ICE 3: Full Adder

ICE 3: Full Adder#

Overview#

So far in this course you have implemented individual components on your FPGA. However, one of the benefits of VHDL is that we can pull multiple components into a single design.

In order to organize and reuse individual components, we are introducing the concept of a Top Level Design.

Important

The Top Level is where you pull in and connect all of the different components before finally connecting them to the IO of the board.

This is how you will manage the complexity of combining numerous components.

You will need to use the concept of a Top Level Design in Lab 2, so this is directly applicable.

Objectives#

The objectives of this in-class exercise are for you to:

  • Implement and test a full adder with half-adders using VDHL

  • Employ a top_level.vhd file

  • Gain more experience using tools (Git, Markdown, Xilinx Vivado)

End State#

Provide a live demo to your instructor.

You are not required to turn in ICE3; however, ensure the following files are pushed to your repository:

  1. VHDL files used (top_basys3, half-adder and testbench)

  2. Constraints (.xdc)

  3. .bit file used to program board

Background#

As discussed in ICE2, a half-adder takes two single-bit inputs and outputs their sum and a carry out; see Fig. 19 below.

A full-adder extends the half-adder to include a Carry In bit, \(C_{in}\).

A truth table, schematic symbol, and logic equations for a full-adder are shown below.

../_images/ice3_image1.jpg

Fig. 18 Full-adder schematic#

\[ S = A \oplus B \oplus C_{in} \]
\[ C_{out} = AB + AC_{in} + BC_{in} \]
Table 2 Full-adder truth table#

\(C_{in}\)

\(A\)

\(B\)

\(C_{out}\)

\(S\)

0

0

0

0

0

0

0

1

0

1

0

1

0

0

1

0

1

1

1

0

1

0

0

0

1

1

0

1

1

0

1

1

0

1

0

1

1

1

1

1

Full-adder from multiple half-adders#

It turns out that we can create a full-adder by combining two half-adders.

Recall from Half-adder a half-adder is:

../_images/ice3_halfaddersummary.png

Fig. 19 Half-adder schematic, truth table, and equations.#

Fig. 20 shows how two half-adders can be combined to function as a full-adder.

../_images/ice3_fullfromtwo.png

Fig. 20 A full-adder from two half-adders#

Our goal will be to implement this in VHDL!

The Project#

Setup Vivado#

Just like in the previous ICE, we will create a fork of the repository.

  • Navigate to USAFA-ECE/ece281-ice3

  • Fork the repository

  • Clone the repository

  • Open the project in Vivado

  • Modify the headers

Clone your repository, open it in Vivado, and modify headers.

Import half adder#

You created a half adder during ICE2. We will reuse that code here.

First, copy and past your completed the halfAdder.vhd file into src/hdl/

Then, follow the instructions in Manually add files to Vivado Project to add halfAdder.vhd to the project as a source. Make sure you see hallfAdder in the sources hierarchy.

The half-adder test bench has already been added for you. We recommend you run the simulation just to verify that things are working properly.

Note

Adding a file to the project will allow Vivado to see it and use it but the file will NOT be automatically added if you migrate the project to a new computer (such as with Git).

In order to do this you must Write a TCL file for use with Git and commit those changes as well. You are not required to do this for the lab.

Commit the addition of halfAdder.vhd with Git

Top Level Design#

Our top level design is the boundary of our system.

It allows us to connect internal components and our I/O.

top_basys3 entity#

Unlike the half-adder, the full-adder has three inputs, though it shares the same two outputs. This has been given to you.

Hint

A std_logic_vector combines multiple std_logic bits into one variable. The different signals within the vector can be referenced by index.

We will use switches for inputs and LEDs for outputs.

entity top_basys3 is
    port(
        -- Switches
        sw  :   in  std_logic_vector(2 downto 0);

        -- LEDs
        led :   out std_logic_vector(1 downto 0)
    );
end top_basys3;

top_basys3 architecture#

Always with layers of abstraction in mind, let’s redraw the schematic with the additional details of internal signals and board inputs and outputs. This is shown in Fig. 21.

../_images/ice3_image5.jpg

Fig. 21 Entity architecture for implementing full adder with two half-adders#

You need to modify the architecture of your top_basys3 file to reflect the schematic in Fig. 20.

