Park University Digital Logic Project Moderate Circuit with A Sequential Sub Circuit Paper – Description
Purpose
The purpose of this project is to use a digital logic simulator and build a moderate circuit with a sequential sub-circuit and a combinational sub-circuit.
Project Description
We will build a 4-bit counter with two Hex Digit displays to show the corresponding decimal values 0 ~ 15 (and back to 0). The circuit uses a counter sub-circuit (step 1) to generate BCD code automatically. The output of the counter sub-circuit will then be fed to a second sub-circuit (step 2) and connected to two Hex Digit displays.
Step 1
BUILD THE COUNTING-UP COUNTER DESCRIBED IN FIG 10-17 (CH10.5) OF THE TARNOFF TEXTBOOK IN LOGISIM. THIS IS A SEQUENTIAL CIRCUIT.
The clock signal is in the Wiring folder [of Logisim].
Use one D flip-flop for each of the four latches and set their trigger attribute to “Rising Edge”. Be aware that the pins of the D flip-flop component in Logisim are ordered differently from the D latch graphic symbol in our textbook. Hover your mouse over each pin of the D flip-flop to know what that pin is for. Use Help menu > Library Reference.
Connect the output Q of each D flip-flop to an output pin. Name the output pins Q0 (latch A, LSB), Q1, Q2, and Q3 (latch D, MSB) respectively. Together Q3 Q2 Q1 Q0 represents a four-bit binary number. It’s a good idea to lay out the four output pins in a line with Q3 on the left (since it’s the MSB) and Q0 on the right.
Poke the clock to run your circuit. The outputs should cycle through 0000~1111 and then restart from 0000.
Add your name, Park ID, and date to your circuit using the Text tool. Save your circuit as Unit4_YourLastName_Step1.circ. Test your circuit to make sure it works properly before proceeding to the next step. Document your testing in the project document.
Step 2
Now design and build a combinational circuit to convert BCD code (4-bit input) to its decimal value expressed in two sets of BCD codes (8-bit output Y7 ~ Y0). Even though it has 8 outputs, this circuit is simpler than the project we built in the previous unit.
THE TRUTH TABLE IS PROVIDED BELOW. THE INTENTION IS TO CONVERT INPUT ABCD (0000 ~ 1111) TO OUTPUTS Y7~Y0 WHICH REPRESENT DECIMAL VALUES 0 ~ 15. THE DECIMAL VALUES WILL BE EXPRESSED IN TWO SETS OF BCD CODES, Y7~Y4 FOR THE TEN’S DIGIT AND Y3~Y0 FOR THE ONE’S DIGIT.
Provide a simplified expression for each of the 8 outputs Y7 ~ Y0. You may use whatever simplification method you like, but show your work with steps (such as k-maps). Hint: Y7, Y6, and Y5 will be the same and end up being 0.
Create a new circuit in Logisim to implement these simplified functions. Only use AND, OR, and NOT gates. Label items (every input/output pin and every AND/OR gate) in your circuit.
Hint: use a constant 0 wire for Y7, Y6, and Y5.
Add your name, Park ID, and date to your circuit using the Text tool. Save your circuit as Unit4_YourLastName_Step2.circ. Test your circuit to make sure it works properly before proceeding to the next step. Document your testing in the project document.
Step 3
Now we’re ready to put the two parts together. The overall connection (see the visual below) will be: Step 1 circuit (counter) -> Step 2 circuit -> two splitters -> two Hex displays. It’s a good idea to save your work following each sub-step.
Save a copy of your counter circuit from Step 1 as Unit4_YourLastName_Step3.circ (i.e. don’t destroy your Step 1 file). We will work in this file to combine our Step 1 and Step 2 circuits.
Load your Step 2 circuit (BCD to 2-digit decimal) into this Step 3 file. First, make sure your Step 2 circuit file is in the same folder on your computer as this Step 3 file. Next use Project menu -> Load Library -> Logisim Library … to add your Step 2 circuit. The loaded circuit will become a new folder under the Explorer Pane, as shown in Figure 1. Click open that folder and add the main component to the current file (the same way you add a built-in component). It will be displayed as a black box with four input pins and 8 output pins. We’ll use the loaded circuit like a Logisim component, the same way a circuit chip can be used anywhere with proper wiring.
Figure 1. The loaded circuit in the Explorer Pane
CONNECT EACH OF THE FOUR OUTPUTS OF YOUR STEP 1 COUNTER TO THE CORRESPONDING INPUT PIN OF THE BLACK BOX (YOUR STEP 2 CIRCUIT). MAKE SURE THEY’RE PAIRED CORRECTLY. THE STEP 1 MSB OUTPUT SHOULD BE CONNECTED TO THE MSB INPUT PIN OF THE BLACK BOX. DO NOT DELETE THE OUTPUT PINS. JUST ADD ADDITIONAL WIRE BEFORE EACH OUTPUT OF YOUR STEP 1 COUNTER CIRCUIT.
Add two Hex Digit Displays to the right side of the black box and align them side by side (Figure 2). The Hex Digit Display component is listed under the Input/Output folder. Be sure to use the “Hex Digit Display”, not the 7-seg display we used last week.
Figure 2. Two Hex Digit Displays Side by Side Showing an Output of 15.
A Hex display takes a single input line carrying 4 bits, not 4 lines of 1-bit input. Because of that, we need to combine Y7~Y0 into two 4-bit lines before connecting the outputs of the black box to the two Hex displays. We will use two splitters. Splitter is the first component listed under the Wiring folder. Figure 3 shows how a splitter combines four 1-bit lines into a 4-bit line for a Hex Digit Display. If it helps, build a test circuit as in Figure 3 to understand the connection before adding splitters to your Step 3 circuit. Use the following attributes for the splitter:
Facing: North
Fan Out: 4
Bit Width In: 4
Figure 3. Use Splitter to combine four 1-bit lines into a 4-bit line
Now add two splitters to your Step 3 circuit between the black box and the two Hex displays. One splitter will combine Y7~Y4 into a 4-bit line before sending them to the Hex display on the left (for the ten’s digit). The other splitter will combine Y3~Y0 for the Hex display on the right (for the one’s digit).
Save your circuit and test it. Document your testing in the project document.
Step 4
Project feedback:
What’s the hardest part of this project for you? Please explain.
How’s your understanding of combinational circuits and sequential circuits after this project? Please explain. Feel free to comment on other aspects of this project.
Notes
In Logisim, a wire carrying 1 bit should only be light green (1) or dark green (0). See Help > Logisim References > Wire bundles > Wire colors.
Use one AND gate for each ANDed term no matter how many variables the term uses. Use one OR gate to OR terms at the same level. Edit the number of input pins an AND/OR gate has to match the number of inputs the gate needs. It can help you track your work, not missing any input connections or adding extra ones.
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