Post by thread:A stupid question but...

Is it possible to multiplex the scanning of 4 push-button switches on the same 4 'data' lines as an LCD module is using ? I've sort of thought along the lines of having all of the switches commoned at one side and then having an OPen -collector arrangement to short the switches to ground at read time. The drawback with this arrangement is that when the switches are NOT being read and one or more switches are closed, data lines will be shorted together. Adding diodes seemed like an obvious step forward but the 0.7V drop takes them to the top end of the TTL logic '0' level. Anyone got any suggestions please ? Also I am looking at using a standard 16x2 LCD with these switches to allow menu selection and configuration settings - has anyone done anything that allows the PIC to communicate with a host processor (in this case it is ACORN podule bus) to download menu choices, selections etc. I am trying to reduce the amount of processing done by the host so that the PIC can deal with most of the menu control if poss........ \\\|/// \\ - - // "Yes it IS safe ( @ @ ) to switch on" +---oOOo-(_)-oOOo------------------+ | Mark Birks | | Hardware Section Leader | | OmniBus Systems, Stanford House, | | Stanford-on-Soar, | | Loughborough, Leicestershire. UK.| | | | Tel/Fax: +44 (0)990 004300/333 | | E-Mail: spam_OUTmarkbTakeThisOuTomnibus.co.uk | | ooo0 | | ( ) 0ooo | +---\ (----( )------------------+ \_) ) / (_/

> >Also I am looking at using a standard 16x2 LCD with these switches to >allow menu selection and configuration settings - has anyone done >anything that allows the PIC to communicate with a host processor (in >this case it is ACORN podule bus) to download menu choices, selections >etc. I am trying to reduce the amount of processing done by the host so >that the PIC can deal with most of the menu control if poss........ > I did this a while ago - for this type of app, throughput isn't usually an issue, so I used a 4 bit port (part of a GAL) on the podule bus to do synchronous serial to the PIC, bnit-bashed by the ARM with req/ack handshaking. higher-end 40 pin PICs have a bus interface, which makes life easier, but read the errata sheets - there were some serious bugs in early versions!

... and the Ge diodes will drop you only ~0.4V compared of Si = ~0.7! Please respond to .....wwlKILLspam@spam@netcomuk.co.uk To: PICLISTKILLspamMITVMA.MIT.EDU cc: (bcc: Jaroslaw Pawlega/Perle) Subject: Re: A stupid question but... O> >The drawback with this arrangement is that when the switches are NOT >being read and one or more switches are closed, data lines will be >shorted together. Adding diodes seemed like an obvious step forward but >the 0.7V drop takes them to the top end of the TTL logic '0' level. Yeah, but the PIC uses CMOS levels, so no problem! Vil max for 16c5x is 0.2vdd = 1.0v at 5v

Hi, > Is it possible to multiplex the scanning of 4 push-button switches on > the same 4 'data' lines as an LCD module is using ? > > I've sort of thought along the lines of having all of the switches > commoned at one side and then having an OPen -collector arrangement to > short the switches to ground at read time. > > The drawback with this arrangement is that when the switches are NOT > being read and one or more switches are closed, data lines will be > shorted together. Adding diodes seemed like an obvious step forward but > the 0.7V drop takes them to the top end of the TTL logic '0' level. > > Anyone got any suggestions please ? > I have previously done it in the following way (Repeat for each data line): VCC | o| o| | / \ 4K7 / \ | PIC--------------O----------------------------LCD Dx | / \ / 47K \ | Gnd When the PIC pin is an output and in the worst case, the output either has to drive against the 47K or the 4K7 resistor. This will be no problem. So the PIC would be able to drive the line to a valid logic value, irrespective of whether the button is pressed. To read the button, change the pin to an input. If the button is not pressed, the 47K will pul the line low. If it is pressed, the line will be at 5V*(47/(47+4.7))=4.5V which is a valid high level (I assume that the LCD draws negligible current on its data inputs). Hope this helps Niki

