Post by thread:[PIC]: Weak pull ups not so weak?

Hi everyone, I have switches connected to Port B of the 18F6720. The inputs are pulled high using the onboard weak pull ups, and the other side of the switches are connected to ground through 10K resistors. When a switch is pressed the input line does not go low enough to trigger the interupt on change. This set up worked fine on the 17C756A. Does anyone know the value of the on board pull ups? I have looked through the data book, but can not find a value for them. Thanks Graham North -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spam_OUTlistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

> -----Original Message----- > From: Graham North [SMTP:.....graham.northKILLspam@spam@CIRRUSRESEARCH.CO.UK] > Sent: Tuesday, March 11, 2003 9:18 AM > To: PICLISTKILLspamMITVMA.MIT.EDU > Subject: [PIC]: Weak pull ups not so weak? > > Hi everyone, > > I have switches connected to Port B of the 18F6720. The inputs are pulled > high using the onboard weak pull ups, and the other side of the switches > are > connected to ground through 10K resistors. > > When a switch is pressed the input line does not go low enough to trigger > the interupt on change. This set up worked fine on the 17C756A. Does > anyone > know the value of the on board pull ups? I have looked through the data > book, but can not find a value for them. > The on board pull ups are current sources rather than simple resistors. My datasheet says that the current is somewhere between 50uA and 400uA. 10k * 400uA = 4V, somewhat above the logic '0' threshold! I suspect the weak pullups got somewhat stronger! Is there a particular reason for having the 10k resistors? Regards Mike ======================================================================= This e-mail is intended for the person it is addressed to only. The information contained in it may be confidential and/or protected by law. If you are not the intended recipient of this message, you must not make any use of this information, or copy or show it to any person. Please contact us immediately to tell us that you have received this e-mail, and return the original to us. Any use, forwarding, printing or copying of this message is strictly prohibited. No part of this message can be considered a request for goods or services. ======================================================================= Any questions about Bookham's E-Mail service should be directed to .....postmasterKILLspam.....bookham.com. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email EraseMElistservspam_OUTTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

>I have switches connected to Port B of the 18F6720. The inputs are >pulled high using the onboard weak pull ups, and the other side of >the switches are connected to ground through 10K resistors. Well I have not looked at the data sheet of the 18F6720, but was does it give as a pull up current for these? The 16F87x data sheet gives 50uA to 400uA, at 5V supply, which with 10k would give 0.5V to 4V as the drop across the resistor. Personally I would have thought 10k resistors excessively high. I would have expected 1k maximum, and probably even lower than that (100-470 ohm). -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservspam_OUTmitvma.mit.edu with SET PICList DIGEST in the body

Between 12.5K and 100K on the 18Fx20 series. Having 10K to ground at the other side of the switch sounds like asking for trouble... Why do you have external resistors at all ? Some kind of protection should the output be set as output by error ? Maybe something like 470 Ohms would work in that case. Or at least something much lower than 20K... Jan-Erik. Graham North wrote: >Hi everyone, > >I have switches connected to Port B of the 18F6720. The inputs are pulled >high using the onboard weak pull ups, and the other side of the switches are >connected to ground through 10K resistors. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email @spam@listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

Graham North wrote: >I have switches connected to Port B of the 18F6720. The inputs are pulled >high using the onboard weak pull ups, and the other side of the switches are >connected to ground through 10K resistors. > >When a switch is pressed the input line does not go low enough to trigger >the interupt on change. This set up worked fine on the 17C756A. Does anyone >know the value of the on board pull ups? I have looked through the data >book, but can not find a value for them. They are quoted as being between 50uA and 400uA in the 18F1x20 data. You can use good old Ohms law to work it out from there. David... ___________________________________________ David Duffy Audio Visual Devices P/L U8, 9-11 Trade St, Cleveland 4163 Australia Ph: +61 7 38210362 Fax: +61 7 38210281 New Web: http://www.audiovisualdevices.com.au ___________________________________________ -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email KILLspamlistservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

If you need/want to protect the ports from beeing set to outputs and someone pressing a key at the same time, use a value low enugh to ensure that the level will be seen as "low" by the PIC in normal cases (port set to input). If you don't need/want that protection, take them away. The reason the weak pull-up are there in the first place, is that you should be able to just connect a switch directly between the port and GND, right ? What is "better" depends, as usual :-) Jan-Erik. Graham North wrote: >> >>Is there a particular reason for having the 10k resistors? >> > >Only that the circuit is inherited from a previous design. Is it best to fit >small resistors, or just use none? -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spamBeGonelistservspamBeGonemitvma.mit.edu with SET PICList DIGEST in the body

> > > >Is there a particular reason for having the 10k resistors? > > > > -----Original Message----- > From: Graham North [SMTP:TakeThisOuTgraham.northEraseMEspam_OUTCIRRUSRESEARCH.CO.UK] > Sent: Tuesday, March 11, 2003 9:52 AM > To: RemoveMEPICLISTTakeThisOuTMITVMA.MIT.EDU > Subject: Re: [PIC]: Weak pull ups not so weak? > > > Only that the circuit is inherited from a previous design. Is it best to > fit > small resistors, or just use none? > Worst case, the maximum input low threshold for PortB is 0.8v (assuming you are running at 5 volts). 0.8/400uA = 2k absolute maximum resistance, half that to give some margin so 1k is a suitable value. The resistors are not actually needed if the pins are never switched to outputs. If this is the case zero ohms would be fine. Mike ======================================================================= This e-mail is intended for the person it is addressed to only. The information contained in it may be confidential and/or protected by law. If you are not the intended recipient of this message, you must not make any use of this information, or copy or show it to any person. Please contact us immediately to tell us that you have received this e-mail, and return the original to us. Any use, forwarding, printing or copying of this message is strictly prohibited. No part of this message can be considered a request for goods or services. ======================================================================= Any questions about Bookham's E-Mail service should be directed to postmasterEraseME.....bookham.com. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email EraseMElistservmitvma.mit.edu with SET PICList DIGEST in the body

