An Introduction to This Helpful Integrated Circuit

A 3 to 8 decoder IC (integrated circuit) is a very useful component in digital logic systems. As the name suggests, it takes a 3-bit binary input and decodes it into an 8-bit output. Let’s take a closer look at what makes this IC so handy.

What is a Decoder?

A decoder is a combinational logic circuit that converts binary information from the input side to a set of output lines. It takes an n-bit binary number and decodes it into 2^n unique output lines.

So a 3 to 8 decoder takes a 3-bit input word and decodes it into 8 (2^3) output lines. Only one of the output lines will be active (HIGH) at any given time, depending on the binary input.

Why Use a 3 to 8 Decoder IC?

Here are some of the benefits of using a premade 3 to 8 decoder IC:

• Saves design time: You don’t have to design the decoder logic from scratch using AND/OR gates. Just drop in the IC.
• Reliable and robust: The IC has been manufactured and tested for correct logic functionality. No need to worry about mistakes in your own decoder design.
• Space saving: A single IC package takes less space on your PCB than discrete AND/OR gates.
• Logic level compatibility: The decoder IC operates at standard logic levels like 5V, 3.3V etc. No need for level shifting.
• Fan out capabilities: The IC can drive more outputs thanAND/OR gates. Handy when you need to activate multiple devices.

Logic Function Table

Here is the logic function table of a 3 to 8 decoder:

As you can see, the 3-bit input binary number is decoded into the decimal equivalent output line. For example, input 110 activates output Y6.

This table demonstrates the useful one-hot output decoding functionality.

Example Application: Memory Address Decoding

One of the most common uses of 3 to 8 decoder ICs is in address decoding for memory systems. For example, let’s say we have a system with a 512 byte memory chip. It has a 9-bit address bus, so 2^9 = 512 memory locations.

We can use a 3 to 8 decoder IC to decode the Most Significant Bits (MSBs) of the address into 8 chip select signals for an 8-chip memory system.

Here is a diagram of a 512 byte memory system using a 3 to 8 decoder:

The A2, A1, A0 inputs come from the MSBs of the address bus. These activate one of the 8 chip select lines at a time, enabling 1 out of 8 memory ICs. The remaining address bits A8-A3 select the byte within the activated memory chip.

This allows easy expansion of memory capacity using a cost-effective decoder IC.

Example Chips

Here are some commonly used 3 to 8 decoder ICs:

• 74HC138: High speed CMOS decoder
• 74LS138: Low power TTL decoder
• CD4514: 4-bit BCD to decimal decoder
• MC14512B: High current decoder driver IC

These provide the core 3 to 8 decoding function with different speed, power and drive capabilities.

How a 3 to 8 Decoder IC Works Internally

Now let’s take a look at what’s inside a 3 to 8 decoder IC and how it works. Here is the internal logic diagram:

It uses 3 input AND gates, one for each output line. The other input of each AND is an enable line that is LOW by default. This forces all outputs to LOW when disabled.

The 3 inputs A2, A1 and A0 connect to the AND gates in a specific pattern to generate the 1 out of 8 output decoding. Only one AND gate will have all 3 HIGH inputs at a time, activating its output.

The 8 outputs feed into output buffers that can drive external devices. Some decoder ICs have inverted outputs as well.

Enable Function

The enable input (/G2) is useful to activate the decoder only when needed. Bringing it LOW disables all outputs. This prevents spurious outputs when the inputs are changing state.

The enable line is sometimes named as Strobe or Chip Select. It provides an extra level of control over the decoding operation.

Advantages of Active LOW Outputs

You may notice that the decoder outputs are active LOW signals instead of HIGH. This inverted output has some advantages:

• Pull-up resistors can keep outputs HIGH when disabled, avoiding floating inputs to other ICs.
• Active LOW signals are better for driving the enable/chip select pins on memory ICs and other devices. Most devices use active LOW enables.
• Inverted outputs also allow combining decoder ICs easily using NOR gates when more decoding outputs are needed.

These advantages make active LOW outputs preferable in decoder ICs.

Using a 3 to 8 Decoder IC in Circuits

Here are some tips for effectively utilizing 3 to 8 decoder ICs in your prototypes and designs:

Add Pull-Up Resistors

Add 4.7k-10k ohm pull-up resistors between the outputs and +5V rail. This keeps the outputs HIGH when disabled instead of floating.

Include Enable Signal

Make sure to use the /Enable or /Strobe input for properly gating the outputs. Drive this pin to logic HIGH when active, and LOW to shut off the outputs.

Drive Inputs Properly

Drive the decoder inputs from a microcontroller or other logic circuitry with adequate current capacity. Avoid slow rise times which may cause incorrect decoding.

Manage Fan-Out

Check the fan-out rating (number of inputs an output can drive) of the decoder IC. Use buffers if driving a large number of inputs to avoid overloading the outputs.

Use Proper Decoupling

Use 100nF bypass capacitors across the power and ground pins of the decoder IC as per standard IC decoupling practices. This will maintain a steady voltage supply during switching.

By following these tips, you can build reliable and robust systems using 3 to 8 decoder ICs.

Common Mistakes to Avoid

Here are some common mistakes to watch out for when using 3 to 8 decoder ICs:

• Forgetting Enable signal – This will activate multiple erroneous outputs
• Overloading outputs – Fan-out limits should be checked
• Floating inputs – Unused inputs must be tied HIGH or LOW
• Slow input edges – Can cause glitches and incorrect decoding
• Power supply spikes – Good decoupling is mandatory
• PCB layout issues – Keep traces short, minimize noise pickup
• Driving LEDs without current limiting resistors – May damage the IC
• Hot plugging ICs – Power off the circuit when plugging in ICs

By being mindful of these issues, faulty behavior can be avoided when using decoder ICs.

Conclusion

The versatile 3 to 8 decoder IC provides an easy way to decode binary information into one of 8 active LOW outputs. This is useful for memory addressing, input multiplexing, seven segment displays and other applications. By selecting the right decoder IC for speed, drive strength and power needs, reliable performance can be ensured. Proper care should be taken to avoid common mistakes like overloading outputs, floating inputs and power spikes. Overall, the 3 to 8 decoder is an indispensable component in many digital logic designs.

Frequently Asked Questions

What are the advantages of an enable input on the 3 to 8 decoder?

The enable input allows you to activate the decoding function only when needed. When not enabled, the outputs are forced to the inactive state. This prevents spurious operation when inputs are changing state. It provides an extra level of control over the decoder.

Can the outputs of two 3 to 8 decoder ICs be combined easily?

Yes, the active LOW outputs can be combined by using NOR gates. The NOR output will be active only if both the original decoder outputs are LOW. This allows easily expanding the number of decodes by using multiple ICs.

Can unused inputs be left floating on a decoder IC?

No, unused decoder inputs should not be left floating. They should be tied HIGH or LOW via a resistor. A floating input can cause unpredictable circuit operation by randomly going HIGH or LOW.

Is a truth table required to understand decoder IC functionality?

The truth table provides a precise representation of the input/output relationships of a decoder IC. While not absolutely necessary, referring to the truth table removes any doubt about the expected outputs for given inputs. It is highly recommended to consult the truth table.

How does a 3 to 8 decoder save resources compared to discrete gates?

A 3 to 8 decoder IC replaces 24 (3 x 8) AND gates and 8 NOR gates in a discrete component implementation. This significantly reduces integrated circuit silicon area. A single IC package also takes less physical PCB space than discrete gates. Overall, the decoder IC saves on both semiconductor and PCB resources.