-bit barrel 3-shifter capable of shifting in both directions. Discuss its symbol and truth table drawing all signals using positive indexes. Control Input P determines the number of bits to shift.

Symbol and truth table

Fig.1. Symbol and truth table of the Barrel_3_shifter_4bit component adapted from the source.

This is a large device and better to use plan C2 and MUX_4 components for designing it. The book reference gives an idea on how to start a first step considering only one shift direction.

 

After having studied P4 tutorials, additional questions can be added to our report. For instance: