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

Chapter 3 problems

      - B3.9 -

  4-bit (nibble) shifter operator

Products

B3.8

B3.10

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. The same project is proposed in D1.9 as a combinational circuit based on logic gates.

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 dont'care about the output values if by any chance both control signals are activated SR = '1', SL = '1'; assuming that this condition will nevel occur. The simbol and operations of this device are represented in Fig. 1.

Use a PIC18F4520 microcontroller, C language and Microchip IDE tools. Test your design using Proteus. Add switches to input data and control signals and LED to visualise the operations.

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.

Picture report

Fig. 2. Timing diagram example.

2. Planning

You will solve the circuit in several steps, adding a few lines of code each time to develop and test immediately to debug any error before adding more code. For instance:

- step #1 read only SL and monitor its current value var_SL in a watch window.

- step #2 read only SR and monitor its current value.

- step #3 read the vector W(3..0) and monitor its current value in a watch window. In this way, your read_inputs() function will be validated.

- step #4 Imagine a given value for the variable var_Y, write it in the corresponding pins and observe LED values, thus validating your function write_outputs().

- step #5 complete the truth table and the project.

 

A) Planning hardware

Copy and adapt a circuit from any of the LAB9 projects and name it Bin_Johnson_4bit.pdsprj. Assign pins to inputs and outputs accordingly to one of the following options (your instructior will tell you which):

Pin assignment option #1:

SL --> RD6; SR --> RB0; W(3..2) ---> RC(1..0); W(1..0) ---> RD(2..1); Y(5) ---> RA3; Y(4..2) ---> RB(7..5);  Y(1) ---> RD4;  Y(0) ---> RA2;

Pin assignment option #2:

SL --> RB0; SR --> RD6; W(1..0) ---> RC(2..1); W(3..2) ---> RD(2..1); Y(5) ---> RA2; Y(4..2) ---> RB(6..4);  Y(1) ---> RA3;  Y(0) ---> RD4;

Pin assignment option #3:

SL --> RC1; SR --> RD2; W(3..2) ---> RB(5..4); W(1..0) ---> RA(3..2); Y(5..3) ---> RC(7..5); Y(2) ---> RA0;  Y(1..0) ---> RD(4..3)

Project location:

 C:\CSD\P9\Shifter_4bit\(files)

 

B) Planning software

Organise the main program in our CSD way. Copy and adapt a circuit from any of the LAB9 projects and name it Shifter_4bit.pdsprj.

Propose a hardware-software diagram naming all the electrical signals, RAM variables and the software functions.  

Picture report

Fig. 3. Hardware-software diagram. 

Describe all the RAM memory variables used in this project and its type.

Explain how to configure the µC in init_system().

Organise using a flowchart the interface function read_inputs().

Picture report

Fig. 4. Reading inputs.

Organise using a flowchart the interface function write_outputs().

Picture report

Fig. 5. Writing outputs.

Infer the truth_table() software function using a behavioural plan B interpretation from the corresponding flowchart.

Picture report

Fig. 6. Shifter_4bit initial discussion on adapting the truth table using plan B flowcharts.

 

3. Developing & 4. Testing (debugging)

- Step #1. Write the Shifter_4bit.c source code translating the function flowcharts. Start capturing only one input as in (LAB9) and visualising it in the watch window as a RAM variable as indicated in the sequence of steps above in the planning section. And only then go to the next step developing & testing more inputs. 

Start a software IDE project for the target microcontroller PIC18F4520 and generate the configuration files ".cof" and ".hex" after compilation. Discuss the project summary: % of ROM used for the code, number of RAM bytes used, etc.

Note: Step-by-step tactical approach for developing and testing the project: Read one input at a time and run to check that the voltage value is correctly captured as a valid digital value in RAM memory. Write one output at a time and run to check that your code is correct to light the LED connected at the output pin.

Add a few lines of code every time, compile and run the test intereactively to check results watching variables.

Picture report

Fig. 7. Example pictures demonstrating how the circuit works.

Measure how long does it take to run the main loop code when using a 4 MHz and a 16 MHz quartz crystal oscillator.

Picture report

Fig. 8. Loop execution time measurements.