Lab 6: BCD-to-decimal decoder and Hexcidecimal-to-Seven-Segment Decoder

2023 - Robucci – removed mux for the sake of moving lab earlier in semester. Mentioned complementary at-home exercise. Added description of seven-segment LED with animations.

Table of Contents

Objective and Overview

In this lab, you will be

• verifying the operation of a BCD-to-decimal decoder (74LS42)
• verifying the operation of a hexadecimal to seven-segment decoder (MC14495)
• learning the operation of a seven-segment LED display

Required Equipment

• 7442: BCD-to-decimal decoder
• MC14495: Hex to Seven Segment Decoder
• Seven-Segment LED Display
• LEDs
• Resisors for current limiting LED current
• DIP Switch (4 switches)
• Resistors 4x 10kΩ (for input pull-up)

Discussion

Input Generation

In this lab the standard Quad DIP switch with pull-up resistors will be used for inputs.

Output Display

First Circuit 10 LED output

For the first circuit, 10 discrete LEDs and 10 current-limiting resistors can be used. The output of the 74LS42 is ACTIVE LOW. There are OVERBARS (e.g. $\overline{3}$) on the pin labels in the datasheet to indicate this.

Second Circuit Seven-Segment LED output

For the second circuit, a Seven-Segment LED Display will be used. It has seven primary segments which can be independently activated to display characters. Additionally, this display has a right display point. This display has many LEDs with their cathodes tied together with the intent of a common connection to a low supply terminal. This configuration is called common-cathode.

(Circuit 1) BCD Decoder 7442

• “Binary-Coded Decimal” Decoder
• https://www.futurlec.com/74LS/74LS42.shtml
• 4 inputs, 10 outputs
• Input 0000 activates Out 0, input 0001 activates Out 1, etc.
• Active low outputs: the selected output is low, while other outputs are high. This means and LED should be connected in a path from the output to positive power supply terminal, with series current-limiting resistor.

(Circuit 2) Hexadecimal-to-seven segment Latch/Decoder LED Driver (MC14495 IC)

• The MC14495 (https://www.futurlec.com/Motorola/MC14495P1.shtml) can be used to display a hexadecimal character on a seven-segment LED display.
• Inputs A,B,C,D are used to signal a 4-bit value.
• The outputs a-g can be directly connected to a seven segment display.

Prelab

For this lab, the instructor is providing a complementary exercise related to the design of the logic for one of the decoders. That exercise will be turned in as part a lab report, and does not nessisarily need to be completed before lab. Therefore there is no pre-lab submission. Once you complete the lab activity, the exerience may serve as a reference for understanding the logic design goal (i.e. the function of the decoder) for the at-home exercise.

You should otherwise use time before lab to plan your placement and wiring, and/or you may start assembly of your circuit before lab.

In-Lab Exercise

BCD-to-decimal

1. Connect the four DIP switch signals to the four inputs A,B,C,D of the 74LS42 BCD decoder.
2. Use ten output indicator circuits (resistor and LED) to observe the ten active low-outputs.
3. Demonstrate the operation of the circuit to the TA.

📝 TA Verification

Have the TA check and record your completion.

Hex to Seven-Segment Decoder

1. Without disrupting the previous circuits, connect the four DIP switch signals to the four inputs A,B,C,D of the MC14485 IC.
2. Connect MC14485 LE (pin 7) to ground
3. Connect MC14485 outputs a-g to appropriate inputs a-g of the seven-segment display.
4. Connect the CC (pin 8) of the seven-segment display to ground.
5. Demonstrate the operation circuit to the TA. Test input values 0-9, as well as 10-15.

📝 TA Verification

Have the TA check and record your completion.

Grading and Submission

50% Completion of In-Lab activity (verified physical circuit demonstration). This lab is grading for completion only, based on demonstration to the TA.
No report is otherwise required for the in-lab activity.

50% At-home exercise on decoder logic design. This is provided by the instructor.