Description
1 Overview
1.1 Purpose
The purpose of this assignment is to practice implementing sequential logic circuits – from an RS Latch up
to a Finite State Machine, using Circuitsim. You will build a register from the ground up. Then you will
build a One Hot State Machine circuit, based on a provided state machine diagram (see Part 2 below). Then
you will build a reduced state machine, using a K-Map to simplify your logic.
Objectives:
1. Understand how a register circuit works
2. To learn how to make a state machine
3. To become familiar with different state machine styles
4. To understand K-maps and simplification
1.2 Task
There are four parts to this assignment.
1. First, you will build a register in CircuitSim from the ground up.
2. Second, you will implement a one hot state machine Circuit in CircuitSim, based on the state transition
diagram found below.
3. Third, you will complete a K-Map to simplify your state machine circuit, and implement the reduced
state machine.
4. Fourth, you will build the program counter (PC) register.
Please read the rest of this document for more detailed instructions and hints.
1.3 Criteria
Your grade is based on: 1) the correct outputs from your circuits; and 2) not using any banned components.
For part 3 (reduced state machine), you will lose points if your circuit does not correspond to your K-Map
or if your circuit is not minimal. The grade you see on Gradescope may not be the final grade you receive,
as we may run additional tests on your submission.
You will submit four files to Gradescope: latches.sim; fsm.sim; kmap.xlsx; and PC.sim.
You may submit your code to Gradescope as many times as you like until the deadline. We will grade your
last submission. We have also provided a local checker that you can test your code with. Please submit
your code to Gradescope at least once prior to the deadline, to ensure you are not encountering any issues
submitting at the last minute.
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2 Instructions
Part 1: For this part of the assignment you will build your own register from the ground up.
• Implement your circuits in the “latches.sim” file
Part 2: Given a simple state diagram, you will build a state machine in CircuitSim using the “one-hot”
style of building state machines.
• The circuit will be implemented in the “One Hot FSM” subcircuit of the “fsm.sim” file
Part 3: Given the same state diagram from part 1, you will be minimizing the logic by using K-Maps.
• Fill out the K-Maps located in the spreadsheet named “kmap.xlsx”
• The reduced circuit will be implemented in the “Reduced FSM” subcircuit of the “fsm.sim” file
Part 4: Create the PC component, which is a 16 bit register.
• Create the PC (Program Counter), a register that satisfies the requirements specified in section 2.4 of
this PDF
• The PC component will be implemented in the “pc.sim” file
Do not change/delete any of the input/output pins.
For parts 2 and 3 of this homework, you must use exactly 1 register. These are found under the Memory
tab in CircuitSim. Failure to do so may result in large point deductions.
2.1 Part 1
For this part of the assignment you will build your own register from the ground up. For more information
about each subcircuit refer to your textbook.
2.1.1 RS Latch
You will start by building a RS latch using NAND gates, as described in your textbook. The RS Latch is
the basic circuit for sequential logic. It stores one bit of information, and it has 3 important states:
1. R=1 S=1 : This is called the Quiescent State. In this state the latch is storing a value, and nothing
is trying to change that value.
2. R=1 S=0 : By changing momentarily from the Quiescent State to this state, the value of the latch is
changed so that it now stores a 1.
3. R=0 S=1 : By changing momentarily from the Quiescent State to this state, the value of the latch is
changed so that it now stores a 0.
Once you set the bit you wish to store, change back to the quiescent state to keep that value stored.
Notice that the circuit has two output pins; one is the bit the latch is currently storing, and the other is the
opposite of that bit.
Note: In order for the RS Latch to work properly, you must not set both R and S to 0 at the same time.
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• Build your circuit in the “RS Latch” subcircuit in the “latches.sim” file
2.1.2 Gated D Latch
Using your RS latch subcircuit, implement a Gated D Latch as described on the textbook.
The Gated D Latch is made up of an RS Latch as well as two additional gates that serve as a control. With
that addition not only can we control what value is stored by the latch, but also when that value will be
saved.
The value of the output can only be changed when Write Enable is set to 1. Notice that the Gated D Latch
subcircuit only has one output pin, so you should disregard the inverse output of your RS Latch.
• Implement this circuit in the “Gated D Latch” subcircuit in the “latches.sim” file
• You are not allowed to use the built-in SR Flip-Flop in CircuitSim to build this circuit
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2.1.3 D Flip-Flop
Using the Gated D Latch circuit you built, create a D flip-flop.
A leader-follower D flip-flop is composed of two Gated D latches back to back, and it implements edge
triggered logic.
Your D flip-flop output should be able to change on the rising edge, which means that the state of the D
Flip-Flop should only be able to change at the exact instant the clock goes from 0 to 1.