Nice one thanks. I'm still a little confused as to how to deal with the situation where the switches are NOT enabled (ie common portion floating) and you are accessing the LCD when TWO or more switches are pressed. Surely this will short two data lines together and invalidate the data to/from the LCD ? Please excuse my ignorance on this - it is my first (very ambitious) PIC project. \\\|/// \\ - - // "Yes it IS safe ( @ @ ) to switch on" +---oOOo-(_)-oOOo------------------+ | Mark Birks | | Hardware Section Leader | | OmniBus Systems, Stanford House, | | Stanford-on-Soar, | | Loughborough, Leicestershire. UK.| | | | Tel/Fax: +44 (0)990 004300/333 | | E-Mail: .....markbKILLspam.....omnibus.co.uk | | ooo0 | | ( ) 0ooo | +---\ (----( )------------------+ \_) ) / (_/ {Quote hidden}> ---------- > From: Jason Wolfson[SMTP:EraseMEjasonspam_OUTTakeThisOuTMA.ULTRANET.COM] > Reply To: pic microcontroller discussion list > Sent: 29 October 1997 18:42 > To: PICLISTspam_OUTMITVMA.MIT.EDU > Subject: Re: A stupid question but... > > Mark, > I did this with a 20x4 LCD and it worked great, > check out my code at http://www.ultranet.com/~jason > > I put the 4 buttons at each corner of the display and you > put up a message to their function depending on what you want when... > Kinda like an ATM machine.... > > Jason > > > > {Original Message removed}

I have a similar problem, using a 4x20 and had a look at the site, but found no code to look at Gordon At 09:30 30/10/97 -0000, you wrote: {Quote hidden}>Nice one thanks. > >I'm still a little confused as to how to deal with the situation where >the switches are NOT enabled (ie common portion floating) and you are >accessing the LCD when TWO or more switches are pressed. Surely this >will short two data lines together and invalidate the data to/from the >LCD ? > >Please excuse my ignorance on this - it is my first (very ambitious) PIC >project. > > \\\|/// > \\ - - // "Yes it IS safe > ( @ @ ) to switch on" >+---oOOo-(_)-oOOo------------------+ >| Mark Birks | >| Hardware Section Leader | >| OmniBus Systems, Stanford House, | >| Stanford-on-Soar, | >| Loughborough, Leicestershire. UK.| >| | >| Tel/Fax: +44 (0)990 004300/333 | >| E-Mail: @spam@markbKILLspamomnibus.co.uk | >| ooo0 | >| ( ) 0ooo | >+---\ (----( )------------------+ > \_) ) / > (_/ > > >> ---------- >> From: Jason Wolfson[SMTP:KILLspamjasonKILLspamMA.ULTRANET.COM] >> Reply To: pic microcontroller discussion list >> Sent: 29 October 1997 18:42 >> To: RemoveMEPICLISTTakeThisOuTMITVMA.MIT.EDU >> Subject: Re: A stupid question but... >> >> Mark, >> I did this with a 20x4 LCD and it worked great, >> check out my code at http://www.ultranet.com/~jason >> >> I put the 4 buttons at each corner of the display and you >> put up a message to their function depending on what you want when... >> Kinda like an ATM machine.... >> >> Jason >> >> >> >> {Original Message removed}

At the main screen click on the shareware option (under the Microchip logo on the RHS). \\\|/// \\ - - // "Yes it IS safe ( @ @ ) to switch on" +---oOOo-(_)-oOOo------------------+ | Mark Birks | | Hardware Section Leader | | OmniBus Systems, Stanford House, | | Stanford-on-Soar, | | Loughborough, Leicestershire. UK.| | | | Tel/Fax: +44 (0)990 004300/333 | | E-Mail: spamBeGonemarkbspamBeGoneomnibus.co.uk | | ooo0 | | ( ) 0ooo | +---\ (----( )------------------+ \_) ) / (_/ {Quote hidden}> ---------- > From: Gordon Zerf[SMTP:TakeThisOuTkerzerEraseMEspam_OUTHARARE.IAFRICA.COM] > Reply To: pic microcontroller discussion list > Sent: 30 October 1997 10:56 > To: RemoveMEPICLISTTakeThisOuTMITVMA.MIT.EDU > Subject: Re: A stupid question but... > > I have a similar problem, using a 4x20 and had a look at the site, > but > found no code to look at > > Gordon > > > At 09:30 30/10/97 -0000, you wrote: > >Nice one thanks. > > > >I'm still a little confused as to how to deal with the situation > where > >the switches are NOT enabled (ie common portion floating) and you are > >accessing the LCD when TWO or more switches are pressed. Surely this > >will short two data lines together and invalidate the data to/from > the > >LCD ? > > > >Please excuse my ignorance on this - it is my first (very ambitious) > PIC > >project. > > > > \\\|/// > > \\ - - // "Yes it IS safe > > ( @ @ ) to switch on" > >+---oOOo-(_)-oOOo------------------+ > >| Mark Birks | > >| Hardware Section Leader | > >| OmniBus Systems, Stanford House, | > >| Stanford-on-Soar, | > >| Loughborough, Leicestershire. UK.| > >| | > >| Tel/Fax: +44 (0)990 004300/333 | > >| E-Mail: markbEraseME.....omnibus.co.uk | > >| ooo0 | > >| ( ) 0ooo | > >+---\ (----( )------------------+ > > \_) ) / > > (_/ > > > > > >> ---------- > >> From: Jason Wolfson[SMTP:EraseMEjasonMA.ULTRANET.COM] > >> Reply To: pic microcontroller discussion list > >> Sent: 29 October 1997 18:42 > >> To: RemoveMEPICLISTEraseMEEraseMEMITVMA.MIT.EDU > >> Subject: Re: A stupid question but... > >> > >> Mark, > >> I did this with a 20x4 LCD and it worked great, > >> check out my code at http://www.ultranet.com/~jason > >> > >> I put the 4 buttons at each corner of the display and you > >> put up a message to their function depending on what you want > when... > >> Kinda like an ATM machine.... > >> > >> Jason > >> > >> > >> > >> {Original Message removed}

