Is is possible to do the shiftOut() command with fewer than 8 bits? I am getting a digital pot working and it requires that I send it special groups of commands (1 bit then 7 bits then 7 bits). I want to do shiftOut to send these commands, as it does the whole clock pin thing. Is this possible? If I feed it only 1 byte will it only spit out 1 byte, or assume the rest are zeroes? Is there a suitable replacement command? Thanks!
Te shiftOut() function is quite simple. You may want to include a copy of its source in your sketch and then rename and modify it to match your requirements. The source is as follows:
void shiftOut(uint8_t dataPin, uint8_t clockPin, uint8_t bitOrder, byte val)
{
int 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);
}
}
Ahh, thanks! I could easily use that.
sorry to hook in to this post, but i was reading someting i can't find a explanation off:
digitalWrite(dataPin, !!(val & (1 << i)));
what does the double exclamations do? quite difficult to google it. 
and where do you find this source code of internal functions?
Simple! The !! casts it as a boolean, even if it's an integer. basically a zero returns a zero and anything else returns a one. So even if it's not the LSB that's a high value, the function will still return a 1. Otherwise you have to reverse bitshift to make the LSB a 1, and that's slower than !!. I am the google-fu master! 
But yeah, where do you get the source code for functions? And I would imagine they would be in avr-c, so maybe the person who posted made up that code.
what does the double exclamations do? quite difficult to google it.
The exlamation point "!" is the C/C++ logical NOT operator. If you apply the operator twice (!!) - anything not zero will be equal to "1" and anyting zero will remain as zero. One alternative to using this trick with the double NOT operator would be something like:
if (val & (1 << i)) digitalWrite(dataPin,HIGH);
else digitalWrite(dataPin, LOW);
and where do you find this source code of internal functions?
The source is part of the Arduino download. Whenever you compile/upload a sketch - the Arduino core (such as the shiftOut function) will also be compiled and linked with your program.
yes. that i know, but on what path do you find them. like /Users/simon/foobar/blabla
If you're working on creating cores for new mcu's you should know where to find the source.
The following is the path for the Arduino core.
...\arduino\hardware\arduino\cores\arduino
Additional cores (e.g. Tiny) should have a directory of its own under cores.
On a mac, you need to right-click the arduino app in finder, and click "show package contents", and then there are a few folders in there (I forget specifics) but you should be able to find stuff pretty quickly.
to be exact:
.../arduino/hardware/arduino/cores/arduino/wiring_shift.c