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ECE2029: Introduction to Digital Circuit Design

Lab 4 – Convert 8-Bit Binary to BCD Display

1. Objective

The objective of this lab is to build complicated combinational logic circuits, such as the binary

to BCD converter. This lab will also be a transition from combinational logic to sequential logic

circuits, such as counters. We will also apply the techniques learned from lectures on

multiplexers and decoders to implement 4-digit 7-segment LED display.

The lab signoff is to use 8 dip switches to input a binary number and the 7-segment display

shows the corresponding number in 4-digit decimal format. It is fine to have leading 0’s in

display, e.g. 810 is shown as “0008” in display. The input number range is (00~FF)16 and the

decimal number on display is (000~255)10.

2. Understand how multiple 7-segment LEDs work on Basys 3 board

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From your experience in Lab 3, all 4-digit of 7-segment display the same number. This is because

all of their inputs a~g are tied together. We can turn each individually off by set the AN0 or AN1

or AN2 or AN3 to 1. Then how can we display 4 different numbers on this display?

The idea is to “trick your eyes”. You will display one digit at one LED for a short period of time,

by turning on only 1 LED and turn off the other 3 LEDs. Then you repeat for the next digit. If

you do this fast enough, human eyes cannot catch the on/off switching activities. The following

information is extracted from the Basys 3 reference manual on page 16.

A scanning display controller circuit can be used to show a four-digit number on this display. This circuit drives the

anode signals and corresponding cathode patterns of each digit in a repeating, continuous succession, at an update

rate that is faster than the human eye can detect. Each digit is illuminated just one-quarter 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 around 45 hertz, most people will begin to see the display

flicker.

In order for each of the four digits to appear bright and continuously illuminated, all four 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 ¼ of the refresh cycle, or 4ms.

In this lab, we will only need 3 decimal digits for BCD display. So the left most LED is

actually turned off by setting AN4 = ‘1’. We will design a decoder and a mux module to

implement the switch of data and AN signals simultaneously.

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3. System Block Diagram

Figure 1. System diagram for BCD display on 7-segment LEDs

Among the blocks in Fig. 1, the binary-to-BCD is given and tested in the prelab. The binary-to7segment display decoder was built in Lab 2. If you did not complete Lab2, you can use the

following Verilog code to implement the binary-to-7segment decoder.

module bcd7seg(input[3:0] Y,

output reg[6:0] disp);

[email protected](Y)

begin

case(Y)

0: disp=7’b0000001;

1: disp=7’b1001111;

2: disp=7’b0010010;

3: disp=7’b0000110;

4: disp=7’b1001100;

5: disp=7’b0100100;

6: disp=7’b0100000;

7: disp=7’b0001111;

8: disp=7’b0000000;

9: disp=7’b0000100;

10: disp=7’b0001000;

11: disp=7’b1100000;

12: disp=7’b0110001;

13: disp=7’b1000010;

14: disp=7’b0110000;

15: disp=7’b0111000;

endcase

end

endmodule

Switches

SW7~SW0

Binary-toBCD

converter

4-to-1

Mux

Binary-to7Segment

Decoder

2-bit

Counter

CLK

(100Hz)

4’b0

//Here 7 bits represent 7 segments (a to g) on the display. Remember we

provide active-low logic to the display. To display 0 , all segments (or

bits) are set to 0 except for g. To display 1, b and c are set to zero, and

rest are all 1s.

//Describing an event that should happen to Y when a certain condition is met.

//The case statement is a decision instruction that executes the statement.

//Declaring outputs as register.

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4. Slow Clock at 100 Hz

In order to rotate the LED display properly, we need to generate a slow clock. As stated in the

Basys 3 reference manual, the refresh clock should be in the range of 60 Hz to 1 KHz. As you

can experiment later in this lab, the LED display is not stable if switching too fast. If the switch

frequency is slower than 45 Hz, you can clearly see the “shifting digits”. We opt to use a slow

clock at 100 Hz, or a clock period of 10ms. The default system clock on the Basys 3 board is 100

MHz or a clock period of 10ns. The following code implements a counter to count the numbers

of clock cycles to 500,000 that is equivalent to a time period of 5ms. If we hold the output signal

‘0’ for 5ms, ‘1’ for another 5ms and repeat, we generate a 100 Hz output clock.

module slowclock (input clk_in,

output reg clk_out);

reg [20:0] period_count = 0;

always @ (posedge clk_in)

if (period_count!= 500000 – 1)

begin

period_count<= period_count + 1;

clk_out <= 0; //clk_out gets 0.

end

else

begin

period_count <= 0;

clk_out <= 1;

end

endmodule

5. 2-bit Counter

In order to send the 4 BCD digits to display iteratively, we need to design a 2-bit counter. The

following code implements a 2-bit counter. We will cover the design of counters and more

complicated sequential logic circuits in class in the following weeks.

module my_counter(input clk,

output [1:0] Q);

reg [1:0] temp = 0;

always @(posedge clk) begin

temp = temp + 1;

end

assign Q = temp;

endmodule

//Clock input of the design, output clock

//We trigger the clock with respect to positive (rising) edge of the clock.

