6. Design a single-precision (i.e., 32-bit) IEEE 754 standard floating-point add

Question

6. Design a single-precision (i.e., 32-bit) IEEE 754 standard floating-point adder with inputs a[31:0] and b[31:0] and output y[31:0]. Your adder need only work for positive numbers and use round toward zero (i.e., truncate). You should also ignore the special cases (given in Table 5.2 of the textbook). (a) Draw a black-box of the system (i.e., box showing inputs and outputs). (b) [35 pts] Draw a schematic of your design. Hints: Remember to use hierarchy (modules and submodules) to build your design. Use building blocks we have discussed, including variable shifters, multiplexers, adders, subtractors, etc. Think about how much the smaller mantissa should be shifted. If the exponents differ by 24 or greater, what happens to the mantissa of the smaller number?

Explanation / Answer

Go to Start --> Programs --> OrCAD Release 9.1 --> Capture CIS

Go to File --> New --> Project...

Give a name for your project.

Select Analog or Mixed-signal Circuit Wizard.
Choose a folder for saving your project.

Click on OK.

For digital simulations Add (double click on) 7400.olb (conatins TTL ICs).
For analog simulations Add (double click on) opamp.olb (conatains Op-Amps including uA741) and diode.olb (contains diodes).

Now make the circuit in the schematic window. Notice the title of the schematic window, it says SCHEMATIC1:PAGE1. This means the name of the schematic is SCHEMATIC1.

Do not place any input signals or voltage markers yet.

In the above example of a Half Adder, it has two inputs A and B, and two outputs Sum and Carry. For each one of them, place a Port using the Place Port button . The following dialog appears:

Connect the ports to the inputs and output. For input, the recommended port is PORTRIGHT-R. For output, the recommended port is PORTLEFT-L.

Double click on the default port names PORTRIGHT-R/L to give them new meaningful names.

Save the design by clicking on .

Open the project window. (Either minimize the schematic window, or choose Window --> <projectname>).

Right click on the .dsn file and chose New Schematic.

Accept the default name ie. SCHEMATIC2. Notice that it appears under the .dsn file.

Now right click on SCHEMATIC2 and click on New Page. Accept the default name.

The new page appears under SCHEMATIC2.

Before proceeding, you MUST SAVE NOW by clicking the button!! Otherwise an error will message will be displayed.

The SCHEMATIC1 contained the Half Adder that we wanted to use as a component. We will now use it in the new schematic SCHEMATIC2. But we have to make SCHEMATIC2 the Root Schematic before we can simulate it. To do this right click on SCHEMATIC2 and click on Make Root.

Now the ordering changes and the mark (/) moves from SCHEMATIC1 to SCHEMATIC2.

Double click on the Page1 of SCHEMATIC2.
The blank schematic page opens.

Now click on the create hierarchical block icon .
Fill in any unique name for Reference.
In Implementation Type, select Schematic View.

In Implementation Name, type SCHEMATIC1, i.e. it must be the same as the one on the Half Adder.

Click OK. Now draw a box in the window with the mouse of any dimensions. The following will appear:

You can drag the ports (A, B, C and D) to comfortable locations within the block (shown in the last figure).

You can repeat step 12 if you want to put more instances of the block. For example, we will make a Full Adder with 2 Half Adders, so we have made two blocks. Keep in mind that reference names should be different like HA1 and HA2, but the Implementation Name is the same for both the Half Adders: SCHEMATIC1.

Now make a circuit with the component blocks and simulate it normally.

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