On Thu, 30 Oct 1997 11:31:36 GMT+0200 "N STEENKAMP [M.ING E&E]" <RemoveMESTEENKMPspam_OUTKILLspamFIRGA.SUN.AC.ZA> writes: {Quote hidden}>Hi, > >> Is it possible to multiplex the scanning of 4 push-button switches >on >> the same 4 'data' lines as an LCD module is using ? >> >I have previously done it in the following way (Repeat for each data >line): > VCC > | > o| > o| > | > / > \ 4K7 > / > \ > | >PIC--------------O----------------------------LCD Dx > | > / > \ > / 47K > \ > | > Gnd > This will work. I usually use this circuit: Vcc | R1 |---R2--> PIC/LCD | | o | o | GND This way the PIC gets full 5V/0V drive regardless of the value of the resistors. When the PIC pins aren't in use for the LCD or the switches, make them inputs or set them high to keep from wasting current through the resistors. If current flow through the switches while they are held closed is a problem, connect the other end of the switches to a port so they can be open or high when not being read. The R2's prevent crosstalk between the lines in any case. This scheme can also be used with any input, not just a switch. I wouldn't recommend it for fast-changing signals though since the resistors will limit the speed. It is ideal for status lines from voice-store chips, DTMF decoders, etc.

Thank you for your help, much appreciated. At 13:24 30/10/97 -0000, you wrote: {Quote hidden}>At the main screen click on the shareware option (under the Microchip >logo on the RHS). > > \\\|/// > \\ - - // "Yes it IS safe > ( @ @ ) to switch on" >+---oOOo-(_)-oOOo------------------+ >| Mark Birks | >| Hardware Section Leader | >| OmniBus Systems, Stanford House, | >| Stanford-on-Soar, | >| Loughborough, Leicestershire. UK.| >| | >| Tel/Fax: +44 (0)990 004300/333 | >| E-Mail: RemoveMEmarkbTakeThisOuTspamomnibus.co.uk | >| ooo0 | >| ( ) 0ooo | >+---\ (----( )------------------+ > \_) ) / > (_/ > > >> ---------- >> From: Gordon Zerf[SMTP:EraseMEkerzerspamspamBeGoneHARARE.IAFRICA.COM] >> Reply To: pic microcontroller discussion list >> Sent: 30 October 1997 10:56 >> To: RemoveMEPICLISTKILLspamMITVMA.MIT.EDU >> Subject: Re: A stupid question but... >> >> I have a similar problem, using a 4x20 and had a look at the site, >> but >> found no code to look at >> >> Gordon >> >> >> At 09:30 30/10/97 -0000, you wrote: >> >Nice one thanks. >> > >> >I'm still a little confused as to how to deal with the situation >> where >> >the switches are NOT enabled (ie common portion floating) and you are >> >accessing the LCD when TWO or more switches are pressed. Surely this >> >will short two data lines together and invalidate the data to/from >> the >> >LCD ? >> > >> >Please excuse my ignorance on this - it is my first (very ambitious) >> PIC >> >project. >> > >> > \\\|/// >> > \\ - - // "Yes it IS safe >> > ( @ @ ) to switch on" >> >+---oOOo-(_)-oOOo------------------+ >> >| Mark Birks | >> >| Hardware Section Leader | >> >| OmniBus Systems, Stanford House, | >> >| Stanford-on-Soar, | >> >| Loughborough, Leicestershire. UK.| >> >| | >> >| Tel/Fax: +44 (0)990 004300/333 | >> >| E-Mail: markbSTOPspamspam_OUTomnibus.co.uk | >> >| ooo0 | >> >| ( ) 0ooo | >> >+---\ (----( )------------------+ >> > \_) ) / >> > (_/ >> > >> > >> >> ---------- >> >> From: Jason Wolfson[SMTP:spamBeGonejasonSTOPspamEraseMEMA.ULTRANET.COM] >> >> Reply To: pic microcontroller discussion list >> >> Sent: 29 October 1997 18:42 >> >> To: KILLspamPICLISTspamBeGoneMITVMA.MIT.EDU >> >> Subject: Re: A stupid question but... >> >> >> >> Mark, >> >> I did this with a 20x4 LCD and it worked great, >> >> check out my code at http://www.ultranet.com/~jason >> >> >> >> I put the 4 buttons at each corner of the display and you >> >> put up a message to their function depending on what you want >> when... >> >> Kinda like an ATM machine.... >> >> >> >> Jason >> >> >> >> >> >> >> >> {Original Message removed}