//Increment period_count by 1.

//If the statement is not true, period_count not equivalent to 499000.

//If above statement is true

period_count gets 0

clk_out gets 1.

//clk is input, two outputs Q[1]–MSB, Q[0]–LSB.

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The purpose of the 2-bit counter is to generate a control signal. Table below shows the function

of the counter in the mux and decoder.

2-bit Counter Output 4-to-1 Mux Output Decoder Output [3:0] AN

00 ONES 1110

01 TENS 1101

10 HUNDREDS 1011

11 THOUSANDS (4’b0) 1111

6. 4-to-1 Mux

In this section, you will design and implement a 4-to-1 mux as in the diagram.

There are many different ways to implement a 4-to-1 mux in Verilog. The following is an

example code for an implementation. You can design your 4-to-1 mux with different signal

names.

module mux4to1( input [3:0] A,

input [3:0] B,

input [1:0] C,

input [3:0] D,

input [1:0]

sel, output

[3:0] Y);

assign Y = (sel==0)?A : (sel==1)?B : (sel==2)?C : D;

endmodule

Note that we only need 3 digits for BCD display in this lab (ONES, TENS and HUNDRES). You

can modify the 4-to-mux code to set one input signal D[3:0] to all 0’s. Alternatively, you can

map the input signal D to all 0’s when you connect the modules together at the top level.

sel[1] sel[0]

A[3:0]

B[3:0]

C[3:0]

D[3:0]

Y[3:0]

//selector switches decide

what data at the input data

lines go through to the

output (Y).

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7. Decoder

As we learned in class, we also need to build a 2-to-4 decoder to turn the 7-segment LED on/off

alternately. The following Verilog code is a standard 2-to-4 decoder.

module decoder2to4 (input [1:0] en,

output reg [3:0] an);

[email protected](en) begin

case (en)

0: an=4’b1110;

1: an=4’b1101;

2: an=4’b1011;

3: an=4’b0111;

endcase

end

endmodule

You need to modify the code above to turn the left-most LED permanently off. Then it only

shows 3 BCD digits in display. Hint: a very small modification will do the trick.

2-to-4 decoder

AN[0]

AN[1]

AN[2]

AN[3]

EN[0]

EN[1]

//When both en[0] and en[1] are 0, an[0] is 0, and rest of the o/ps are 1.

//When en[0] is 0 and en[1] is 1, an[1] is 0, and rest of the o/ps are 1.

//Declaring output (4, each used to enable each 7-segment display)

as registers.

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8. Put the system together

Finally, we will need to have a top-level design to put all. This will be your top module, add all

modules as sources to this as shown below:

module bcddisplay4(input clk, input [7:0] sw,

output [3:0] an,

output [6:0] seg);

parameter zero = 4’b0000;

wire clk_out;

wire [3:0] mux_out;

wire [1:0] counter_out;

wire [3:0] ones, tens, hundreds;

binary_to_BCD u0(sw, ones, tens, hundreds);

mux4to1 u1(ones, tens, hundreds, zero, counter_out, mux_out);

slowclock u2(clk, clk_out);

my_counter u3(clk_out, counter_out);

decoder2to4 u4(counter_out, an);

bcd7seg u5(mux_out, seg);

endmodule

The .xdc file (constraint file) is similar to Lab3, except for the addition of clock signal. It should include

8 input switches (sw[0] to sw[7]), clock signal (clk), 7 segment display (seg[0] to seg[7]), and 4 enables

(an[0] to an[3]).

Generate a programming (.bit) file and download your design to the Basys 3 board. Show the lab

TA that your completed design is working for sign-off.

//See Figure 1. Clock and 8-switches are inputs.2 outputs

(enable, one each for each 7-segment display, and; seg [6:0]

for a to g).

//Left most display is set to 0 because the range for 8-bit is 0 to 255.

//Declaring everything (all modules) that connects input and output modules as wires.

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# Lab 4 – Convert 8-Bit Binary to BCD Display

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