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The CD4511 IC: A Versatile CMOS BCD to 7-Segment Decoder/Driver

The CD4511 IC A Versatile CMOS BCD to 7-Segment DecoderDriver

Introduction

The CD4511 is a CMOS integrated circuit that can decode 4-bit binary coded decimal (BCD) inputs into 7-segment outputs for displaying numeric information. It contains an on-chip oscillator, latch, and high current output drivers, making it a versatile choice for many digital display applications. In this article, we will take a deep dive into the features and capabilities of the CD4511 IC.

Key Features of the CD4511

Some of the key features of the CD4511 BCD to 7-segment decoder/driver include:

  • Decodes 4-bit BCD to drive 7-segment displays
  • Includes on-chip oscillator
  • Internal latch stores BCD code
  • High current output drivers (15mA sink/source)
  • Wide supply voltage range: 3V – 15V
  • Low power consumption
  • High noise immunity CMOS design
  • 18-pin DIP package

The on-chip oscillator provides clock signals for timing and multiplexing, while the latch allows BCD inputs to be stored until updated. The high current drivers can directly control common cathode 7-segment LED or LCD displays.

Block Diagram and Pinout

The internal block diagram of the CD4511 is shown below:

The 18 pins of the CD4511 are described in the table below:

PinDescription
1Lamp Test Input
2Display Blank Input
3Latch Clock Input
4Latch Reset
5VSS (Ground)
6Oscillator Frequency Adjust
7Oscillator Input
8Oscillator Output
9Digit Driver Output A
10Digit Driver Output B
11Digit Driver Output C
12Digit Driver Output D
13Segment Driver a
14Segment Driver b
15Segment Driver c
16Segment Driver d
17Segment Driver e
18VDD (+5V Supply)

The BCD inputs are provided on pins 9-12, while the 7-segment outputs are on pins 13-17. The oscillator circuit is connected to pins 6-8.

Internal Operation

The CD4511 contains a 4-bit latch and BCD to 7-segment decoder logic. The latch is controlled by the clock and reset pins. When the latch clock is pulsed, the BCD input is loaded into the latch.

The decoder logic then converts the latched BCD value into the appropriate 7-segment drive signals. These segment output signals remain constant until the latch is updated with a new BCD value.

An on-chip oscillator provides a clock signal that can be used to multiplex between different BCD inputs. The digit driver outputs (pins 9-12) are intended to control the anodes of separate 7-segment displays.

By strobing these digit outputs sequentially, we can display multiple digits by time-multiplexing. The onboard oscillator generates the necessary timing signals for this multiplexing function.

Oscillator Design

The oscillator circuit built into the CD4511 IC is designed to operate at frequencies between 50Hz to 5kHz. This is appropriate for multiplexing 2 to 5 digits at a refresh rate visible to the human eye.

The oscillator frequency is determined by the external timing resistor (Rosc) connected from pin 7 to ground. The formula for calculating the oscillation frequency is:

fosc = 1 / (0.7(Rosc + 2.5kΩ))

Where fosc is in Hz when Rosc is in ohms.

For example, to generate a 2kHz clock we can use:

Rosc = 1 / (0.7 * (2000Hz) ) - 2.5kΩ = 1.43kΩ

The closest standard value resistor is 1.5kΩ.

We can also adjust the frequency slightly by connecting a trimmer potentiometer to pin 6. This allows “fine tuning” the oscillation frequency.

Typical Application Circuit

A typical application circuit for the CD4511 to drive a 4-digit 7-segment LED display is shown below:

This configuration multiplexes between four 7-segment displays using the onboard oscillator and digit driver outputs. The 470Ω resistors limit the segment current through each LED.

To display a number we simply load the appropriate BCD digits into the CD4511 latch for each display position. The oscillating digit strobes will then multiplex, showing the desired value on the 7-segment displays.

Advantages and Disadvantages

Some of the main advantages of using the CD4511 IC include:

  • Integrates decoding, driving, timing, and multiplexing functionality
  • Eliminates need for separate 7-segment decoder ICs
  • Internal oscillator simplifies design
  • Wide supply voltage range
  • Directly interfaces with common cathode LED displays

Some disadvantages or limitations include:

  • Limited to 4-bit BCD input
  • Fixed oscillator frequency
  • Only sinks segment current (no source capability)
  • Outputs cannot be combined to drive larger displays

FAQ

What is the maximum supply voltage for the CD4511?

The CD4511 can operate from a 3V to 15V power supply. The absolute maximum rating is 18V.

How much current can the outputs drive?

Each output pin can sink or source up to 15mA. This is sufficient to drive most common cathode 7-segment LED displays.

Can the CD4511 be used with common anode displays?

No, the CD4511 only sinks current and cannot be used directly with common anode displays. An external transistor interface would be required.

What is the difference between the CD4511 and CD4543?

The CD4543 is also a BCD to 7-segment decoder, but it has higher current sinking capability of up to 25mA per output. The CD4543 does not contain an onboard oscillator.

How many external components are needed with the CD4511?

A minimum of 1 resistor for the oscillator timing, 4 current limiting resistors for the segments, and a supply bypass capacitor are needed. For multiple digit displays, additional current limiting resistors for each digit are also required.

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