What is a 4 Bit Decoder IC?
A 4 bit decoder IC is an integrated circuit that converts a 4 bit binary number into its associated output. It has 4 inputs that represent the 4 bits and up to 16 outputs that are activated based on the input code.
Decoders are used in a wide range of digital logic applications where binary information needs to be converted into control signals to drive various components. Some common uses of 4 bit decoders include:
- Memory address decoding
- 7 segment display driving
- Demultiplexing
- Output port expansion
How Does a 4 Bit Decoder IC Work?
A 4 bit decoder has 4 input pins labeled A, B, C and D that represent the 4 bits of the binary number. These input bits feed into the internal logic gates that activate the correct output pin based on the binary input code.
For example, if the input code is 0110, it will activate output pin 6. Each output pin corresponds to one of the 16 possible input binary codes from 0000 to 1111.
The truth table below shows the input codes and corresponding output pins for a 4 bit decoder:
| Inputs | Outputs |
|---|---|
| A B C D | D3 D2 D1 D0 |
| 0 0 0 0 | 1 0 0 0 |
| 0 0 0 1 | 0 1 0 0 |
| 0 0 1 0 | 0 0 1 0 |
| 0 0 1 1 | 0 0 0 1 |
| 0 1 0 0 | 0 0 0 0 |
| 0 1 0 1 | 0 0 0 0 |
| 0 1 1 0 | 0 0 0 0 |
| 0 1 1 1 | 0 0 0 0 |
| 1 0 0 0 | 0 0 0 0 |
| 1 0 0 1 | 0 0 0 0 |
| 1 0 1 0 | 0 0 0 0 |
| 1 0 1 1 | 0 0 0 0 |
| 1 1 0 0 | 0 0 0 0 |
| 1 1 0 1 | 0 0 0 0 |
| 1 1 1 0 | 0 0 0 0 |
| 1 1 1 1 | 0 0 0 0 |
The decoder IC uses these truth table logic to map the inputs to the correct output pin.
Common 4 Bit Decoder ICs
Some popular 4 bit decoder ICs are:
- 74LS139 – Dual 2-to-4 line decoder
- 74HC139 – Faster CMOS version of 74LS139
- 74LS238 – 3-to-8 line decoder
- 74HC238 – Faster CMOS version of 74LS238
- CD4514 – 4-to-16 line decoder
- CD4028 – 4-to-10 line decoder
These provide 2, 3 or 4 bit decoding in various package types. The 74LS139 is one of the most widely used 4 bit decoders.
Applications of 4 Bit Decoders
Here are some common applications of 4 bit decoder ICs:
- 7 segment LED display driving – A 4 bit decoder can directly drive a 7 segment LED display. The outputs of the decoder are connected to the segments of the display.
- Memory address decoding – In memory address bus, decoders are used to activate the correct memory location based on the address bits.
- Demultiplexing – Decoders can be used to route input signals to various outputs, acting like a demultiplexer.
- Output port expansion – A 4 to 16 line decoder can expand 4 bit I/O ports to drive up to 16 devices.
Pros and Cons of 4 Bit Decoders
Pros:
- Simple TTL/CMOS IC package for basic decoding needs
- No external gates required
- Available in through-hole DIP and surface mount packages
Cons:
- Limited by number of bits (only 4 bits)
- Require external latch for holding data
- Propagation delay can be slower compared to discrete gates
Conclusion
4 bit decoders are versatile digital logic ICs that can convert binary information into selection of output lines. Their simple package and wide availability makes them useful for addressing, decoding, multiplexing and driving applications. With a basic understanding of their internal logic and truth tables, 4 bit decoders can be easily deployed in a wide range of electronic designs.
Frequently Asked Questions
Here are some common questions about 4 bit decoder ICs:
Q: What is the difference between a BCD decoder and a binary decoder?
A: A BCD (Binary Coded Decimal) decoder is designed to accept BCD codes as inputs rather than binary. BCD uses 4 bits to represent decimal digits from 0 to 9, rather than 0 to 15 as in binary. So a BCD decoder will have 10 outputs instead of 16.
Q: How do I select a particular decoder IC for my application?
A: Consider voltage levels, speed, number of outputs needed and package type. CMOS decoders like 74HC139 are fast with decent voltage tolerance. DIP through-hole packages are easier to breadboard while SMD packages are smaller.
Q: Can I cascade two 4 bit decoders to make an 8 bit decoder?
A: Yes, two 4 bit decoders can be cascaded by connecting the upper 4 bits to one and lower 4 bits to the other decoder IC. Their outputs need to be combined through gates to create a full 8 bit decoder.
Q: Why do some decoders have active low outputs?
A: Some decoders have output enable pins that invert the outputs. This active low operation allows AND/OR gating of the outputs for more flexibility but need inverters on each output.
Q: How do I drive a 7 segment display with a 4 bit decoder?
A: Simply connect the A,B,C,D inputs to the 4 bit data bus and each output bit to the appropriate segments a,b,c,d,e,f,g on the display via current limiting resistors. A strobe signal can latch the data.
