Project 16 74HC595

1. Introduction To put it simply, 74HC595 is a combination of 8-digit shifting register, memorizer and equipped with tri-state output. Here, we use it to control 8 LEDs.

You may wonder why use a 74HC595 to control LED? Well, think about how many I/O it takes for an Arduino to control 8 LEDs? Yes, 8.

For an Arduino, it has only 20 I/O including analog ports. So, to save port resources, we use 74HC595 to reduce the number of ports it needs. Using 74HC595 enables us to use 3 digital I/O port to control 8 LEDs!

The 74HC595 devices contain an 8-bit serial-in, parallel-out shift register that feeds an 8-bit D-type storage register. The storage register has parallel 3-state outputs. Separate clocks are provided for both the shift and storage register.

The shift register has a direct overriding clear (SRCLR) input, serial (SER) input, and serial outputs for cascading. When the output-enable (OE) input is high, the outputs are in the high-impedance state. Both the shift register clock (SRCLK) and storage register clock (RCLK) are positive-edge triggered. If both clocks are connected together, the shift register always is one clock pulse ahead of the storage register.

2. Hardware Required

• 74HC595 chip*1

• REV4 board *1

• Red M5 LED*4

• Green M5 LED*4

• 220Ω resistor*8

• Breadboard*1

• USB cable *1

• Breadboard jumper wires*several

3. Circuit Connection Note: for pin 13 OE port of 74HC595, it needs to connect to GND.

• Connection for REV4

• Connection for 2560 R3

The circuit may seem complicated, but soon you will find it easy!

4. Sample Code

int data = 2;// set pin 14 of 74HC595as data input pin SI 
int clock = 5;// set pin 11 of 74hc595 as clock pin SCK
int latch = 4;// set pin 12 of 74hc595 as output latch RCK 
int ledState = 0;
const int ON = HIGH;
const int OFF = LOW;

void setup()
{
    pinMode(data, OUTPUT);
    pinMode(clock, OUTPUT);
    pinMode(latch, OUTPUT);
}

void loop()
{
    for(int i = 0; i < 256; i++)
    {
        updateLEDs(i);
        delay(500);
    }
}

void updateLEDs(int value)
{
    digitalWrite(latch, LOW);//
    shiftOut(data, clock, MSBFIRST, ~value);// serial data “output”, high level first
    digitalWrite(latch, HIGH);// latch
}

5. Result After downloading the program, you can see 8 LEDs display 8-bit binary number.