I have ordered a bunch of shift registers (CD4021B) in order to play with them on the arduino while trying to find ways to read more inputs with fewer pins.
I wired everything up, correctly (I quadriple checked) and fired the code using http://arduino.cc/en/Reference/ShiftIn
//define where your pins are
int latchPin = 8;
int dataPin = 9;
int clockPin = 7;//Define variables to hold the data for shift register.
//starting with a non-zero numbers can help troubleshoot
byte incoming = 254;void setup() {
//start serial
Serial.begin(9600);pinMode(latchPin, OUTPUT);
pinMode(clockPin, OUTPUT);
pinMode(dataPin, INPUT);}
void loop() {
//Pulse the latch pin:
//set it to 1 to collect parallel data
digitalWrite(latchPin,1);
//set it to 1 to collect parallel data, wait
delayMicroseconds(100);
//set it to 0 to transmit data serially
digitalWrite(latchPin,0);incoming = shiftIn(dataPin, clockPin, MSBFIRST);
Serial.println(incoming, BIN);
delay(500);
}
It seemed to work, but it behaved wrong. I had another arduino acting as input (lowering and highering some pins on the shift register) and my data didn't match.
After a few hours of pondering I tried the code given in the tutorials section (http://www.arduino.cc/en/Tutorial/ShiftIn).
In the tutorial, the author declares his own shiftIn() function:
////// ----------------------------------------shiftIn function
///// just needs the location of the data pin and the clock pin
///// it returns a byte with each bit in the byte corresponding
///// to a pin on the shift register. leftBit 7 = Pin 7 / Bit 0= Pin 0byte shiftIn(int myDataPin, int myClockPin) {
int i;
int temp = 0;
int pinState;
byte myDataIn = 0;pinMode(myClockPin, OUTPUT);
pinMode(myDataPin, INPUT);
//we will be holding the clock pin high 8 times (0,..,7) at the
//end of each time through the for loop//at the begining of each loop when we set the clock low, it will
//be doing the necessary low to high drop to cause the shift
//register's DataPin to change state based on the value
//of the next bit in its serial information flow.
//The register transmits the information about the pins from pin 7 to pin 0
//so that is why our function counts down
for (i=7; i>=0; i--)
{
digitalWrite(myClockPin, 0);
delayMicroseconds(0.2);
temp = digitalRead(myDataPin);
if (temp) {
pinState = 1;
//set the bit to 0 no matter what
myDataIn = myDataIn | (1 << i);
}
else {
//turn it off -- only necessary for debuging
//print statement since myDataIn starts as 0
pinState = 0;
}//Debuging print statements
//Serial.print(pinState);
//Serial.print(" ");
//Serial.println (dataIn, BIN);digitalWrite(myClockPin, 1);
}
//debuging print statements whitespace
//Serial.println();
//Serial.println(myDataIn, BIN);
return myDataIn;
}
and incorporates his shiftIn in the example he provides (http://arduino.cc/en/Tutorial/ShftIn11).
With the second shiftIn, the one provided in the tutorial, everything works fine.
What are the differences between the two? Is this a bug in the software or is the CD4021B incompatible with arduino's shiftIn?
I would like to sort this out so that we can update the reference page.
Thanks in advance 
ps: first post here, being messing with arduino for two-three months now, I love it!