Lab 3 Solution

Preparation




 
Read this document in its entirety.




 
Draw a circuit showing how you might build a 4⇥1 mux from 2⇥1 muxes.




 
Read Example 2.23 in the main textbook on pages 72–73 and prepare the truth table for a 4-bit binary number to hexadecimal 7-segment display. That is, in addition to 0-9, the display also shows A, b, C, d, E, and F.




 
Read the 2-page reference manual (attached to the end of this document) to better understand the seven-segment display connections.




 
Multiplexer Design




2.1 Building a 4⇥1 Mux




Inside your ECE2700 directory, create a new folder called Lab3. Start up Xilinx and create a new project called MuxExp inside the Lab3 directory.




Create a new file called mux4x1.v. Inside, design a 4⇥1 mux using the provided 2⇥1 mux (mux2x1.v on Canvas).




Be sure to declare your 4⇥1 mux as follows: module mux4x1(I3, I2, I1, I0, S1, S0, D), where I3, I2, I1, and I0 are data inputs, S1 and S0 are control inputs, and D is the output. Make sure to design your mux such that when S0 = 0, S1 = 0, D = I0, and when S0 = 1, S1 = 1, D = I3.




Test your design using the bench file mux4test.v from Canvas. Show your results to the TA.




2.2 Using the Muxes




Create a new file called shifter.v. You are to design a simple shifter using the 4⇥1 mux(es) you designed in earlier. The shifter has four 1-bit data inputs (i3, i2, i1, i0), two 1-bit control inputs (s1, s0), and four 1-bit data outputs (d3, d2, d1, d0). The following table describes the desired behaviors of the shifter. Be sure to declare your shifter as follows: module shifter(i3, i2, i1, i0, s1, s0, d3, d2, d1, d0). Observe that i3 is the leftmost bit in the input.




s1
s0
Behavior
Description
Example
0
0
Hold
Output is the same as input.
Input: 1010, Output: 1010
0
1
Shift left
Output is the input shifted left by one bit.
Input: 0101, Output: 1010






The least significant bit is shifted in with a 0.
Input: 1111, Output: 1110
1
0
Shift right
Output is the input shifted right by one bit.
Input: 0101, Output: 0010






The most significant bit is shifted in with a 0.
Input: 1111, Output: 0111
1
1
Rotate Right
Output is the input rotated right by one bit.
Input: 1011, Output: 1101






1



Hint: Remember that a mux is simply a circuit that selects the output from one of the inputs. Think about what the data and control inputs need to be for the shifter to work correctly.




Test your design using the bench file testshifter.v from Canvas. Show your results to the TA.




3 Decoder Design




3.1 Simple 7-Segment Display




For this part of the lab you will write code for the 7-segment display and load it onto the Basys board. Create a new project called SevenSeg. Add a new source file seg7.v and write out your binary number to hexadecimal 7-segment display using the following declaration: seg7(w, x, y, z, a, b, c, d, e, f , g), where w, x, y, and z are the 4-bit binary input and a, b, c, d, e, f , and g are the outputs. (If you are unsure what to do, remember that you have prepared a truth table. From the truth table, you can convert to Boolean equations and write them directly in Verilog.) Run simulations to make sure your design is correct.




Locate SW0, SW1, SW2, and SW3 on the Basys board. These switches will represent the 4-bit binary number input. As mentioned earlier, the output (corresponding hex value) will be displayed on the 7-segment display. From reading the reference manual and looking inside MainBasys.ucf, you will see that although the 7-segment display contains 4 digits, there are only 7 pin assignments for the 7-segment LEDs, not 28.




NET "seg<0" LOC = "p25"; # Bank = 3, Signal name = CA




NET "seg<1" LOC = "p16"; # Bank = 3, Signal name = CB




NET "seg<2" LOC = "p23"; # Bank = 3, Signal name = CC




NET "seg<3" LOC = "p21"; # Bank = 3, Signal name = CD




NET "seg<4" LOC = "p20"; # Bank = 3, Signal name = CE




NET "seg<5" LOC = "p17"; # Bank = 3, Signal name = CF




NET "seg<6" LOC = "p83"; # Bank = 1, Signal name = CG




This is to reduce the number of pins on the board. You can determine which of the 4 digits (or all of them) are displayed by sending appropriate signals to the following anodes.




