Abstract

The shiftIn() gets 8 bit of data and assembles them into 1 byte of data.

Source Code

The shiftIn() is defined in hardware/arduino/avr/cores/arduino/wiring_shift.c as below.

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uint8_t shiftIn(uint8_t dataPin, uint8_t clockPin, uint8_t bitOrder) {
    uint8_t value = 0;
    uint8_t i;
 
    for (i = 0; i < 8; ++i) {
        digitalWrite(clockPin, HIGH);
        if (bitOrder == LSBFIRST)
            value |= digitalRead(dataPin) << i;
        else
            value |= digitalRead(dataPin) << (7 - i);
        digitalWrite(clockPin, LOW);
    }
    return value;
}

The inputs are dataPin, clockPin and bitOrder and they all are uint8_t, returns uint8_t.

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uint8_t shiftIn(uint8_t dataPin, uint8_t clockPin, uint8_t bitOrder) {
    uint8_t value = 0;
    uint8_t i;

It loops to get 8 bits of data. First it makes the clockPin HIGH using digitalWrite().

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for (i = 0; i < 8; ++i) {
    digitalWrite(clockPin, HIGH);

Put the received data, which is got by digitalRead(), into the value.

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    if (bitOrder == LSBFIRST)
        value |= digitalRead(dataPin) << i;
    else
        value |= digitalRead(dataPin) << (7 - i);
    digitalWrite(clockPin, LOW);
}

When the bitOrder is LSBFIRST, this function assembles data from LSB. In the i-th loop, it puts the received data to the i-th bit of value.

When the bitOrder is not LSBFIRST, this function assembles data from MSB. In the i-th loop, it puts the received data to the (7 - i-th) bit of value.

After receiving a bit, it outputs LOW to the clockPin.

Finally it returns the value.

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    return value;
}

Version

Arduino AVR Boards 1.8.6

Last Update

March 21, 2023