How to Use 74HC139: Examples, Pinouts, and Specs

The 74HC139 is a high-speed CMOS dual 2-to-4 line decoder/demultiplexer. It is designed to convert binary inputs into a single active low output, making it an essential component for digital logic circuits. The IC contains two independent decoders, each with two inputs and four outputs, enabling efficient data routing and signal selection. Its compact design and versatility make it ideal for applications in address decoding, memory selection, and data distribution.

• Address decoding in microprocessor systems
• Memory chip selection
• Data routing in digital systems
• Signal demultiplexing in communication circuits

• Supply Voltage (Vcc): 2V to 6V
• Input Voltage (VI): 0V to Vcc
• Output Voltage (VO): 0V to Vcc
• Low-Level Output Current (IOL): 8mA (at 4.5V Vcc)
• High-Level Output Current (IOH): -8mA (at 4.5V Vcc)
• Propagation Delay: ~15ns at 5V
• Operating Temperature Range: -40°C to +125°C
• Package Types: DIP-16, SOIC-16, TSSOP-16

The 74HC139 comes in a 16-pin package. Below is the pinout and description:

1. Power Supply: Connect the Vcc pin (16) to a positive voltage source (2V to 6V) and the GND pin (8) to ground.
2. Enable Input: Each decoder has an enable pin (1G for decoder 1 and 2G for decoder 2). These pins must be set to logic LOW to activate the corresponding decoder.
3. Input Selection: Provide binary inputs to the A and B pins of the desired decoder (1A, 1B for decoder 1; 2A, 2B for decoder 2). The binary combination determines which output (Y0-Y3) is activated.
4. Output Behavior: The selected output pin will go LOW (active), while all other outputs remain HIGH.

• Ensure the enable pin is LOW for the decoder to function. If the enable pin is HIGH, all outputs will remain HIGH regardless of the inputs.
• Use pull-up or pull-down resistors on unused inputs to prevent floating states.
• Avoid exceeding the maximum voltage ratings to prevent damage to the IC.
• Decoupling capacitors (e.g., 0.1µF) are recommended between Vcc and GND to stabilize the power supply.

The following example demonstrates how to use the 74HC139 with an Arduino UNO to control LEDs connected to the outputs of decoder 1.

• Connect the 74HC139's Vcc to the Arduino's 5V pin and GND to the Arduino's GND.
• Connect the 1G pin to Arduino pin 7 (enable control).
• Connect 1A and 1B to Arduino pins 8 and 9, respectively.
• Connect LEDs (with current-limiting resistors) to the outputs 1Y0, 1Y1, 1Y2, and 1Y3.

// Define pins for the 74HC139 decoder
const int enablePin = 7; // Enable pin 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 by setting enablePin LOW
  digitalWrite(enablePin, LOW);
}

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

1. All Outputs Remain HIGH:

   • Ensure the enable pin (1G or 2G) is set to LOW.
   • Verify that the power supply is correctly connected to Vcc and GND.

2. Incorrect Output Selection:

   • Check the binary inputs (A and B) for proper logic levels.
   • Ensure there are no loose connections or floating input pins.

3. Outputs Not Driving Loads:

   • Verify that the connected load does not exceed the IC's current rating (8mA per output).
   • Use external transistors or drivers for higher current loads.

Q: Can I use the 74HC139 with a 3.3V microcontroller?
A: Yes, the 74HC139 operates with supply voltages as low as 2V, making it compatible with 3.3V systems.

Q: What happens if both decoders are enabled simultaneously?
A: Each decoder operates independently, so enabling both will not cause interference. However, ensure that the outputs are not shorted together.

Q: Can the outputs drive LEDs directly?
A: Yes, but ensure the current through each LED does not exceed 8mA. Use appropriate current-limiting resistors.