SN74HC595: 8-Bit Shift Register Explained

The SN74HC595 is an 8-bit serial-in, parallel-out shift register with a storage register and tri-state outputs. It is widely used for expanding the number of digital output pins on a microcontroller (e.g., Arduino, Raspberry Pi, STM32) when the available pins are limited.

🔹 1. What is the SN74HC595?

• It is an 8-bit shift register that takes serial data (one bit at a time) and outputs it in parallel (to 8 output pins).
• It includes a latch function, which holds the data until it is updated.
• The tri-state output allows multiple shift registers to be cascaded.

📌 Why use it?

• Expands microcontroller output pins (uses only 3 control pins to control 8 outputs).
• Can cascade multiple SN74HC595s to control more outputs.
• Commonly used for LED displays, 7-segment displays, and multiplexing applications.

🔹 2. SN74HC595 Pinout & Functions

The SN74HC595 has 16 pins.

📌 Key Control Pins:

• SH_CP (Pin 11) → Shift register clock (loads serial data bit-by-bit).
• ST_CP (Pin 12) → Latch clock (updates all 8 outputs).
• OE (Pin 13) → Output Enable (LOW to enable, HIGH to disable outputs).
• DS (Pin 14) → Serial data input (data fed into the shift register).
• Q7′ (Pin 9) → Serial out (for cascading more shift registers).

🔹 3. How SN74HC595 Works

1. Data is sent serially (bit by bit) into DS (Pin 14).
2. SH_CP (Shift Clock) moves data through the shift register.
3. ST_CP (Latch Clock) transfers the stored bits to the output pins (Q0-Q7).
4. OE (Output Enable) controls whether the outputs are active.

📌 Multiple SN74HC595s can be cascaded by connecting Q7' (Pin 9) of one IC to DS (Pin 14) of the next.

🔹 4. Using SN74HC595 with Arduino

🛠 Required Components

• 1x SN74HC595 Shift Register
• 8x LEDs
• 8x 220Ω Resistors
• 1x Arduino Board
• Jumper Wires

🛠 Wiring

🔹 5. Arduino Code to Control LEDs

This Arduino sketch shifts binary data into SN74HC595 to control 8 LEDs.

#define DS 2     // Data pin
#define SH_CP 3  // Shift Clock pin
#define ST_CP 4  // Latch Clock pin

void setup() {
    pinMode(DS, OUTPUT);
    pinMode(SH_CP, OUTPUT);
    pinMode(ST_CP, OUTPUT);
}

void loop() {
    for (int i = 0; i < 256; i++) {
        digitalWrite(ST_CP, LOW);  // Latch disabled
        shiftOut(DS, SH_CP, MSBFIRST, i);  // Send data
        digitalWrite(ST_CP, HIGH); // Latch enabled
        delay(500);
    }
}

📌 Explanation:

• shiftOut() sends 8 bits of data serially to SN74HC595.
• ST_CP (Latch Clock) updates all outputs at once.
• The loop cycles through 256 patterns (0b00000000 to 0b11111111).

🔹 6. Cascading Multiple SN74HC595 Shift Registers

You can connect multiple SN74HC595s together to control 16, 24, or more outputs.

🛠 Cascading Wiring

📌 Data shifts from the first SN74HC595 to the second one, allowing more outputs.

✅ Arduino Code for Two SN74HC595 Shift Registers

#define DS 2  
#define SH_CP 3  
#define ST_CP 4  

void setup() {
    pinMode(DS, OUTPUT);
    pinMode(SH_CP, OUTPUT);
    pinMode(ST_CP, OUTPUT);
}

void loop() {
    digitalWrite(ST_CP, LOW);
    shiftOut(DS, SH_CP, MSBFIRST, 0b10101010); // First register
    shiftOut(DS, SH_CP, MSBFIRST, 0b11001100); // Second register
    digitalWrite(ST_CP, HIGH);
    delay(500);
}

📌 What happens?

• The first shift register controls the first 8 outputs.
• The second shift register controls the next 8 outputs.

🔹 7. Applications of SN74HC595

✅ LED Displays – Used in multiplexed LED matrix.
✅ 7-Segment Displays – Drives multiple digits with minimal pins.
✅ Motor Controllers – Expands H-Bridge drivers.
✅ Relays & Actuators – Controls high-power devices with low pin usage.
✅ Home Automation – Expands I/O in smart control systems.

🔹 8. SN74HC595 vs. Other Shift Registers

📌 Verdict: SN74HC595 is best for general output expansion, but for high-current applications, use TPIC6B595.

🎯 Conclusion

• The SN74HC595 expands digital outputs using only 3 control pins.
• It is widely used in LED displays, multiplexing, and motor control.
• Multiple SN74HC595s can be cascaded to control dozens of outputs.
• Arduino + SN74HC595 = Simple and Effective I/O Expansion.