# Digital Display Circuits

## Digital Circuits

• #### Question 1

What is the purpose of a seven-segment decoder circuit? What is a “seven-segment” display, and why do we need a decoder circuit to drive it? Research the part number for a typical seven-segment decoder circuit (either CMOS or TTL).

• #### Question 2

 Don’t just sit there! Build something!!

Learning to analyze digital circuits requires much study and practice. Typically, students practice by working through lots of sample problems and checking their answers against those provided by the textbook or the instructor. While this is good, there is a much better way.

You will learn much more by actually building and analyzing real circuits, letting your test equipment provide the “answers” instead of a book or another person. For successful circuit-building exercises, follow these steps:

1. Draw the schematic diagram for the digital circuit to be analyzed.
2. Carefully build this circuit on a breadboard or other convenient medium.
3. Check the accuracy of the circuit’s construction, following each wire to each connection point, and verifying these elements one-by-one on the diagram.
4. Analyze the circuit, determining all output logic states for given input conditions.
5. Carefully measure those logic states, to verify the accuracy of your analysis.
6. If there are any errors, carefully check your circuit’s construction against the diagram, then carefully re-analyze the circuit and re-measure.

Always be sure that the power supply voltage levels are within specification for the logic circuits you plan to use. If TTL, the power supply must be a 5-volt regulated supply, adjusted to a value as close to 5.0 volts DC as possible.

One way you can save time and reduce the possibility of error is to begin with a very simple circuit and incrementally add components to increase its complexity after each analysis, rather than building a whole new circuit for each practice problem. Another time-saving technique is to re-use the same components in a variety of different circuit configurations. This way, you won’t have to measure any component’s value more than once.

• #### Question 3

A seven segment decoder is a digital circuit designed to drive a very common type of digital display device: a set of LED (or LCD) segments that render numerals 0 through 9 at the command of a four-bit code:

The behavior of the display driver IC may be represented by a truth table with seven outputs: one for each segment of the seven-segment display (a through g). In the following table, a “1” output represents an active display segment, while a “0” output represents an inactive segment:

 D C B A a b c d e f g Display 0 0 0 0 1 1 1 1 1 1 0 “0” 0 0 0 1 0 1 1 0 0 0 0 “1” 0 0 1 0 1 1 0 1 1 0 1 “2” 0 0 1 1 1 1 1 1 0 0 1 “3” 0 1 0 0 0 1 1 0 0 1 1 “4” 0 1 0 1 1 0 1 1 0 1 1 “5” 0 1 1 0 1 0 1 1 1 1 1 “6” 0 1 1 1 1 1 1 0 0 0 0 “7” 1 0 0 0 1 1 1 1 1 1 1 “8” 1 0 0 1 1 1 1 1 0 1 1 “9”

A real-life example such as this provides an excellent showcase for techniques such as Karnaugh mapping. Let’s take output a for example, showing it without all the other outputs included in the truth table:

 D C B A a 0 0 0 0 1 0 0 0 1 0 0 0 1 0 1 0 0 1 1 1 0 1 0 0 0 0 1 0 1 1 0 1 1 0 1 0 1 1 1 1 1 0 0 0 1 1 0 0 1 1

Plotting a Karnaugh map for output a, we get this result:

Identify adjacent groups of 1’s in this Karnaugh map, and generate a minimal SOP expression from those groupings.

Note that six of the cells are blank because the truth table does not list all the possible input combinations with four variables (A, B, C, and D). With these large gaps in the Karnaugh map, it is difficult to form large groupings of 1’s, and thus the resulting “minimal” SOP expression has several terms.

However, if we do not care about output a’s state in the six non-specified truth table rows, we can fill in the remaining cells of the Karnaugh map with “don’t care” symbols (usually the letter X) and use those cells as “wildcards” in determining groupings:

With this new Karnaugh map, identify adjacent groups of 1’s, and generate a minimal SOP expression from those groupings.

• #### Question 4

A seven segment decoder is a digital circuit designed to drive a very common type of digital display device: a set of LED (or LCD) segments that render numerals 0 through 9 at the command of a four-bit code:

The behavior of the display driver IC may be represented by a truth table with seven outputs: one for each segment of the seven-segment display (a through g). In the following table, a “1” output represents an active display segment, while a “0” output represents an inactive segment:

 D C B A a b c d e f g Display 0 0 0 0 1 1 1 1 1 1 0 “0” 0 0 0 1 0 1 1 0 0 0 0 “1” 0 0 1 0 1 1 0 1 1 0 1 “2” 0 0 1 1 1 1 1 1 0 0 1 “3” 0 1 0 0 0 1 1 0 0 1 1 “4” 0 1 0 1 1 0 1 1 0 1 1 “5” 0 1 1 0 1 0 1 1 1 1 1 “6” 0 1 1 1 1 1 1 0 0 0 0 “7” 1 0 0 0 1 1 1 1 1 1 1 “8” 1 0 0 1 1 1 1 1 0 1 1 “9”

Write the unsimplified SOP or POS expressions (choose the most appropriate form) for outputs a, b, c, and e.

• #### Question 5

Examine the datasheet for a 7447 BCD-to-7-segment decoder/driver IC, and identify what input conditions need to be met in order to cause it to display any decimal digit from 0 to 9.

• #### Question 6

Liquid crystal display (LCD) elements require the application of AC voltage rather than DC voltage to prevent certain undesirable effects. Since logic circuits typically operate on DC power (VCC or VDD and Ground), there must be some clever way of generating the necessary AC from DC logic power in order to drive these power-thrifty display devices. Indeed, it just so happens that Exclusive-OR gates do the trick quite nicely:

Consider the square wave voltage source in this schematic as a source of alternating “high” and “low” logic states, 5 volts and 0 volts respectively. Determine what sort of voltage exists across the liquid crystal fluid with the switch in the open position as well as the closed position, and from this determine which switch position results in a darkened LCD versus a transparent LCD.

