What is the use of port_x_pullups ( )

[srikrishna]

I know external pull up (like pull up and pull down resistor). But can not understand what is internal pull up and its function.

The ccs c manual says

port_a_pullups (value)

It sets the input pullups. TRUE will activate, and a FALSE will
deactivate.

I can not understand the function of pull up. Anyone describe me briefly?

[Ttelmah]

This depends on your PIC.

Down to reading it's data sheet.

Now a lot of PIC's have a tiny 'programmable current source' on some pins. It's a MOSFET, and a couple of resistors, able to deliver a small current into the pin when turned on. The current is small. Typically equivalent to perhaps a 50KR resistor. On some PIC's this is just enabled/disabled for the whole port. On others there are individual enables for each bit. For chips that have it on the whole port, the syntax is simply:

port_a_pullups(TRUE);

to enable the whole port. On chips that have individual enables, the syntax is:

port_a_pullups(0b00001111);

or a similar pattern to enable it on individual bits (A0..A3 as shown).

Relatively few PIC's have the pullup on port A. The data sheet will tell you if yours does. Most do have some form on port B. Some have it on other pins as well.

A few of the higher PIC's (PIC24's etc.). have programmable pull-downs as well.

So the first thing to do is read the data sheet for the chip you are using.
It's under the section 'I/O Ports'.

For example, for a PIC16F1826:

[srikrishna]

And what its utilities?

[RF_Developer]

[temtronic]

AS RF D says they can be used instead of real external resistors saving space and cost. However I've never used them in over 2 decades of 'playing with PICs'.
The cost 'savings' isn't really great. Maybe 1 cent per pin at most. So yes, IF you make 1,000s of boards it might add up to a dollar or two BUT forget to enable the internal pullups and it'll cost you big $$$ in R&D time to recode/recompile/reinstall...
The space 'savings' isn't much either and frankly I don't do itty, bitty, sub micro sized PCBs. I prefer my 4by6 inch PCB with lots of layout room where I can SEE PIC and pieces.
The HUGE reason I don't do it is some PICs have them, some have whole port pullups, some are individually enabled. Now IF you ONLY use ONE PIC for all projects and products (I only use 3), then you'll always (well hopefully) remember to enable the pullups. If you forget to do that even ONCE, then the cost to redo the software is HUGE compared to the small cost of installing 8 resistors that visually show you there are pullups. Also YOU get to choose the value of pullup. 'Soft' like 50K for power savings, 'hard' like 10K for consistent switch response.
The feature of internal PIC pin pullups is a nice idea but for me it's not one that I have used or care to. It'd be interesting to here from those that do use it and what what level of products it's in.

Jay

[newguy]

If management is counting pennies on every board produced, the internal pullups are attractive. If you are space constrained on your PCB, the internal pullups are attractive. If you are layer constrained on your PCB (which has implications regarding layout size/sprawl), again due to counting pennies, then internal pullups are attractive.

[PCM programmer]

You use the 18F2550.
Look in this section on page 379 of the 18F2550 data sheet:

[Ttelmah]

A typical example of their use is one I did recently.

Had an SPI interfaced chip, that the customer after initial design, decided might not be present on some boards. The chip is guaranteed to return all 0's in the top byte after a data read. Simply programmed the pull-up 'on' for the data line, and then at the initial read, check the byte. If it is all zeros the chip is present, if not I just set a flag to stop talking to the chip.
Without the pull-ups there would have had to be a redesign to add a resistor, or the data on this line might be unreliable. With it, problem solved....

[SherpaDoug]

The software controlled pull-ups can also be useful when power savings is extremely important. You can have a pull-up when you need it but shut it off when you don't.
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