How to Use 74xx595 / Shift Register: Examples, Pinouts, and Specs

The 74HC595 is an 8-bit serial-in, parallel-out shift register with output latches, manufactured by Texas Instruments (TI). It is widely used for serial-to-parallel data conversion, allowing microcontrollers to control multiple outputs using fewer pins. The device features a storage register and a shift register, enabling data to be shifted in serially and latched for stable parallel output.

• Expanding the number of output pins in microcontroller-based systems
• Driving LED arrays, 7-segment displays, or dot matrix displays
• Controlling relays or other digital outputs
• Applications requiring daisy-chaining of multiple shift registers for extended outputs

The following are the key technical details of the 74HC595:

The 74HC595 has 16 pins, as described in the table below:

The 74HC595 is straightforward to use in a circuit. Below are the steps and considerations for proper usage:

1. Power Supply: Connect the Vcc pin (16) to a 5V or 3.3V power supply and the GND pin (8) to ground.
2. Data Input: Connect the DS pin (14) to the microcontroller's data output pin.
3. Clock Signals:
   • Connect the SH_CP pin (11) to a microcontroller pin for the shift register clock.
   • Connect the ST_CP pin (12) to another microcontroller pin for the storage register clock (latch).
4. Output Enable: Connect the OE pin (13) to ground to enable the outputs. If dynamic control is needed, connect it to a microcontroller pin.
5. Outputs: Use the Q0-Q7 pins (15, 1-7) to drive LEDs, relays, or other devices.
6. Optional Daisy-Chaining: To expand outputs, connect the Q7' pin (9) of the first shift register to the DS pin (14) of the next shift register.

Below is an example of how to connect and control the 74HC595 with an Arduino UNO to drive 8 LEDs:

• 74HC595 Pin 14 (DS) → Arduino Pin 11 (MOSI)
• 74HC595 Pin 11 (SH_CP) → Arduino Pin 12
• 74HC595 Pin 12 (ST_CP) → Arduino Pin 8
• 74HC595 Pin 13 (OE) → GND
• 74HC595 Pin 10 (MR) → Vcc
• 74HC595 Pin 16 (Vcc) → 5V
• 74HC595 Pin 8 (GND) → GND
• Q0-Q7 Pins → LEDs (with current-limiting resistors)

// Define pins for the 74HC595 connections
const int dataPin = 11;  // DS pin of 74HC595
const int clockPin = 12; // SH_CP pin of 74HC595
const int latchPin = 8;  // ST_CP pin of 74HC595

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

void loop() {
  // Example: Light up LEDs in a binary counting pattern
  for (int i = 0; i < 256; i++) {
    digitalWrite(latchPin, LOW); // Disable output while shifting data
    shiftOut(dataPin, clockPin, MSBFIRST, i); // Send data to shift register
    digitalWrite(latchPin, HIGH); // Latch data to output pins
    delay(500); // Wait for 500ms
  }
}

• Use current-limiting resistors (e.g., 220Ω) on the output pins when driving LEDs to prevent damage.
• Keep the clock signals clean and free of noise to ensure reliable operation.
• For daisy-chaining, ensure the Q7' pin is properly connected to the next shift register's DS pin.

1. Outputs Not Responding:
   • Ensure the OE pin (13) is connected to GND.
   • Verify that the latch pin (ST_CP) is toggled HIGH after shifting data.
2. Incorrect Output States:
   • Check the timing of the clock signals (SH_CP and ST_CP).
   • Ensure the data is shifted in the correct order (MSB or LSB first).
3. No Output on Daisy-Chained Registers:
   • Verify the Q7' pin of the first register is connected to the DS pin of the next register.
   • Ensure the clock and latch signals are shared across all registers.

Q: Can the 74HC595 drive high-current loads like motors?
A: No, the 74HC595 can only source or sink up to 6mA per pin. Use external transistors or drivers for high-current loads.

Q: How many 74HC595 chips can be daisy-chained?
A: Theoretically, there is no limit, but practical constraints like signal integrity and timing delays may limit the number to around 8-10 chips.

Q: Can I use the 74HC595 with a 3.3V microcontroller?
A: Yes, the 74HC595 operates with supply voltages as low as 2V. Ensure the output current requirements are met.

By following this documentation, you can effectively integrate the 74HC595 into your projects for reliable and efficient serial-to-parallel data conversion.