Introduction to 7 Segment Display Driver ICs

7 segment display driver ICs are integrated circuits designed to interface microprocessors or microcontrollers with 7 segment displays. They convert binary coded decimal (BCD) inputs to drive the individual segments of the display. Some common 7 segment driver ICs include:

• 7447 – Basic 7 segment decoder/driver
• 7448 – BCD to 7 segment decoder with active low outputs
• 74HC595 – Serial in, Parallel out shift register that can drive 7 segments

7447 Pin Diagram and Description

The 7447 IC has the following pin configuration:

• Pin 1-7 (A-G): Cathode outputs of the 7 segments
• Pin 8: Decimal point output (DP)
• Pin 9-12: BCD inputs D0-D3
• Pin 13: Lamp test input (LT)
• Pin 14: Ripple blanking input (RBI)
• Pin 15,16: Power supply pins

The 7447 takes 4 bit BCD input at pins 9-12 and decodes it to drive the 7 segments at pins 1-7. An active low at the RBI pin blanks the display. LT pin lights all the segments when low.

Circuit Diagram and Operation

Here is a typical 7447 circuit driving a common cathode 7 segment display:

The 4 bit BCD data is applied to pins 9-12. This is decoded and drives the segments through current limiting resistors. A transistor can be used instead for higher current displays. The decimal point pin 8 drives the DP segment directly.

Sample Code and Waveforms

Here is a sample Arduino code to test 7447 with counter output:

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int latchPin = 11; //Pin connected to ST_CP of 74HC595 int clockPin = 9; //Pin connected to SH_CP of 74HC595 int dataPin = 12; //Pin connected to DS of 74HC595 void setup() { pinMode(latchPin, OUTPUT); pinMode(clockPin, OUTPUT); pinMode(dataPin, OUTPUT); } void loop() { for(int number = 0; number < 1024; number++) { digitalWrite(latchPin, LOW); shiftOut(dataPin, clockPin, MSBFIRST, number); digitalWrite(latchPin, HIGH); delay(500); } }

This generates a counting pattern on the display. The waveforms will be as follows:

Common Applications

• Driving single and multiple 7 segment LED/LCD displays
• Displaying digits, counters, small alphanumeric info
• Embedded systems, IoT devices to display stats
• ATM, electronic items, clocks, meters etc.

Advantages and Disadvantages

Advantages:

• Simple 3 to 8 line decoding
• No need for segment driving transistors
• Internal output buffers for driving segments
• Easy interface with microcontrollers

Disadvantages:

• Fixed integrated logic, not programmable
• Limited drive current (25mA max)
• Prone to damage from static discharge

Conclusion

The 7447 IC provides a easy way to interface 7 segment LED or LCD displays in digital circuits and systems. With its built-in decoding logic, it simplifies the design and reduces component count. Some care needs to be taken regarding the drive current capabilities and static protection. Overall, it is a versatile and useful driver IC for most applications involving display of numeric data.

Frequently Asked Questions

Q1. How is 7447 different from 7442 IC?

A1. The 7442 is a BCD to decimal decoder IC which drives only 4 outputs (0 to 9). The 7447 has built-in logic to drive 7 segments and decimal point in addition to decoding BCD input.

Q2. Can 7447 directly drive 7 segment displays?

A2. No, the output current of 7447 pins is limited to 25mA maximum. For common cathode displays, transistors are needed to provide the drive current. For common anode displays, resistors can be used.

Q3. How many 7447 ICs needed for a 4 digit 7 segment display?

A3. Only one 7447 is required. The BCD data corresponding to the 4 digits has to be multiplexed to feed the single 7447 serially. The 4 common cathode or anode terminals will control which digit lights up.

Q4. Is 7447 suitable for multiplexed displays?

A4. Yes, 7447 can be easily interfaced with multiplexed 7 segment displays by controlling the cathode or anode of each digit and feeding the BCD data correspondingly.

Q5. What is the function of LT and RBI pins in 7447?

A5. LT pin lights up all the segments when driven low, useful for testing the display. RBI blanks the display by inhibiting the outputs when driven low.