NET "an<3" LOC = "p26"; # Bank = 3, Signal name = AN3




NET "an<2" LOC = "p32"; # Bank = 3, Signal name = AN2




NET "an<1" LOC = "p33"; # Bank = 3, Signal name = AN1




NET "an<0" LOC = "p34"; # Bank = 3, Signal name = AN0




Be Careful! The inputs for the 7-segment LEDs and the anodes are active low. In other words, to display an 8 on the rightmost digit, you will set a, b, c, d, e, f , and g all to 0. In addition, you will set the input corresponding to an<0 to 0 and set all the other inputs to the anodes to 1.




Add a new file called SevenSeg.v with the following declaration: SevenSeg(w, x, y, z, a, b, c, d, e, f , g, n3, n2, n1, n0). Here, n0 corresponds to the signal sent to anode 0 (AN0). Write necessary code so that only the rightmost digit lights up. Recall that you will need to add a .ucf file to indicate pin assignments. Generate the corresponding bit file, test your design and show the results to the TA.




 
TA Checko↵




 
(20 points) Pre-lab.




 
(40 points) 4⇥1 mux design.




 
(60 points) Shifter design.




 
(80 points) 7-segment display.










2





Overview







The PmodSSD offers a single two-digit seven-segment display device (7sd) that can attach directly to any Digilent system board. The 7sd uses high-bright LEDs that can are easily readable with less than 5mA of current, so they can be driven directly from most system boards.




Features include:



 
two high bright seven-segment displays

 
a 6-pin system connector

 
small form factor (0.80” x 0.80”).







Functional Description




The two digits on the common cathode seven-segment LED display are each composed of seven segments arranged in a “figure 8” pattern, with an LED embedded in each segment. Segment LEDs can be individually illuminated, so any one of 128 patterns can be displayed on a digit by illuminating certain LED segments and leaving the others dark. Of these 128 possible patterns, the ten corresponding to the decimal digits are the most useful.




The cathodes of the seven LEDs forming each digit are tied together into one “common cathode” circuit node, but the LED anodes remain separate. The common cathode signals are available as two “digit enable” input signals to the display. The anodes of similar segments on both digits are connected into seven circuit nodes that are available from the Pmod connector pins (so, for example, the two “D” anode signals from the two digits are connected to the P4 pin on J1). These seven anode signals are available as inputs to the 2-digit display. This signal connection scheme















560




P1
AA




P2
AB




P3
AC




P4
AD
AA1
AA2




GND


AB1
AB2
Vcc


AC1
AC2


AD1
AD2








AE1
AE2




AF1
AF2
P1
AE
AG1
AG2
P2
AF
C1
C2
P3
AG




P4
C




GND
GND




Vcc
Vcc










Seven-Segment Display Connection Diagram










creates a multiplexed display, where the anode signals are common to both digits but they can only illuminate the segments of the digit whose corresponding cathode signal is asserted.




A scanning display controller circuit can be used to show a two-digit number on the display. This circuit drives the anode signals and corresponding cathode patterns of each






Doc: 502-126 page 1 of 2



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PmodSSD Reference Manual Digilent, Inc.







digit in a repeating, continuous succession, at an update rate that is faster than the human eye can respond. Each digit is illuminated one-half of the time, but because the eye cannot perceive the darkening of a digit before it is illuminated again, the digit appears continuously illuminated. If the update or “refresh” rate is slowed to a given point (around 45 hertz), then most people will begin to see the display flicker.




In order for each of the four digits to appear bright and continuously illuminated, both digits should be driven once every 1 to 16ms (for a refresh frequency of 1KHz to 60Hz). For example, in a 60Hz refresh scheme, the entire display would be refreshed once every 16ms, and each digit would be illuminated for ½ of the refresh cycle, or 8ms. The controller must assure that the correct anode pattern is present when the corresponding cathode signal is driven. To illustrate the process, if Cat1 is asserted while AB and AC are asserted, then a “1” will be displayed in digit position 1. Then, if Cat2 is asserted while AA, AB and AC are asserted, then a “7” will be displayed in digit position 2. If Cat1 and AB, AC are driven for 8ms, and then Cat2 and AA, AB, AC are driven for 8ms in an endless succession, the display will show “17”. An example timing diagram for a two-digit controller is shown below.






Refresh period:
Digit period:


1ms to 16ms


Refresh /
2
Cat1




















Cat2










Anodes
Digit 0
Digit 1


Digit 0












































An un-illuminated seven-segment display, and nine illumination patterns corresponding to decimal digits












Individual anodes


F
A










E
G
C




Two-digit Seven










Segment Display










D




Common cathode





















































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