LSB/MSB and shiftOut

Question

Here is the shiftOut function code from wiring_shift.c

void shiftOut(uint8_t dataPin, uint8_t clockPin, uint8_t bitOrder, uint8_t val)
{
     uint8_t i;

     for (i = 0; i < 8; i++)  {
           if (bitOrder == LSBFIRST)
                 digitalWrite(dataPin, !!(val & (1 << i)));
           else      
                 digitalWrite(dataPin, !!(val & (1 << (7 - i))));

           digitalWrite(clockPin, HIGH);
           digitalWrite(clockPin, LOW);            
     }
}

Does this mean that least significant bit is on the left and most significant bit is on the right? Or LSBFIRST means that will be the last outputted bit so it will be "first" in outputted sequence?

One more thing

here is a sample code:

shiftOut(dataPin, clockPin, LSBFIRST, B00101001);
shiftOut(dataPin, clockPin, LSBFIRST, B11010011);

Why does this

union 
{
     word uniform;
     byte separate[2];
} state;

state.uniform = 54057; // aka 11010011 00101001
shiftOut(dataPin, clockPin, LSBFIRST, state.separate[1]);
shiftOut(dataPin, clockPin, LSBFIRST, state.separate[0]);

have different output but if i swap last two lines it works correctly?

score 0 · Accepted Answer · answered Jan 23 '20 at 09:44

Integer values of the Arduino have the MSB left and the LSB right. The shiftOut() function let's you choose, how the order should be timewise. If you provide LSBFIRST, the function will start the transmit with the LSB (on the right) and then go left to the MSB. Otherwise it will start with the MSB and then go right to the LSB. So this effects only the time order of the bits, in that they get transmitted.

score 0 · Answer 2 · answered Jan 23 '20 at 09:45

There is no "left" or "right" in a binary number, only in the human representation of that number.

It is traditional for us to place the most significant bit on the left, purely because that his how we represent other numbers - like 1234 (1 is the most significant digit, the thousands).

Given the pseudo-number 12345678 if you shift out LSBFIRST the 8 gets output first, then the 7, then the 6, etc. In MSBFIRST the 1 gets output first, then the 2, then the 3, etc.

LSB/MSB and shiftOut

2 Answers2