Description

Jinhoon Bae
bae00003@postech.ac.kr
Contact the TAs at csed311-ta@postech.ac.kr
Contents
• Combinational logic
• Register-Transfer Level
• Finite State Machine
• Assignments
• Combinational logic (ALU)
• FSM (vending machine)
2
• Represents Boolean function (time(clk)-independent)
• Output is a function of inputs only
• output = f(input)
• Example: 4-to-1 mux

• Implementation with Verilog (mux)
• If you use “always”, simply giving “(*)” as the sensitivity list is equivalent to the code below
Using Assign Using Always

4
• RTL schematic

Register-Transfer Level
• The design method to implement a synchronous circuit with an HDL (Hardware Description Language)
• Synchronous circuit consists of:
• Register: a memory element synchronized by the clock signal
• Combinational logic: logical function

RTL – example
• Toggler example

• Moore Machine
• Mealy Machine
• Moore Machine
• Outputs only depend on the current state.
• Mealy Machine
• Outputs depend on the current state and the current inputs
• Mealy Machine
• Mealy Machine can be made synchronous

Assignment 1.a
• Implement ALU (Arithmetic Logic Unit) in Verilog
• A skeleton code and testbench will be provided
• Lab1/ALU/*
• Input: (A, B, FuncCode)
• A: left operand (16-bit signed binary)
• B: right operand (16-bit signed binary)
• FuncCode: operator (4-bit binary)
• Output: (C, OverflowFlag)
• C: operation result (16-bit signed binary)
• OverflowFlag: overflow flag (1-bit binary)
Assignment 1.a (cont’d)
• ALU operations
FuncCode Operation Comment FuncCode Operation Comment
0000 A + B Signed Addition 1000 A B Bitwise XOR
0001 A – B Signed Subtraction 1001 ~(A B) Bitwise XNOR
0010 A Identity 1010 A << 1 Logical Left Shift
0011 ~A Bitwise NOT 1011 A >> 1 Logical Right Shift
0100 A & B Bitwise AND 1100 A <<< 1 Arithmetic Left Shift
0101 A | B Bitwise OR 1101 A >>> 1 Arithmetic Right Shift
0110 ~(A & B) Bitwise NAND 1110 ~A + 1 Two’s Complement
0111 ~(A | B) Bitwise NOR 1111 0 Zero
Assignment 1.a (cont’d)
• Overflow detection
• For addition and subtraction, you should detect overflow and set the flag.
Overflow Flag Singed Addition Signed Subtraction
0 Correct Result Correct Result
1 Wrong Result Wrong Result
• For other operations, OverflowFlag is always zero.
Assignment 1.b
• Implement a simple vending machine RTL in Verilog
• A simple Finite State Machine (FSM)
• Your vending machine should cover all use-cases (in the next slides)
• A skeleton code and testbench will be provided
• Lab1/vending_machine/*

• Vending machine interface
INPUT
Signal Description Number of bit(s)
i_input_coin Insert Coin 1 for each type of coins
i_select_item Select Item 1 for each type of items
i_return_trigger Return change 1
clk clock 1
reset_n reset 1
OUTPUT
Signal Description Number of bit(s)
o_output_item Indicate dispensed items 1 for each type of items
o_available_item Indicate item availability 1 for each type of item
o_return_coin Indicate type of coin (change) 1 for each type of coins
• Vending machine use-case
Assumption: infinite item, change
Sequence
1. Insert money (available money unit: 100, 500, 1000 won) and initialize waiting time(=100)
2. Vending machine shows all available items where (item cost <= current money)
2.a. decrease waiting time(-1)
3.a. Insert money within the waiting time
3.a.1. Go to 2 and initialize waiting time(=100)
3.b. Select an item within the waiting time
3.b.a. Case1: the item is available
3.b.a.1. The item is dispensed
3.b.a.2. Go to 2 and initialize waiting time(=100)
3.b.b. Case 2: the item is unavailable
3.b.b.1. Nothing happens. Waiting time is not reset.
3.c. No input within the waiting time 3.c.1 Return changes
a*. Whenever press the return button
a*.1. Return changes a*.2. Go to 1
• If your vivado is not displaying the output of $display or $monitor, then try using vending_machine_tb_f.v. (output will be written to output.txt)
• If your simulation keeps running, the testbench module is waiting for the output signals. (o_output_item or o_return_coin)
• If your vending machine code passes all tests, you will get the message “Passed = 23, Failed = 0”

Evaluation Criteria (Source code)
• ALU
• Number of tests passed from the shared TB
• Vending machine
• Number of tests passed from the shared TB as well as additional TB not shared
• Modularization
• Your implementation should include at least two modules
• Using modules lowers implementation complexity
• Similar to the functions in other programming languages
• It would be helpful to practice before implementing CPUs
• Understanding of combinational/sequential logic
• E.g.) Appropriate use of blocking/non blocking assignment
Evaluation Criteria (Report)
• You can write report in Korean or English
• Report should include (1) introduction, (2) design, (3) implementation, (4) discussion, and (5) conclusion
• ALU
• Report not evaluated, source code only
• Vending machine
• Combinational logic design and implementation
• Sequential logic design and implementation
• FSM type of your design
• Moore or mealy machine
Assignment tips
#1: When modeling sequential logic, use nonblocking assignments.
#2: When modeling latches, use nonblocking assignments.
#3: When modeling combinational logic with an always block, use blocking assignments.
#4: Do not mix blocking and nonblocking assignments in the same always block.
#5: Do not make assignments to the same variable from more than one always block.
#6: Use $strobe to display values that have been assigned using nonblocking assignments.
#7: Do not make assignments using #0 delays.
Assignment Submission
• Submit your assignment to LMS with filename:
• Lab1_{TeamID}_{StudentID1}_{StudentID2}.pdf
• PDF file of your report
• Lab1_{TeamID}_{StudentID1}_{StudentID2}.zip
• Zip file of your source code (without testbench)
• One directory including two subdirectories (ALU, vending_machine) when unzipped
• ALU (1 week), vending machine (1 week) merged into 2 weeks to help students to distribute time freely
Announcement
• Please check your team in PLMS
• If you have any questions, please post them on the Q&A board in PLMS
• Today, we will provide a simple test bench, and on Thursday, we will offer a test bench that includes a wider variety of cases. We will grade based on that test bench.
Reference
• http://www.sunburst-design.com/papers/CummingsSNUG2000SJ_NBA.pdf