• #### Question 7

A feature found on most 7-segment decoder/driver ICs is called ripple blanking. Describe what this feature is, and why it is used. Hint: a good source of information on this subject is a datasheet for a 7-segment decoder/driver IC.

• #### Question 8

It is usually necessary to have more than one display digit for a digital system. The most obvious and direct way of driving multiple 7-segment display units is to use an equal number of BCD-to-7-segment decoders like this:

If we are driving the decoder ICs with a microprocessor or microcontroller, this direct technique unfortunately uses a lot of I/O pins. In this particular case, with three 7-segment displays, we would need to use twelve output pins on the microcontroller for the three BCD numbers:

Due to limited pin count on most MPU and MCU chips, I/O lines are precious. It would be a shame to waste so many on a simple function such as driving display digits when we could use them for other tasks such as interfacing with memory devices, receiving real-world data from sensors, driving discrete control devices such as lights and solenoids, or communicating with other MPU/MCU systems. But if each digit requires four output lines for the BCD number, how can we possibly use less than twelve output lines on the processor?

One clever way to do just this exploits persistence of human vision, by driving only one digit at a time. Examine the following circuit, then explain how this “multiplexed” display system works with so few output lines. Also identify what steps the MCU/MPU must take to successfully drive all three digits so the display looks continuous:

• #### Question 9

Digital computers communicate with external devices through ports: sets of terminals usually arranged in groups of 4, 8, 16, or more. These terminals may be set to high or low logic states by writing a program for the computer that sends a numerical value to the port. For example, here is an illustration of a microcontroller being instructed to send the hexadecimal number 2B to port A and A9 to port B:

Suppose we wished to use the first seven bits of each port (pins 0 through 6) to drive two 7-segment, common-cathode displays, rather than use BCD-to-7-segment decoder ICs:

Write the necessary hexadecimal values to be output at ports A and B to generate the display “42” at the two 7-segment display units.

• #### Question 10

The MM58342 high-voltage display driver IC from National Semiconductor serves as an interface between either a microprocessor or microcontroller and a high-voltage vacuum fluorescent (VF) display panel. The IC reads and conditions 20 bits of data to drive 20 “grids” in such a display. When combined with a similar driver driving the anodes of the same VF display, individual pixels (or combinations of pixels) may be controlled (lit).

An interesting feature of this IC is that it receives the 20 bits of data serially (one at a time), through a single input pin:

Read the datasheet for this device, then comment on why you think a serial (rather than parallel) data input format was chosen. Also describe the sequence of operation for loading data into this IC and outputting that data to the 20 output lines.

• #### Question 11

One new technology entering the market is organic light-emitting diodes, or OLEDs. Describe what these are, and why they hold so much promise for electronic display device elements.

• #### Question 12

Two electronics students attempt to build 7-segment display circuits, one using a 7447 decoder/driver IC and the other using a 7448. Both students connect their ICs to common-cathode 7-segment displays as such:

The student using the 7448 notices the LED segments glowing faintly, but the patterns are not correct for the digits that are supposed to be displayed. The student using the 7447 has an even worse problem: no light at all! Both have checked and re-checked their wiring, to no avail. It seems as though all the connections are in the right place.

What do you think the problem is? Hint: consult datasheets for both chips to find clues!

• #### Question 13

An obsolete display technology that still finds enthusiastic followers in the hobbyist world, called Nixie tubes, relies on a BCD-to-10 decoder to drive one of ten different metal cathodes inside a neon-filled glass bulb. For each BCD code, exactly one of the cathode figures inside the “Nixie tube” will glow, causing that numeral to shine with a pink-orange light. The tube receives power through a common anode (usually over 150 volts DC).

A friend of yours is trying to build his own Nixie tube display circuit, but is experiencing problems. He wants to use a 7442 BCD-to-10 decoder to drive ten discrete transistors, each one handling the current for a digit in the tube. Being cautious, your friend decides to connect just one of the Nixie tube digits to a transistor, and then to the 7442, to see if the idea works (before connecting all ten).

Unfortunately, that one digit begins to glow the moment the high-voltage DC supply is turned on, even before the 7442 chip receives power! And after that, the 7442 gets warm to the touch, which is not good.

Thinking he has damaged the 7442 IC, your friend turns to you for advice. Did he do anything wrong here? Explain what advice you would give to him.

• #### Question 14

Liquid crystal display (LCD) technology used to have very narrow viewing angles. Anyone who remembers the first LCD displays on portable personal computers will recall how you could only see the display if you viewed it perpendicular to the display surface, or at a very slight angle from perpendicular.

Modern LCD technology is much better, is still not as good as viewing printed paper, the “gold standard” for non-emissive display. One term frequently used to describe the quality of viewing with regard to angle is Lambertian. Define what “Lambertian” means with regard to display surfaces.

• #### Question 15

One method of driving pixels in a grid-based display is to organize the pixels into rows and columns, then select individual pixels for illumination by the intersection of a specific row line and a specific column line. In this example, we are controlling an 8 × 8 grid of LEDs with two 8-bit (1-byte) ports of a microcontroller:

Note that a high state is required on one of port B’s pins to activate a row, and a low state is required on one of port A’s pins to activate a column, because the LED anodes connect to port A and the LED cathodes connect to port B.

Determine the hexadecimal codes we would need to output at ports A and B to energize the LED in the far lower-left corner of the 8 × 8 grid.

Port A =

Port B =