• Implement this circuit in the “D Flip-Flop” subcircuit in the “latches.sim” file.
2.1.4 Register
Using the D Flip-Flop you just created, build a 4-bit Register. Your register should also use edge-triggered
logic. The value of the register should change on the rising edge.
• This circuit will be implemented in the “Register” subcircuit in the “latches.sim” file
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2.2 Part 2
For this part of the assignment you are given the state machine transition diagram of a new video game that
is currently in the early stages of production. The game has a start state, game state, victory state, and
defeat state. Our game manager has one input button which is either pressed (G) or not pressed (G!). This
computer has six outputs (A, B, C, D, E, F) that affect the screen, music, and other details in the game
plays for the user.
• You will be implementing this state transition diagram as a circuit using the one-hot style. Remember
for one-hot you will have a register with the number of bits being the number of states of the FSM.
Each bit corresponds to a state, and you are in that state if the corresponding bit is a 1. Since you can
only be in one state at a time, exactly one of the bits can be 1 at each time (except when the machine
starts up, when all the bits will be 0).
• You must implement this using one-hot. A template file fsm.sim has been given to you. Implement
the state machine in the provided “one-hot state machine” subcircuit.
• Note that there are 3 inputs to the “One Hot FSM” subcircuit: CLK, G, and RST. The CLK input
turning off and on repeatedly will be used to represent clock ticks in your circuit. The G input
corresponds to whether or not G is on, as in the diagram above. RST resets the circuit (clears all the
bits of your state register).
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2.3 Part 3
Take a look at this state machine transition diagram:
• First, produce the K-maps for the state diagram above on the provided spreadsheet named “kmap.xlsx”.
Use the K-maps to produce the reduced Boolean expressions for the state machine.
The inputs for each K-map are:
– S0 = Current state least significant bit
– S1 = Current state most significant bit
– G = Input button
The outputs make K-maps for are:
– N0 = Next State least significant bit
– N1 = Next State most significant bit
– A, B, C, D, E, F
Please Note: This State Machine is a Moore State Machine, meaning that the output values are
determined solely by the current state (you should not use the N1 and N0 outputs or the G input for
determining the values for A, B, C, D, E, F.
– You will fill out one K-map per output and one per next state bit for a total of 8 K-maps (A, B,
C, D, E, F, N0, N1). The respective K-maps are located in the kmap.xlsx file.
– Your K-map must give the best solution groupings possible to receive full credit. This means you
must select the optimal values for any don’t cares (if applicable) in your K-maps to do this.
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– It may be helpful to check with others on Piazza to see if your circuit is optimal. In order to do
this without giving away your answer you may share the number of AND and OR gates used.
– IMPORTANT: The K-maps will be autograded. Because of this, there are a set of restrictions
to how you must fill your K-maps to ensure you get full credit:
∗ When you fill the row and column headers for your K-maps, you may only use the following
variable names: S0, S1, and G. To negate a variable, you must use an apostrophe. Two
adjacent variables with nothing in between are interpreted as an AND.
Example label: S0
0G
∗ When writing the Boolean expressions resulted from your K-map groupings, you must use
the same rules as the previous bullet point, but also use ”+” for OR.
Example grouping: For the Boolean expression (NOT S0) OR (S1 AND G), write S0
0+S1G
∗ When filling in the cells of your K-map table, you must use 0, 1, and x.
• Implement this circuit in the “Reduced FSM” subcircuit of the provided fsm.sim file. You will lose
points if your circuit does not correspond to your K-map or if your circuit is not minimal. You should
use only the minimal components possible to implement the state machine.
• HINT: We recommend you make a truth table for the state machine to help organize the logic, and
then transfer your answers to the K-maps. We’ve provided truthtable.xlsx to help with this.
Note: You are not required to complete truthtable.xlsx or submit it anywhere; it is only provided
for convenience when making your K-maps.
2.4 Part 4
You will need to build the PC (Program Counter) subcircuit. The PC register is a core component of
computers to aid in program sequences.
The PC is a 16 bit register that holds the address of the next instruction to be executed. The contents of
the PC must be updated depending on which MUX selectors and input signals are selected. There are three
scenarios for updating the PC:
1. The contents of the PC are incremented by 1.
Selected when PCMUX = 0b00
2. The result of the ADDR is the address of the next instruction. The output from the ADDR should be
stored in the PC.
Selected when PCMUX = 0b01
3. The value on the BUS is the address of the next instruction. The value on the BUS should be stored
into the PC.
Selected when PCMUX = 0b10
The PC should only be loaded on a rising clock edge when the proper input signal is on. Ensure that you
don’t reach the unused case (PCMUX = 0b11) of the circuit, or else spooky stuff might happen (undefined
behavior). All the proper inputs have been provided (and the reset has already been connected).
