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• Tested outputting to lots of LEDs via shift registers

  Lee Sampson • 09/27/2022 at 19:29 • 2 comments

  I wanted to figure out the best way to run many LEDs quickly without using loads of IO pins. Here I tested using shift registers. I tried two methods, shiftOut() and SPI. I ran a test setting 16bits and recorded the time. Results:

  - shiftOut() = 204 μs

  - SPI = 16 μs

  Clearly SPI is much faster. Below are both methods.

  SPI Schematic:

  SPI test code:

  #include <SPI.h>
  
  //int pll_d=11; //piedino data
  //int pll_c=13; //piedino clock
  int pll_l=10; //piedino latch
  #define DATAOUT 11//MOSI
  #define DATAIN 12//MISO
  #define SPICLOCK 13//SCK
  
  int timer = 0;
  
  uint16_t ledNumbers[17] = { 0b0000000000000000, 
                              0b1000000000000000,
                              0b0100000000000000,
                              0b0010000000000000,
                              0b0001000000000000,
                              0b0000100000000000,
                              0b0000010000000000,
                              0b0000001000000000,
                              0b0000000100000000,
                              0b0000000010000000,
                              0b0000000001000000,
                              0b0000000000100000,
                              0b0000000000010000,
                              0b0000000000001000,
                              0b0000000000000100,
                              0b0000000000000010,
                              0b0000000000000001};
  
  
  void setup () {
  
    pinMode(DATAOUT, OUTPUT);
    pinMode(DATAIN, INPUT);
    pinMode(SPICLOCK,OUTPUT);
    pinMode(pll_l, OUTPUT);
  
    SPI.begin();
    Serial.begin(9600);
    SPI.setDataMode(SPI_MODE0);
    SPI.setBitOrder(LSBFIRST);
    SPI.setClockDivider(SPI_CLOCK_DIV2);
  
  
    timer = micros();
  
    digitalWrite(pll_l,LOW);
    SPI.transfer(ledNumbers[0]);
    SPI.transfer(ledNumbers[0] >> 8);
    digitalWrite(pll_l,HIGH);
    
    timer = micros() - timer;
    Serial.println(timer);
  
  }
  
  void loop() {
    // put your main code here, to run repeatedly:
  
    for(int x = 0; x < 16; x++){
      turnOn(ledNumbers[x + 1]);
      delay(500);
    }
    
  }
  
  void turnOn(uint16_t bitNum){
  
    digitalWrite(pll_l,LOW);
    SPI.transfer(bitNum);
    SPI.transfer(bitNum >> 8);
    digitalWrite(pll_l,HIGH);
  
  }
  

  ---

  
  

  shiftOut() Schematic:
  

  shiftOut() Code:

  int DS_pin = 11;
  int STCP_pin = 12;
  int SHCP_pin = 8;
  
  uint16_t ledOutput = 0;
  
  uint16_t ledNumbers[17] = { 0b0000000000000000, 
                              0b1000000000000000,
                              0b0100000000000000,
                              0b0010000000000000,
                              0b0001000000000000,
                              0b0000100000000000,
                              0b0000010000000000,
                              0b0000001000000000,
                              0b0000000100000000,
                              0b0000000010000000,
                              0b0000000001000000,
                              0b0000000000100000,
                              0b0000000000010000,
                              0b0000000000001000,
                              0b0000000000000100,
                              0b0000000000000010,
                              0b0000000000000001};
                        
  
  void setup()
  {
    Serial.begin(115200);
  
    pinMode(DS_pin,OUTPUT);
    pinMode(STCP_pin,OUTPUT);
    pinMode(SHCP_pin,OUTPUT);
  
    turnOn(4);
    delay(500);
    turnOn(8);
    delay(500);
    turnOn(12);
    delay(500);
    turnOn(16);
    delay(500);
  
    turnOff(4);
    delay(500);
    turnOff(8);
    delay(500);
    turnOff(12);
    delay(500);
    turnOff(16);
  }
  
  void loop()
  {
  
  
  }
  
  void turnOn(int bitNum)
  {
    ledOutput |= ledNumbers[bitNum];
    digitalWrite(SHCP_pin, LOW);
    shiftOut(DS_pin, STCP_pin, LSBFIRST, ledOutput); 
    shiftOut(DS_pin, STCP_pin, LSBFIRST, ledOutput  >> 8);
    digitalWrite(SHCP_pin, HIGH);
  }
  
  void turnOff(int bitNum)
  {
    ledOutput &= ~ledNumbers[bitNum];
    digitalWrite(SHCP_pin, LOW);
    shiftOut(DS_pin, STCP_pin, LSBFIRST, ledOutput); 
    shiftOut(DS_pin, STCP_pin, LSBFIRST, ledOutput  >> 8);
    digitalWrite(SHCP_pin, HIGH);
  }
  

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