How to Use SHIFT REGISTER: Examples, Pinouts, and Specs

The 74*595 is an 8-bit serial-in, parallel-out shift register with a storage register and tri-state outputs. It is widely used in digital electronics to expand the number of output pins available on a microcontroller or microprocessor. By cascading multiple shift registers, users can control a large number of outputs using only a few pins.

• Driving LED arrays or 7-segment displays
• Expanding GPIO pins on microcontrollers
• Controlling relays or other digital outputs
• Data storage and transfer in digital circuits
• Multiplexing and demultiplexing signals

The 74*595 shift register is a versatile component with the following key specifications:

The 74*595 has 16 pins, as described in the table below:

The 74*595 shift register is easy to use in digital circuits. Below are the steps and considerations for using it effectively:

1. Power Supply: Connect the Vcc pin to a 5V or 3.3V power source (depending on your system) and the GND pin to ground.
2. Data Input: Connect the DS pin to the data source (e.g., a microcontroller pin).
3. Clock Pins:
   • Connect the SH_CP pin to the clock signal for shifting data into the register.
   • Connect the ST_CP pin to the clock signal for latching the data to the output pins.
4. Output Enable: Connect the OE pin to ground to enable the outputs. If you want to disable the outputs, connect OE to Vcc.
5. Outputs: Use the Q0–Q7 pins to drive your desired load (e.g., LEDs, relays).
6. Cascading: To cascade multiple shift registers, connect the Q7' pin of the first register to the DS pin of the next register.

Below is an example of how to connect and control a 74*595 shift register with an Arduino UNO to drive 8 LEDs:

• Connect the 74*595 Vcc pin to the Arduino 5V pin and GND to Arduino GND.
• Connect the DS pin to Arduino pin 11.
• Connect the SH_CP pin to Arduino pin 12.
• Connect the ST_CP pin to Arduino pin 8.
• Connect the OE pin to GND.
• Connect LEDs to Q0–Q7 through 220-ohm resistors.

// Define pins for the 74*595 shift register
const int dataPin = 11;  // DS pin
const int clockPin = 12; // SH_CP pin
const int latchPin = 8;  // ST_CP pin

void setup() {
  // Set pins as outputs
  pinMode(dataPin, OUTPUT);
  pinMode(clockPin, OUTPUT);
  pinMode(latchPin, OUTPUT);
}

void loop() {
  // Example: Turn on LEDs one by one
  for (int i = 0; i < 256; i++) {
    digitalWrite(latchPin, LOW); // Prepare to send data
    shiftOut(dataPin, clockPin, MSBFIRST, i); // Send data to shift register
    digitalWrite(latchPin, HIGH); // Latch data to outputs
    delay(200); // Wait for 200ms
  }
}

• Current Limiting: Use resistors (e.g., 220 ohms) in series with LEDs to limit current and prevent damage to the shift register.
• Clock Signal: Ensure the clock signals (SH_CP and ST_CP) are clean and free of noise to avoid data corruption.
• Cascading: When cascading multiple shift registers, ensure the clock and latch signals are shared across all registers.

1. Outputs Not Working:
   • Ensure the OE pin is connected to GND to enable the outputs.
   • Verify that the Vcc and GND pins are properly connected.
2. Incorrect Output Data:
   • Check the timing of the clock signals (SH_CP and ST_CP).
   • Ensure the data is being sent in the correct order (MSB or LSB first).
3. LEDs Not Lighting Up:
   • Verify that the LEDs are connected in the correct orientation (anode to Qx, cathode to GND).
   • Check the current-limiting resistors.

Q: Can I use the 74*595 with a 3.3V microcontroller?
A: Yes, the 74*595 operates with supply voltages as low as 2V, making it compatible with 3.3V systems.

Q: How many shift registers can I cascade?
A: Theoretically, you can cascade as many as you want, but practical limitations like signal degradation and timing constraints may arise after 8–10 registers.

Q: What is the purpose of the Q7' pin?
A: The Q7' pin outputs the last bit of the shift register and is used to cascade additional shift registers.

Q: Can I use the 74*595 to drive high-current loads?
A: No, the 74*595 can only source or sink up to 6mA per pin. Use external transistors or drivers for high-current loads.