Question

Write a code in C to shift serially the content of the register labelled x through the bit #1 of Port A starting with least significant bit. Include the port configuration. Do not include any directive.           

Answer 1

According to the given question, we havr one main function called shiftout which takes 3 inputs- your DS or data pin, your SH_CP or clock pin, and a byte data that contains 8 bits for the shift register. To enable multiple shift registers we have to simply wrap our main function around a for loop and trigger our ST_CP or latch pin low/high accordingly while shifting the byte from our dataBuffer to our data byte.

Code:

//Pin connected to ST_CP of 74HC595
int latchPin = 0;

//Pin connected to SH_CP of 74HC595
int clockPin = 3;

//Pin connected to DS of 74HC595
int dataPin = 2;

// Number of digits attached
int const numberOfRegisters = 3;

// Numbers mapped on our seven segment display
byte one = 0b00010010;
byte two = 0b01100111;
byte three = 0b01110110;
byte four = 0b11010010;
byte five = 0b11110100;
byte six = 0b11110101;
byte seven = 0b00010110;
byte eight = 0b11110111;
byte nine = 0b11110110;
byte zero = 0b10110111;
byte test = 0b10000000;
byte empty = 0b00000000;
byte numbers[] = {zero,one,two,three,four,five,six,seven,eight,nine};

// Internal Vars
byte data;
byte dataBuffer[numberOfRegisters];

void setup() {
// put your setup code here, to run once:
  
addToBuffer(123);
}

void loop() {
// put your main code here, to run repeatedly:
}

void addToBuffer(int digit){
// clear buffer
memset(dataBuffer, 0, sizeof(dataBuffer));

int c = 0;
while( digit > 0 ){
byte b = numbers[digit % 10]; // modulus 10 of our input
dataBuffer[c] = b;
digit /= 10;
c++;
}
  
writeBuffer();
}

void writeBuffer(){
digitalWrite(latchPin, 0);
  
for (int a = sizeof(dataBuffer) - 1; a >= 0 ; a--) {
shiftOut(dataPin, clockPin, dataBuffer[a]);
}
  
digitalWrite(latchPin, 1);
}

void shiftOut(int myDataPin, int myClockPin, byte myDataOut) {
// This shifts 8 bits out MSB first,
//on the rising edge of the clock,
//clock idles low

//internal function setup
int i=0;
int pinState;
pinMode(myClockPin, OUTPUT);
pinMode(myDataPin, OUTPUT);

//clear everything out just in case to
//prepare shift register for bit shifting
digitalWrite(myDataPin, 0);
digitalWrite(myClockPin, 0);

//for each bit in the byte myDataOut�
//NOTICE THAT WE ARE COUNTING DOWN in our for loop
//This means that %00000001 or "1" will go through such
//that it will be pin Q0 that lights.
for (i=0; i<=7; i++) {
digitalWrite(myClockPin, 0);

//if the value passed to myDataOut and a bitmask result
// true then... so if we are at i=6 and our value is
// %11010100 it would the code compares it to %01000000
// and proceeds to set pinState to 1.
if ( myDataOut & (1<<i) ) {
pinState= 1;
}
else {
pinState= 0;
}

//Sets the pin to HIGH or LOW depending on pinState
digitalWrite(myDataPin, pinState);
//register shifts bits on upstroke of clock pin
digitalWrite(myClockPin, 1);
//zero the data pin after shift to prevent bleed through
digitalWrite(myDataPin, 0);
}

//stop shifting
digitalWrite(myClockPin, 0);
}