Post by thread:[PIC]: can input be connected directly to VDD

Can an input of a PIC be connected directly to VDD to set a High level ? What about other digital IC's such as 74HCXXX or FPGA's ? One of my guys here insists that input should be tied to VDD through a resistor. Thanks, Tal -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

At 10:58 AM 1/24/03 -0800, Tal wrote: >Can an input of a PIC be connected directly to VDD to set a High level ? >What about other digital IC's such as 74HCXXX or FPGA's ? > >One of my guys here insists that input should be tied to VDD through a >resistor. Your guy is correct. Think about what happens if, somehow, that PIC pin gets turned into an output. Many things can cause this - an ESD event that corrupts the TRIS register is only one such possibility. In general, input only pins on CMOS devices can tie directly to GND or VDD without intervening resistors. Un-used pins on TTL devices should tie HI via a 1K or so resistor. The way I prefer to deal with un-used I/O PIC pins is to leave them un-connected and set as outputs. dwayne -- Dwayne Reid <spam_OUTdwaynerTakeThisOuTplanet.eon.net> Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 18 years of Engineering Innovation (1984 - 2002) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

At 10:58 AM 1/24/03 -0800, you wrote: >Can an input of a PIC be connected directly to VDD to set a High level ? >What about other digital IC's such as 74HCXXX or FPGA's ? > >One of my guys here insists that input should be tied to VDD through a >resistor. Input-only can be tied to Vdd (except for old bipolar TTL parts). Otherwise (bidirectional microcontroller pins) best to use a series resistor. Same with ground. You can safely tie an open drain output I/O to ground, but other bi-di should be through an R. Best regards, Spehro Pefhany --"it's the network..." "The Journey is the reward" .....speffKILLspam@spam@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

It can, whether it should or not is another matter. Some might worry that if the pin is accidentally set as an output and driven low, you will be shorting out your power supply. Josh -- A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools. -Douglas Adams Tal wrote: > > Can an input of a PIC be connected directly to VDD to set a High level ? > What about other digital IC's such as 74HCXXX or FPGA's ? > > One of my guys here insists that input should be tied to VDD through a > resistor. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

I hope you didn't take a strong position on this... Give him a pat on the back for insisting on good design practice. If he knows the difference between a soft-pullup and a hard one, give him a raise also... CL {Quote hidden}> > Can an input of a PIC be connected directly to VDD to set a > High level ? > What about other digital IC's such as 74HCXXX or FPGA's ? > > One of my guys here insists that input should be tied to VDD through a > resistor. > > Thanks, > > Tal > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

On Fri, 24 Jan 2003, Tal wrote: *>Can an input of a PIC be connected directly to VDD to set a High level ? *>What about other digital IC's such as 74HCXXX or FPGA's ? *> *>One of my guys here insists that input should be tied to VDD through a *>resistor. It's ok for a PIC but not for older 74xx. 74[H]Cxx inputs can be connected directly to Vdd. FPGAs are usually cmos, so yes, unless the manufacturer says otherwise. You would use a pullup if you plan to drive the pin low at some time or other (f.ex. MCLR on a PIC). Peter -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

> Can an input of a PIC be connected directly to VDD to set a High level ? > What about other digital IC's such as 74HCXXX or FPGA's ? No problem as long as the connection is close to the IC's power pin. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Thanks James. Tal {Quote hidden}> -----Original Message----- > From: James Newton, webhost [EraseMEjamesnewtonspam_OUTTakeThisOuTpiclist.com] > Sent: Sunday, January 26, 2003 12:41 AM > To: Pic Microcontroller Discussion List > Cc: talspam_OUTZAPTA.COM > Subject: Re:[PIC]: can input be connected directly to VDD > > > source= > http://www.piclist.com/postbot.asp?id=piclist\2003\01\24\135928a > > This is FAQ > http://www.piclist.com/logic/xtrapins "What should I do with pins that I am not using? " This covers all sides of the issue and I think it does so fairly. --- James Newton: PICList.com webmaster, former Admin #3 @spam@jamesnewtonKILLspampiclist.com 1-619-652-0593 phone http://www.piclist.com/member/JMN-EFP-786 PIC/PICList FAQ: http://www.piclist.com -- http://www.piclist.com hint: To leave the PICList KILLspampiclist-unsubscribe-requestKILLspammitvma.mit.edu>