The Random Thoughts of a Geek heading for Banbury
The Random Thoughts of a Geek heading for Banbury

Week 6: Shift Registers

74HC595 Shift Register

This week we will be having a play with a 74HC595, the Data sheet lists it as an 8-bit serial-in, serial or parallel-out shift register with output latches; 3-state.” In other words, you can use it to control 8 outputs at a time while only taking up a few pins on your microcontroller.

You can connect multiple registers together to extend the number of outputs beyond 8.

As you can see from the ‘Featured image I used my LEDs with integrated Resistors.

/*     ---------------------------------------------------------
 *     |  Arduino Experimentation Kit Example Code             |
 *     |  CIRC-05 .: 8 More LEDs :. (74HC595 Shift Register)   |
 *     ---------------------------------------------------------
 * We have already controlled 8 LEDs however this does it in a slightly
 * different manner. Rather than using 8 pins we will use just three
 * and an additional chip.

//Pin Definitions
//Pin Definitions
//The 74HC595 uses a serial communication 
//link which has three pins
int data = 2; 
int clock = 3;
int latch = 4;

//Used for single LED manipulation
int ledState = 0;
const int ON = HIGH;
const int OFF = LOW;

 * setup() - this function runs once when you turn your Arduino on
 * We set the three control pins to outputs
void setup()
  pinMode(data, OUTPUT);
  pinMode(clock, OUTPUT);  
  pinMode(latch, OUTPUT);  

 * loop() - this function will start after setup finishes and then repeat
 * we set which LEDs we want on then call a routine which sends the states to the 74HC595
void loop()                     // run over and over again
  int delayTime = 100; //the number of milliseconds to delay between LED updates
  for(int i = 0; i < 256; i++){

 * updateLEDs() - sends the LED states set in ledStates to the 74HC595
 * sequence
void updateLEDs(int value){
  digitalWrite(latch, LOW);     //Pulls the chips latch low
  shiftOut(data, clock, MSBFIRST, value); //Shifts out the 8 bits to the shift register
  digitalWrite(latch, HIGH);   //Pulls the latch high displaying the data

 * updateLEDsLong() - sends the LED states set in ledStates to the 74HC595
 * sequence. Same as updateLEDs except the shifting out is done in software
 * so you can see what is happening.
void updateLEDsLong(int value){
  digitalWrite(latch, LOW);    //Pulls the chips latch low
  for(int i = 0; i < 8; i++){  //Will repeat 8 times (once for each bit)
  int bit = value & B10000000; //We use a "bitmask" to select only the eighth 
                               //bit in our number (the one we are addressing this time through
  value = value << 1;          //we move our number up one bit value so next time bit 7 will be
                               //bit 8 and we will do our math on it
  if(bit == 128){digitalWrite(data, HIGH);} //if bit 8 is set then set our data pin high
  else{digitalWrite(data, LOW);}            //if bit 8 is unset then set the data pin low
  digitalWrite(clock, HIGH);                //the next three lines pulse the clock pin
  digitalWrite(clock, LOW);
  digitalWrite(latch, HIGH);  //pulls the latch high shifting our data into being displayed

//These are used in the bitwise math that we use to change individual LEDs
//For more details
int bits[] = {B00000001, B00000010, B00000100, B00001000, B00010000, B00100000, B01000000, B10000000};
int masks[] = {B11111110, B11111101, B11111011, B11110111, B11101111, B11011111, B10111111, B01111111};
 * changeLED(int led, int state) - changes an individual LED 
 * LEDs are 0 to 7 and state is either 0 - OFF or 1 - ON
 void changeLED(int led, int state){
   ledState = ledState & masks[led];  //clears ledState of the bit we are addressing
   if(state == ON){ledState = ledState | bits[led];} //if the bit is on we will add it to ledState
   updateLEDs(ledState);              //send the new LED state to the shift register

This circuit should be a useful for increasing the number of outputs that I can have on Swarmbot, and my pick and place machine.

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