3 Testing
To test your circuits locally, navigate to the directory with the latches.sim and fsm.sim files and run the
tester JAR file with
java -jar hw03-tester.jar
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Make sure to run the local autograder in a terminal in Docker.
To test your K-maps, please submit your kmap.xlsx to the “Homework 3: K-maps” assignment on Gradescope.
4 Deliverables
Submit the following files to the “Homework 3: State Machines” assignment on Gradescope:
• latches.sim
• fsm.sim
• PC.sim
Additionally, submit the following files to the “Homework 3: K-maps” assignment on Gradescope:
• kmap.xlsx
Note: The autograder may not reflect your final grade on the assignment. We reserve the
right to run additional tests during grading.
5 Rules and Regulations
5.1 General Rules
1. Starting with the assembly homeworks, any code you write must be meaningfully commented. You
should comment your code in terms of the algorithm you are implementing; we all know what each
line of code does.
2. Although you may ask TAs for clarification, you are ultimately responsible for what you submit. This
means that (in the case of demos) you should come prepared to explain to the TA how any piece of
code you submitted works, even if you copied it from the book or read about it on the internet.
3. Please read the assignment in its entirety before asking questions.
4. Please start assignments early, and ask for help early. Do not email us the night the assignment is due
with questions.
5. If you find any problems with the assignment it would be greatly appreciated if you reported them to
the author (which can be found at the top of the assignment). Announcements will be posted if the
assignment changes.
5.2 Submission Conventions
1. All files you submit for assignments in this course should have your name at the top of the file as
a comment for any source code file, and somewhere in the file, near the top, for other files unless
otherwise noted.
2. When preparing your submission you may either submit the files individually to Canvas/Gradescope
or you may submit an archive (zip or tar.gz only please) of the files. You can create an archive by right
clicking on files and selecting the appropriate compress option on your system. Both ways (uploading
raw files or an archive) are exactly equivalent, so choose whichever is most convenient for you.
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3. Do not submit compiled files, that is .class files for Java code and .o files for C code. Only submit the
files we ask for in the assignment.
4. Do not submit links to files. The autograder does not understand it, and we will not manually grade
assignments submitted this way as it is easy to change the files after the submission period ends.
5.3 Submission Guidelines
1. You are responsible for turning in assignments on time. This includes allowing for unforeseen circumstances. If you have an emergency let us know IN ADVANCE of the due time supplying documentation (i.e. note from the dean, doctor’s note, etc). Extensions will only be granted to those who contact
us in advance of the deadline and no extensions will be made after the due date.
2. You are also responsible for ensuring that what you turned in is what you meant to turn in. After
submitting you should be sure to download your submission into a brand new folder and test if it
works. No excuses if you submit the wrong files, what you turn in is what we grade. In addition, your
assignment must be turned in via Canvas/Gradescope. Under no circumstances whatsoever we will
accept any email submission of an assignment. Note: if you were granted an extension you will still
turn in the assignment over Canvas/Gradescope.
5.4 Syllabus Excerpt on Academic Misconduct
Academic misconduct is taken very seriously in this class. Quizzes, timed-labs and the final examination are
individual work.
Homework assignments are collaborative, In addition many if not all homework assignments will be evaluated
via demo or code review. During this evaluation, you will be expected to be able to explain every aspect of
your submission. Homework assignments will also be examined using computer programs to find evidence
of unauthorized collaboration.
What is unauthorized collaboration? Each individual programming assignment should be coded by you.
You may work with others, but each student should be turning in their own version of the assignment.
Submissions that are essentially identical will receive a zero and will be sent to the Dean of Students’ Office
of Academic Integrity. Submissions that are copies that have been superficially modified to conceal that
they are copies are also considered unauthorized collaboration.
You are expressly forbidden to supply a copy of your homework to another student via electronic means. This includes simply e-mailing it to them so they can look at it. If you supply
an electronic copy of your homework to another student and they are charged with copying,
you will also be charged. This includes storing your code on any site which would allow other
parties to obtain your code such as but not limited to public repositories (Github), pastebin,
etc. If you would like to use version control, use github.gatech.edu
5.5 Is collaboration allowed?
Collaboration is allowed on a high level, meaning that you may discuss design points and concepts relevant
to the homework with your peers, share algorithms and pseudo-code, as well as help each other debug code.
What you shouldn’t be doing, however, is pair programming where you collaborate with each other on a
single instance of the code. Furthermore, sending an electronic copy of your homework to another student
for them to look at and figure out what is wrong with their code is not an acceptable way to help them,
because it is frequently the case that the recipient will simply modify the code and submit it as their own.
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Figure 1: Collaboration rules, explained colorfully
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