Declare half-adder component#

The half-adder component must match the halfAdder entity declaration in halfAdder.vhd These declarations go in between the architecture and begin statements. Reference ICE2 testbench if you don’t remember the syntax.

architecture top_basys3_arch of top_basys3 is

    -- declare the component of your top-level design
    component halfAdder is
        port (
            i_A : in std_logic;
            i_B : in std_logic;
            o_S : out std_logic;
            o_Cout : out std_logic
            );
        end component halfAdder;

  -- declare any signals you will need

Just like the test bench pulls in your component to simulate inputs and outputs for it, a top level design will bring in components and wire them together. If you aren’t sure what signals you may need to create, the general rule is to create a signal for any wire not connected directly to an input or an output.

Instantiate half-adder components#

Now that we have made the halfAdder component available to the top_basys3 entity, we need to instantiate occurrences of the components and declare the logic to connect them to each other, and to inputs/outputs.

Remember, we are trying to build what is in Fig. 21.

Notice that we have TWO instantiations of halfAdder and we have given them unique names (halfadder1_inst and halfAdder2_inst). The or gate can be directly declared in “CONCURRENT STATEMENTS”

-- PORT MAPS --------------------
    halfAdder1_inst: halfAdder
    port map(
        i_A     => sw(0),
        i_B     => sw(1),
        o_S     => w_S1,
        o_Cout  => w_Cout1
    );

    halfAdder2_inst: halfAdder
    port map(
        i_A     => -- TODO
        i_B     =>
        o_S     =>
        o_Cout  =>
    );
    ---------------------------------

    -- CONCURRENT STATEMENTS --------
    led(1) <= -- TODO
    ---------------------------------

Test design#

We already have a completed halfAdder_tb.vhd file, which is awesome, because it means we can test our system at multiple layers!

Your job is to complete the top_basys3_tb.vhd file and test the system as a whole.

Edit testbench#

Open top_basys3_tb.vhd. Note the header has top_basys3.vhd as a REQUIRED FILE. Revisit ICE2 for more detailed instructions if you need them.

Declare your top_basys3 “component” in the test bench, create signals to simulate the inputs and outputs, and connect them in a port map.

Then create your test cases within the test process. Here is an example of the first line:

    begin

        w_sw <= o"0"; wait for 10 ns;
            assert w_led = "00" report "bad 000" severity failure;
        w_sw <= o"1"; wait for 10 ns;
            assert w_led = "01" report "bad 001" severity failure;
        --You must fill in the remaining test cases.

Similar to Lab 1 we can set the enitre vector with a single value, but because there are only three bits, we need to use octal instead of hex. The leading o is how you express an octal number in VHDL.

How many test cases will you need? Create them all 😄

Simulate project#

Your half-adder should be working already, so focus on the top_basys3_tb. Use your waveform to debug if any of the assert statements fail.

Implement Design#

Constraints File#

Unlike our pervious exercise, you will not need to replace the highlighted portions. We made our entity inputs and outputs reflect the default constraint file naming convention.

## Switches
set_property PACKAGE_PIN V17 [get_ports {sw[0]}]
set_property IOSTANDARD LVCMOS33 [get_ports {sw[0]}]

Uncomment all lines needed for this design (sw{0}, sw{1}, sw{2}, led{0}, and led{1}.

Test bitstream on FPGA#

Synthesize, implement, generate.

Squirt the bitstream to the board, and test!

Wrapping up#

See GitHub real fast if you need some tips.

README#

Take a screenshot of your waveform, save it to an the top level of your directory (or you can use an img/ folder).

Make the screenshot appear in your README with the following Markdown syntax:

![description of my waveform](imagename.png)

Add a ## Documentation section to the README. This is the only one you need for the entire ICE; you don’t need one in the file headers.

## Documentation

My statement here.

Add the image and README.md to git and commit.

Other files#

  • Run git status

  • Double check that you have the image, your README.md and all .vhd and .bit files committed to your git repo.

  • Then push them to your repo.

  • Make sure all your work appears in GitHub as you expect.

  • Submit the assignment on Gradescope.

  • Demo your working board to an instructor.

Congratulations, you have completed In Class Exercise 3!

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