Understanding the 74HC595 shift register IC

The 74HC595 is an 8-bit serial-in, parallel-out shift register IC, frequently used in electronics to expand the number of output pins on a microcontroller, making it possible to control multiple devices with minimal pin usage. Operating within a voltage range of 2V to 6V, it supports fast data transfer with a typical propagation delay of only 10ns, allowing for efficient control of devices such as LEDs, motors, and relays. With a maximum clock frequency of 25 MHz, it can shift data at high speeds, making it ideal for projects where space and pin limitations are a concern. The shift register can control up to 8 devices per IC, and by cascading multiple 74HC595s, users can control dozens or even hundreds of devices with just a few microcontroller pins. In this article, we will explore the functional principles of the 74HC595, from basic data shifting to advanced applications like multiplexing and serial communication. We will also highlight technical characteristics, including its ability to source or sink 35mA per output, and provide practical code examples to illustrate its use in real-world scenarios.

What is the 74HC595 shift register?

The 74HC595 IC is part of the 74HC family, designed for high-speed CMOS (Complementary Metal-Oxide-Semiconductor) logic. It is used to expand the number of digital output pins available from a microcontroller. Through serial communication, the 74HC595 allows you to control multiple outputs with just a few pins.

The IC contains an 8-bit shift register, a storage register, and an output driver. By shifting in data serially, you can control up to 8 output pins, typically used to light LEDs, drive displays, or control other devices.

Pinout and functionality

The 74HC595 has 16 pins. Below is the pinout for the IC:

Pin	Function
1	QH’ (Serial Out)
2	QH (Serial In)
3	OE (Output Enable)
4	MR (Master Reset)
5	SH_CP (Shift Clock Pin)
6	ST_CP (Storage Clock Pin)
7	Q0 – Q7 (Parallel Output Pins)
8	VSS (Ground)
9	Q7′ (Serial Out)
10-16	VCC (Supply Voltage)

How does the 74HC595 work?

The 74HC595 operates by serially shifting data into the shift register via the DS (Serial Data Input)</ pin. The data is shifted in on each rising edge of the SH_CP (Shift Clock Pin). Once the data is shifted in, it can be latched into the output register using the ST_CP (Storage Clock Pin). This means that the data is available on the output pins and can be used to control connected devices like LEDs, displays, or motors.

Applications of the 74HC595

The 74HC595 shift register is widely used in applications where many outputs are needed but the number of available pins on the microcontroller is limited. Below are a few examples of applications:

• LED Control: Use the 74HC595 to control multiple LEDs with a single microcontroller pin.
• Driving 7-Segment Displays: The 74HC595 can be used to control multiple 7-segment displays for digital readouts.
• Relay Control: It can be used to drive relays for switching high-current devices.
• Motor Control: By connecting multiple 74HC595s, you can control several motors or other actuators.

Example code for using the 74HC595

Here’s an example Arduino code that uses the 74HC595 to control 8 LEDs:

        int dataPin = 2;   // DS pin connected to 74HC595
        int latchPin = 3;   // ST_CP pin connected to 74HC595
        int clockPin = 4;   // SH_CP pin connected to 74HC595

        void setup() {
          pinMode(dataPin, OUTPUT);
          pinMode(latchPin, OUTPUT);
          pinMode(clockPin, OUTPUT);
        }

        void loop() {
          shiftOut(dataPin, clockPin, LSBFIRST, B11111111); // Turn on all LEDs
          delay(1000);
          shiftOut(dataPin, clockPin, LSBFIRST, B00000000); // Turn off all LEDs
          delay(1000);
        }
        

Characteristics of the 74HC595

The table below summarizes the key characteristics of the 74HC595 shift register:

Feature	Specification
Supply Voltage	2V to 6V
Output Current (per pin)	+6mA (High), -6mA (Low)
Input Voltage	0V to VCC
Package Types	DIP, SOP, TSSOP
Shift Register Size	8 Bits

Comparing 74HC595 with other shift registers

The 74HC595 is often compared with other shift registers like the 74LS595 and the 74HCT595. The main difference lies in their voltage compatibility and speed. Here’s a comparative table:

Feature	74HC595	74LS595	74HCT595
Voltage Range	2V to 6V	4.5V to 5.5V	4.5V to 5.5V
Speed (Max Clock Frequency)	25 MHz	15 MHz	20 MHz
Logic Family	HC (High-Speed CMOS)	LS (Low Power Schottky)	HCT (High-Speed CMOS, TTL Compatible)

Conclusion

The 74HC595 shift register is an essential component for expanding the output capability of microcontrollers. Its ability to control multiple devices using a limited number of pins makes it a versatile and cost-effective solution in many electronics projects. Whether you’re controlling LEDs, displays, or relays, the 74HC595 provides an easy-to-implement and reliable solution for your needs.