Overview of the 74138
The 74138 is a popular 3 to 8 line decoder IC chip used in many digital logic applications. It takes a 3-bit binary input and converts it into 1 of 8 possible output lines. Some key features of the 74138 include:
- 3 binary inputs labeled A, B, C
- 8 active low outputs labeled Y0 through Y7
- Enable input that allows the outputs to be disabled
- Active low output enable
The truth table for the 74138 is:
| A | B | C | Outputs |
|---|---|---|---|
| 0 | 0 | 0 | Y0 |
| 0 | 0 | 1 | Y1 |
| 0 | 1 | 0 | Y2 |
| 0 | 1 | 1 | Y3 |
| 1 | 0 | 0 | Y4 |
| 1 | 0 | 1 | Y5 |
| 1 | 1 | 0 | Y6 |
| 1 | 1 | 1 | Y7 |
Applications of the 74138
Some common applications of the 74138 3 to 8 decoder include:
Memory Addressing
- Decoding address lines from a microprocessor to access memory or IO locations
- Allows expanding the addressable range using minimal address lines
Demultiplexing
- Converting a single data line into 1 of 8 output lines
- Used for controlling individual segments of an LED or LCD display
Driving Peripherals
- Selecting 1 of 8 peripherals using a 3-bit select code
- Enables connecting multiple devices to a limited number of microcontroller pins
Circuit Design with the 74138
Here is a simple circuit using the 74138 to decode a 3-bit value and drive one of 8 LEDs:
The 74138 is powered by 5V, the A/B/C inputs come from a microcontroller or other logic chip. The selected output pin sinks to ground, lighting the corresponding LED.
Some key considerations when using the 74138 in a circuit:
- Add pull-up resistors on the output lines to keep them high when not selected
- Pay attention to voltage levels – the inputs and output pins may be damaged by excessive voltage
- Disable unused outputs by connecting the enable pin to ground
- Minimize long wires to avoid noise pickup and crosstalk between signals
Typical specifications
Here are some key parameters from the 74138 datasheet:
- Supply voltage: 4.5-5.5V
- Input voltage: 3.8-5.5V
- Output current: 25mA (max)
- Propagation delay: 13ns
- Operating temperature: 0-70°C
- Package: 16-pin DIP, SOIC
Always double check the specs on the exact 74138 chip being used, parameters can vary between manufacturers.
Frequently Asked Questions
What is the difference between the 74138 and 74139?
The main difference is that the 74139 has active HIGH outputs, while the 74138 has active LOW outputs. So the 74139 will drive the output pin high when selected.
Can I extend the 74138 to more outputs?
Yes, two 74138 chips can be cascaded to create a 6 to 16 decoder. This is done by connecting the enable pin of the second chip to the last output of the first.
Do I need pull-up resistors on the 74138 outputs?
Pull-up resistors are highly recommended when driving LEDs or logic inputs to prevent the outputs from floating when not selected. 10K resistors are commonly used.
What precautions should I take when using the 74138?
Avoid exceeding the power supply voltage ratings or drawing too much current from the outputs. Use capacitors across the power pins for stability. Minimize noise through good circuit board layout.
Are there similar decoder ICs to the 74138?
Some other popular decoder chips include the 74HC138, CD4514, and 74LS138 which are CMOS versions and work well at lower voltages.
