Save the need of pull up

[edi]

I have a very cost sensitive project so I want to save the need of external pull ups resistors on inputs pins.
Is there a trick of eliminate the needs of external pull ups for inputs from switches to PIC pins that does not have internal pu?

Thanks.

[Ttelmah]

It depends totally on the 'nature' of the external signals.
Probably not.
But in some cases (loads that are not 'switches'), it may be possible to momentarily drive them by switching the pins to 'output_high', and then to float, followed by a read.
On switches, this will cause excessive currents, but on some capacitive/resistive signals, this may be done.
Remember that quite a few of the newer small PIC's have far more internal puil-up's available, and even in some cases pull-down's.

Best Wishes

[PCM programmer]

Read this thread:
http://www.ccsinfo.com/forum/viewtopic.php?t=21418
I would never voluntary design a product with no pull-ups.

However, SMD resistors are cheap. A reel of 4.7K, 0603 (1608 metric),
5%, 100ppm, 0.1w, thick film resistors costs only $5.00 USD. That's for
5000. Let's say you have 10 pull-ups, that's only 1 cent USD.

[Ttelmah]

The comment "ha'porth of tar" might well apply.

[temtronic]

While there 'might' be a possible trick, you will NOT save a penny! Company 'bean counter' wanted me to shave every penny I could off a PICbased product 10 years ago,saved all of $2-$3 per board.When he got the R&D bill for that he went nuts! Something rarely considered is the amount of time($$) that goes into properly designing a reliable product.Saving a few cents now WILL cost you big bucks when unreliable products with random problems come back for repair."Old school" philosophy, design product like it's going to the moon.One way trip and it MUST work when it gets there.

OK, one 'trick' that does work is if possible, use the same value resistors for several purposes.Say 10K for pullups, instead of 4k7s here, 10Ks there.

Another(though I haven't checked prices recently) is to use 10r resistors as jumpers instead of 0r resistors.Long ago 0r resistors in 1/4w pkg were more money.

Another money saver,in the long run, is to use a more powerful PIC for all products.The 'Swiss army knife' philosophy has saved me lots of money over the years.Sure the 18F46K22 is overkill for most projects but by having a 'core' of well known and tested functions,subsystems, etc. saves R&D time which costs a LOT more than PICs and parts.We always need more I/O pins, speed, peripherals and having a common PCB also helps save money, as does a common case.heck, cases cost more than PICs these days!

The bottom line, is well the bottom line, you have to see the cost of the WHOLE product, PICs,parts,R&D, labour, etc. NOT just a few resistors.

hth
jay

[Ttelmah]

and add to this, the 'new' PIC's with lots of features, are often cheaper!....