> I have switches connected to Port B of the 18F6720. The inputs are > pulled high using the onboard weak pull ups, and the other side of the > switches are connected to ground through 10K resistors. > > When a switch is pressed the input line does not go low enough to > trigger the interupt on change. This set up worked fine on the 17C756A. > Does anyone know the value of the on board pull ups? I have looked > through the data book, but can not find a value for them. The data sheets usually lists the min/max current the pullups can source to ground. In any case, there is something else seriously wrong if a mechanical switch to ground can't pull the input to the logic low level. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email RemoveMElistservEraseMEEraseMEmitvma.mit.edu with SET PICList DIGEST in the body

> Only that the circuit is inherited from a previous design. Is it best > to fit small resistors, or just use none? Loose 'em. Sorry, I didn't catch you had series resistors when I wrote my previous reply. The only reason for a series resistor might be if the the line goes off board where it might pick up noise. Even then, 10Kohms is way to much. If you need to put serious protection filtering on the line, then the interal pullups will probably not be appropriate. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email RemoveMElistservspam_OUTKILLspammitvma.mit.edu with SET PICList DIGEST in the body

> Does anyone > know the value of the on board pull ups? I have > looked through the data > book, but can not find a value for them. What if you wired the pulled-up input pin to ground and measured the current through the wire - couldn't you then calculate the pull-up value using Ohms Law? __________________________________________________ Do you Yahoo!? Yahoo! Web Hosting - establish your business online http://webhosting.yahoo.com -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email RemoveMElistservTakeThisOuTspammitvma.mit.edu with SET PICList DIGEST in the body

You don't have to "wire" anything anywhere, leave the port open and just connect your mAmp meter between the pin and ground. Jan-Erik. Rick Regan wrote: >> Does anyone >> know the value of the on board pull ups? I have >> looked through the data >> book, but can not find a value for them. > > What if you wired the pulled-up input pin to ground > and measured the current through the wire - couldn't > you then calculate the pull-up value using Ohms Law? -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email EraseMElistservspamspamBeGonemitvma.mit.edu with SET PICList DIGEST in the body

>> What if you wired the pulled-up input pin to ground >> and measured the current through the wire - couldn't >> you then calculate the pull-up value using Ohms Law? > > You don't have to "wire" anything anywhere, leave > the port open and just connect your mAmp meter > between the pin and ground. Better yet, read the data sheet to find out what the range of current *could* be, not what it happens to be on your part that day. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email RemoveMElistservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

At 10:35 AM 3/11/03 +0000, Michael Rigby-Jones wrote: >The resistors are not actually needed if the pins are never switched to >outputs. If this is the case zero ohms would be fine. Series resistors are always a good idea when connecting to switches that might get zapped with an ESD hit. One side of the switch should go directly to ground, the other side through a resistor to the PIC pin. The reason for using a series resistor is to limit the peak current and slow the rise time of the ESD spike that actually hits the PIC pin. In your case, having a strong pull-up of 400 uA limits your options. Assuming a VIlo of 0.8V, the largest series resistor value you can tolerate is 2K. Finally, if your switches are arranged such that they can never have a static discharge hit them (behind a Lexan sheet, etc), then you may not even need the resistors as others have suggested. But if a ESD hit can contact any portion of the switch, the series resistors will help protect the PIC. dwayne -- Dwayne Reid <dwaynerSTOPspamspam_OUTplanet.eon.net> Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 19 years of Engineering Innovation (1984 - 2003) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' 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#nomail Going offline? Don't AutoReply us! email spamBeGonelistservSTOPspamEraseMEmitvma.mit.edu with SET PICList DIGEST in the body

> -----Original Message----- > From: Dwayne Reid [SMTP:KILLspamdwaynerspamBeGonePLANET.EON.NET] > Sent: Tuesday, March 11, 2003 5:40 PM > To: EraseMEPICLISTEraseMEMITVMA.MIT.EDU > Subject: Re: [PIC]: Weak pull ups not so weak? > > At 10:35 AM 3/11/03 +0000, Michael Rigby-Jones wrote: > > >The resistors are not actually needed if the pins are never switched to > >outputs. If this is the case zero ohms would be fine. > > Series resistors are always a good idea when connecting to switches that > might get zapped with an ESD hit. One side of the switch should go > directly to ground, the other side through a resistor to the PIC pin. > > The reason for using a series resistor is to limit the peak current and > slow the rise time of the ESD spike that actually hits the PIC pin. > Agreed, although this is not overly relevant in this situation as the OP circuit has the resistors between the switch and ground where it is unlikely to protect the PIC in the event of an ESD hit. Regards Mike ======================================================================= This e-mail is intended for the person it is addressed to only. The information contained in it may be confidential and/or protected by law. If you are not the intended recipient of this message, you must not make any use of this information, or copy or show it to any person. Please contact us immediately to tell us that you have received this e-mail, and return the original to us. Any use, forwarding, printing or copying of this message is strictly prohibited. No part of this message can be considered a request for goods or services. ======================================================================= Any questions about Bookham's E-Mail service should be directed to @spam@postmaster@spam@spam_OUTbookham.com. -- http://www.piclist.com hint: To leave the PICList spamBeGonepiclist-unsubscribe-requestKILLspammitvma.mit.edu>