Input/Output - Advanced

shiftIn()

Shifts in a byte of data one bit at a time. Starts from either the most (i.e. the leftmost) or least (rightmost) significant bit. For each bit, the clock pin is pulled high, the next bit is read from the data line, and then the clock pin is taken low.

NOTE: if you're interfacing with a device that's clocked by rising edges, you'll need to make sure that the clock pin is low before the call to shiftOut(), e.g. with a call to digitalWrite(clockPin, LOW).

This is a software implementation; see also the SPI function, which provides a hardware implementation that is faster but works only on specific pins.

// SYNTAX
shiftIn(dataPin, clockPin, bitOrder)

// EXAMPLE USAGE

// Use digital pins D0 for data and D1 for clock
int dataPin = D0;
int clock = D1;

uint8_t data;

setup() {
    // Set data as INPUT and clock pin as OUTPUT before using shiftIn()
    pinMode(dataPin, INPUT);
    pinMode(clock, OUTPUT);

    // shift in data using MSB first
    data = shiftIn(dataPin, clock, MSBFIRST);

    // Or do this for LSBFIRST serial
    data = shiftIn(dataPin, clock, LSBFIRST);
}

loop() {
    // nothing to do
}

shiftIn() takes three arguments, 'dataPin': the pin on which to input each bit, clockPin: the pin to toggle to signal a read from dataPin, bitOrder: which order to shift in the bits; either MSBFIRST or LSBFIRST (Most Significant Bit First, or, Least Significant Bit First).

shiftIn() returns the byte value read.