# ISE from Xilinx – Project in Verilog and Schematic

1. Introduction and Objetives

In this work you should employ what you have learned in theory about memory
elements and their application in shift register circuits.

Shift register arise from the frequent need of shifting one position all bits of a register, to
the left and to the right. It is possible to have linear or circular shift according to the way in
which the output and input of the register are connected.

2. Shift Register

a) Design, using type D Flip-Flops, a shift register serial-in parallel-out of 4 bits (you
should have an input “Serial in”, and 4 outputs, QAQBQCQD). Represent the logic diagram
of the designed circuit:

Fill in the following table for the expected state of the outputs immediately after the
rising edge of the “Clock” signal: (at the beginning the outputs are all at “O” and in the four
following “Clock” rising edges, the input “Serial in” has the bits shown in the table):
Sistemas Digitais – Preparation of Lab. Work n.° 4 2
b) Do the simulation in Xilinx ISE, and check the outputs QA to QD. The “Serial in”
input is the data input. The “Clock” input signal should vary periodically between “0” and
1″.

Implement the same circuit in a Verilog module, with behavioral level description (not
logic gate level). Simulate it.

3. Constant commutation

Keeping the previous circuit, it is now intended to control, by using a switch, the
alternating functioning between a shift register and constant commutation (the outputs
change at each clock cycle between all “1” and all “0”). In this sense, if the switch is at “l”,
the circuit should work as shift register, and if the switch is at “O”, the circuito works in
constant commutation. For that purpose, you should take into account that there are Preset
and Clear inputs on the D-type F lip-F lop (see the truth table in the datasheet). Sugestion:
using a type T F lip-F lop (implemented by using a type JK one), and two OR gates, allow to
provide a solution to the problem.