Basics of Decoder

Decoder is a combinational logic circuit that converts abinary code into the desired output signals. It is called decoder because it performs the reverse process of encoder. The process of converting binaryinput code into desirable output is known as decoding.

Figure 1 is the logic symbol of decoder with ‘n’ inputs and ‘m’ outputs. In short, it is multiple inputs and multiple outputs device with proper conversion system.

Note that decoder performs

the reverse operation of the encoder. A decoder has ‘n’ number of input lines and ‘m’ number of output lines. There are 2n possible input combinations and the number of outputs (m) are less than or equal to 2n.

Decoder converts an n bit code to single active output. The information present in one code is obtained back into the desired code. In general Decoder is circuit which convert specific code

into more general form of code.

A decoder is a logic circuit that accepts a set of inputs that represents a binary number and activates only the output that corresponds to the input binary number.

In other words, a decoder circuit looks at its inputs, determines which binary number is present and activates the one output that corresponds to that number; all other outputs remain inactive.

The basic function of a decoder is used to detect the presence of a specified combination of bits (code) on its inputs and to indicate the presence of that code by a specific output level. In its general form, a decoder has n input lines to handle n bits and from one out of 2^n output lines to indicate the presence of one or more n bit combinations.

3 to 8 (Binary to Octal) decoder

In three to eight decoder, there are three inputs and eight outputs, as shown in figure 2. S0 is the least significant variable, while S2 is the most significant variable. The three inputs are decoded into eight outputs namely Y0, Y1,…Y7 . The function table indicates, binary values at the input from eight combinations activate the corresponding output line.

This 3 to 8 line decoder is also known as binary to octal decoder.

In three to eight decoder, there are three inputs and eight outputs, as shown in fig. 3 A 3 to 8 line decoder generates 8 product terms corresponding to each binary code applied at the three inputs.

Thus there is a need of eight 3-input AND gates and 3 NOT gates to generate the complement of input along with un-complemented input. It is possible to verify the truth table by providing input condition and checking whether desired product term is activated on not.

In many decoders, there is an enable function, a LOW level on input E1,E2 and a HIGH level on input E3, is required in order to make the enable gate output HIGH and to enable the decoding action.

Input binary code makes only the respective output High and other outputs will be inactive i.e. Low. When select inputs are 000 then the decoder generates only Y0=1 and other outputs are 0. From the truth table it is clear that the input binary code decides which output is to be activated.

Special decoder

Let us now get familiar with Special Decoders. The binary decoders studied so far generates only one output of many in response to the binary inputs to the decoder. Whereas special decoder activates more than one outputs simultaneously for the applied input binary code.

Many digital types of equipment require some means for displaying information. One of the simplest and most popular methods for displaying numeric digits utilizes seven segment configuration to for numbers 0 to 9 and sometimes the hex characters A to F.

Special decoders are designed to drive the Seven Segment display.

Each segment is made up of material that emits light when current is passed through it. Most commonly used devices include light emitting diode (LEDs). The seven segment display has 7 LEDs for the segments and one additional LED for dot i.e. used as decimal point.

There are two configurations for seven segment display. These are Common Anode and Common Cathode Display.

BCD to seven segment decoder

Designer can select either common anode or common cathode depending on the application. The logic symbol of BCD to seven segment decoder / driver has 4 inputs and eight outputs is shown in fig. 12. This BCD to seven segment decoder generates a seven segment code in response to binary code for driving the seven segment display.

In order to display a decimal digit, it necessary for the decoder to activate the desired segment. Let us now prepare a table for displaying a decimal digit and corresponding activated segments. For displaying digit 0, LED segments a,b,c,d,e and f must be activated and segments g and dp are OFF.

Let us now find out activated segments for displaying digit 1. Here only b and c segments must be activated and other segments are deactivated. For displaying digit 2, LED segments a,b,g,e,d are activated and so on.

Let us now study the actual interfacing of BCD to seven segment decoder/driver with the seven segment display as shown in fig. 13. Here limiting resistors are connected between them. The resistor value decides the brightness of the segment.

Let us look at the commercially available BCD to seven segment decoder / driver ICs. IC 7446 and 7447 has open collector output and the decoder requires active LOW output to drive the common anode display configuration of 7 segment display. Whereas IC 7448 is recommended for driving common cathode displays.

7446 - Active low , open collector - 30v, 40mA.
7447 - Active low, open collector - 15v, 40mA.
7448 - Active High, pull up Resistor- 5.5v, 6mA.
7449 - Active High, open collector- 5.5v, 8mA.

IC 7446/47 is a TTL IC for driving a common anode display as shown in this figure 15. Common anode terminal is connected to +Vcc. Decoder IC has open collector outputs where seven segments are connected through the current limiting resistors. Logic circuit inside the IC converts BCD input to a required output.

IC 7448 is a TTL IC for driving a common cathode display as shown in this figure 16. Common cathodes of all LED segments are connected to common ground.The decoder IC has its own current limiting resistor on the chip. Logic circuit inside the IC converts BCD input to a required output.

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