i have a 74166 PISO IC and trying to make multi inputs to single inputs for GPIO
Not able to get a proper design or document relating this !!!!
74166 74595 etc are available but not this 74166
Please Guide . . Thanks !! 
Here you go, all there is to know about it:
If you have google on your internets over there, you can also find info with serach terms like
arduino 74166
HTH
a7
Ya ya Sir already mentioned that I dint found its interaction arduino
Only few detals about the characteristics are avalable like pin detail etc
I just want proper code script to handle this and real connections
The data sheet has all you need to know to write the code. It is used as a parallel to serial converter, sort of the opposite of the 74595. Unless you are trying to cascade multiple parts ignore the serial in.
The sequence would be
Latch parallel data into shift register
Shift data out 1 bit at a time
Do whatever with the data
Repeat
The pins interactions to do this are in the data sheet. I haven't used one in decades, so I don't have any code.
Oh OK then I have to go through that !!
Is there any difference between using GPIO pins of Arduino and using a single input pin with multiple inputs of 74166 ??
I mean is there any speed difference or more complications ?
If this parallel to serial out is there then why MEGA model is having so many input pins ??
Why not :
pin numbers are like UNO
Less space & small size
MEGA with much memory and less space and less number of pins would be a great hardware design then . . ???
Designers can choose whatever hardware they want to work with.
There are many factors that would go into choosing a resource limited microprocessor and fixing that with external ICs and software versus using a microprocessor that had plenty of real GPIO in the first place.
Some or all or none of those factors may be important to you and the kinds of things you've managed to do.
Here, for example, you are stuck because no one, it seems, has written a library for you, and no one, it seems, is just going to dash off the code. For you.
So, time being money and even if your time is worth very little, you'd be happily getting on with whatever you are doing if you just used a MEGA…
instead of having a different kind of fun - read the dtatsheet, read the code for similar parts that do have libraries, scratch your head, get help with your attempts answer so forth. Write some code, get it working, use that progress in your real work.
It all comes down to what kind of fun you wanna have, and what kind of work effort you want to invest.
a7
Another question is why use a 74166? It is a very old part and there are many other options now. Unless you are trying to interface to some ancient device that uses it I would use a I2C port expander. Fewer pins, libraries available, a lot of users to answer questions.
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