How to Use 74HC139N: Examples, Pinouts, and Specs

The 74HC139N is a high-speed CMOS dual 2-to-4 line decoder/demultiplexer. It features two independent decoders, each with two binary inputs and four mutually exclusive outputs. The device is designed for use in high-performance digital systems and is compatible with TTL logic levels. The 74HC139N is commonly used in applications requiring address decoding, data routing, or device selection.

• Address decoding in memory systems
• Data demultiplexing in communication systems
• Selection of multiple devices or peripherals
• Logic signal routing in digital circuits

• Supply Voltage (Vcc): 2V to 6V
• Input Voltage (VI): 0V to Vcc
• Output Voltage (VO): 0V to Vcc
• Maximum Output Current (IO): ±25mA per pin
• Propagation Delay: ~16ns at Vcc = 5V
• Power Dissipation: 500mW (maximum)
• Operating Temperature Range: -40°C to +125°C
• Logic Family: HC (High-Speed CMOS)

The 74HC139N is a 16-pin IC with the following pinout:

1. Power Supply: Connect the Vcc pin (pin 16) to a voltage source between 2V and 6V, and connect the GND pin (pin 8) to ground.
2. Enable Inputs: Each decoder has an active LOW enable input (G1 for decoder 1 and G2 for decoder 2). To activate a decoder, set its enable input to LOW.
3. Input Selection: Provide binary inputs (A and B) to select one of the four outputs. For example:
   • A = 0, B = 0 → Y0 is active (LOW)
   • A = 0, B = 1 → Y1 is active (LOW)
   • A = 1, B = 0 → Y2 is active (LOW)
   • A = 1, B = 1 → Y3 is active (LOW)
4. Outputs: The selected output will be LOW, while all other outputs remain HIGH.

• Ensure that the enable inputs (G1 and G2) are properly controlled to avoid unintentional activation of the decoders.
• Use pull-up or pull-down resistors on unused inputs to prevent floating states.
• Avoid exceeding the maximum voltage and current ratings to prevent damage to the IC.

The following example demonstrates how to use the 74HC139N with an Arduino UNO to control a set of LEDs.

• Connect the Vcc pin (pin 16) to the Arduino's 5V pin.
• Connect the GND pin (pin 8) to the Arduino's GND.
• Connect the enable input (G1, pin 1) to the Arduino's digital pin 7.
• Connect inputs A1 (pin 2) and B1 (pin 3) to Arduino digital pins 8 and 9, respectively.
• Connect outputs Y0_1 to Y3_1 (pins 4–7) to LEDs with current-limiting resistors.

// Define pins for the 74HC139N
const int enablePin = 7; // Enable input for decoder 1
const int inputA = 8;    // Input A for decoder 1
const int inputB = 9;    // Input B for decoder 1

void setup() {
  // Set pin modes
  pinMode(enablePin, OUTPUT);
  pinMode(inputA, OUTPUT);
  pinMode(inputB, OUTPUT);

  // Enable the decoder (active LOW)
  digitalWrite(enablePin, LOW);
}

void loop() {
  // Cycle through all outputs (Y0 to Y3)
  for (int i = 0; i < 4; i++) {
    digitalWrite(inputA, i & 0x01); // Set A based on LSB of i
    digitalWrite(inputB, (i >> 1) & 0x01); // Set B based on MSB of i
    delay(1000); // Wait 1 second before switching
  }
}

1. No Output Activation:

   • Ensure the enable input (G1 or G2) is set to LOW.
   • Verify that the power supply voltage is within the specified range.
   • Check for loose or incorrect connections.

2. Multiple Outputs Active:

   • Verify that the inputs (A and B) are not floating. Use pull-up or pull-down resistors if necessary.
   • Ensure that only one decoder is enabled at a time.

3. Overheating:

   • Check that the output current does not exceed the maximum rating of ±25mA per pin.
   • Use current-limiting resistors for connected devices like LEDs.

Q: Can I use the 74HC139N with a 3.3V system?
A: Yes, the 74HC139N is compatible with supply voltages as low as 2V, making it suitable for 3.3V systems.

Q: What happens if both decoders are enabled simultaneously?
A: Both decoders will function independently, but ensure that their outputs are not connected to the same load to avoid conflicts.

Q: Are the outputs active HIGH or active LOW?
A: The outputs are active LOW, meaning the selected output will go LOW while the others remain HIGH.