Lab 2: Verilog Synthesis and FSMs

Introduction

In this lab you will be introduced to behavioural Verilog, and one of the more important tasks of the tools, logic synthesis. Completing this lab will give you experience describing hardware quickly, and at a high level. In addition you will take a close look at the logic that is produced by the CAD tools, and any inefficiencies produced in the translation from text to FPGA implementation.

In this lab we have given you the framework to build a two digit combination lock. Your job will be to implement three separate modules and complete the lock. The inputs to your lock will consist of a rotary dial and two push buttons. The rotary dial, represented by the signals FPGA_ROTARY_INCA and FPGA_ROTARY_INCB, is used to select a digit of the combination. The rotary dial push button, represented by the signal FPGA_ROTARY_PUSH is used to enter the digit selected. LEDs are used to indicate that the lock has been opened, and also that an incorrect combination has been entered.

Prelab

Note: Please make sure to complete the items in this section before coming to lab. You most likely will not finish the lab during your section if you do not do the prelab.

For the prelab, complete the following items:

• Read this entire handout thoroughly. In particular, the Lab Details section gives you lots of valuable details on the circuit you must build. Please also read the Verilog FSM Tutorial.
• Download the lab files. lab2.tar.gz
• Write your Verilog ahead of time. You are responsible for implementing Lab2Lock and Lab2Counter.
• Answer the prelab questions.

Questions

1. How many D-type flip-flops do you think the synthesis tools will infer in Lab2Lock? Think about this carefully. What will D-type flip-flops be used for in this module?

2. Given the verilog you came up with for Lab2Counter, what do you think the logic synthesis tools will produce? Suggest a circuit that implements Lab2Counter. Draw an RTL diagram of this circuit on the back of the checkoff sheet. You need not go in to great detail - simply show a very high level structure of the circuit.

Design Details

This section describes in detail what you will need to design for this lab.

Overview

A diagram of the circuit we want you to implement is below.

Most of the modules have been provided for you; you are only responsible for implementing Lab2Lock and Lab2Counter.

One possible point of confusion is the existence of two separate reset signals, LockReset and SystemReset. This separation exists to allow the lock to be reset independently of the rest of the system, such as the counter, in the event that the user would like to input another combination. The Lab2Lock module is reset by both the SystemReset and the LockReset.

ButtonParse

The module has been built for you.

Raw inputs from push buttons are often very noisy, oscillating for a short time before settling on a value. This module filters is input into a clean 1-cycle pulse. It is a fairly complex circuit; you do not need to understand the internal details of how it works.

Refer to the following table for the ButtonParse port specification.

RotaryEncoder

The module has been built for you.

The RotaryEncoder module parses raw input from the rotary encoder (which is the wheel found on the lower right corner of the ML505 board). This module decodes changes in the 2-bit state of the encoder in to pulses of either the Up or the Down signal. It is important to note that due to the encoder works, the signal will be pulsed 4 times for each click of the wheel.

Refer to the following table for the RotaryEncoder port specification.

Lab2Counter

We will be using the Lab2Counter in conjunction with the Rotary Encoder as a means of inputting the digits of the combination into our combination lock. We will increment the Count output of the lock when the wheel is clicked once counter-clockwise, and decrement the wheel is spun clockwise. Slightly complicating the task of designing this module is the fact that, as noted in the specification of the RotaryEncoder, we only want the Count output to change once for every 4 clicks of the Increment or Decrement signal. You will have to figure out a way to deal with this in your design.

Lab2Lock

This module is responsible for maintaining the state of the lock and generating outputs which indicate the lock's status (these will be pulled out to LEDs).

This module should detect DIGIT1 and DIGIT2 being entered in the correct order. You should use the verilog localparam statement in order to define the values of these constants inside your module. The following snippet shows how this is done:

/* Inside the Lab2Lock module */
localparam DIGIT1 = 4'h2;
localparam DIGIT2 = 4'h3;

Please use the above values for the combination to your lock to make it easier to test.

You will build this module as a finite state machine. In particular, you should build a Moore machine to accomplish this task. A Moore machine depends only on its current state to generate its output (not, for instance, on both its current state and its current inputs). Therefore, your Lab2Lock will have the following high level organization:

Remember the tips we have given you for building FSMs in verilog. If you're not absolutely sure about how to proceed, please review the comprehensive Verilog FSM Tutorial.

Both the port specification and the state transition diagram for this module follow.

Analysis

You should now have a complete and working combination lock. In order to build this circuit you did not need to specify the individual implementation of each functional unit. Instead, you provided the synthesis tool with a high level textual representation of the behaviour you wanted, and the synthesizer did it's best to match this description. You now have enough experience to have seen how drastically high level synthesis can affect development times. You will now take a look at how well the synthesizer does it's job.

1. In the prelab you speculated a possible circuit for synthesized Lab2Counter. Find this module in the RTL schematic and compare the circuit created by the synthesis tool to your own.

2. Collect some general information about the design

   1. Number of occupied SLICEs
   2. Number of SLICE LUTs
   3. Number of SLICE registers (used as flip-flops)

3. Open the Lab2 circuit in FPGA Editor and answer the following

   1. Where is the circuit placed on the chip?
   2. Why do you think the CAD tools placed the circuit this way?
   3. Is the circuit closely packed or distributed throughout the FPGA fabric?