Mike Keitz suggested placing the pull-up directly on the switch, rather than at the PIC, which is sound advice ... except ... that my first thought when I read the original request is that "this is what the Port B weak pull-ups of various PICs are for". Presuming that parts count is a prime consideration, it makes sense to use these where they are provided. The voltage divider effect is of concern but the maximum specified value for the weak pull-ups is 400µA corresponding to the logic low maximum specification of 800mV if a 2K resistor is used in series with the pushbutton. At worst, the 2K resistor will draw 2.5mA at 5V from a port output, well within its drive capacity for as long as it needs to be asserted to send data to the LCD. In this case of course, the I/O pins should idle as LOW outputs or inputs. The weak pull-ups are disabled on pins while they are outputs. Current flow through the switches while they are held closed is unlikely to be a problem for pushbuttons, particularly if these steps are taken. Well, that's my theory anyway! Paul B.

Hi Gordon! In future, spare a moment to make your message a _bit_ shorter? It wasn't totally necessary to include the entire thread! Thanks! Cheers! Gordon Zerf wrote: {Quote hidden}> Thank you for your help, much appreciated. > > At 13:24 30/10/97 -0000, you wrote: > >At the main screen click on the shareware option (under the Microchip > >logo on the RHS). > > > > \\\|/// > > \\ - - // "Yes it IS safe > > ( @ @ ) to switch on" > >+---oOOo-(_)-oOOo------------------+ > >| Mark Birks | > >| Hardware Section Leader | > >| OmniBus Systems, Stanford House, | > >| Stanford-on-Soar, | > >| Loughborough, Leicestershire. UK.| > >| | > >| Tel/Fax: +44 (0)990 004300/333 | > >| E-Mail: EraseMEmarkbEraseMEomnibus.co.uk | > >| ooo0 | > >| ( ) 0ooo | > >+---\ (----( )------------------+ > > \_) ) / > > (_/ > > > > > >> ---------- > >> From: Gordon Zerf[SMTP:@spam@kerzer@spam@spam_OUTHARARE.IAFRICA.COM] > >> Reply To: pic microcontroller discussion list > >> Sent: 30 October 1997 10:56 > >> To: spamBeGonePICLISTKILLspamMITVMA.MIT.EDU > >> Subject: Re: A stupid question but... > >> > >> I have a similar problem, using a 4x20 and had a look at the site, > >> but > >> found no code to look at > >> > >> Gordon > >> > >> > >> At 09:30 30/10/97 -0000, you wrote: > >> >Nice one thanks. > >> > > >> >I'm still a little confused as to how to deal with the situation > >> where > >> >the switches are NOT enabled (ie common portion floating) and you are > >> >accessing the LCD when TWO or more switches are pressed. Surely this > >> >will short two data lines together and invalidate the data to/from > >> the > >> >LCD ? > >> > > >> >Please excuse my ignorance on this - it is my first (very ambitious) > >> PIC > >> >project. > >> > > >> > \\\|/// > >> > \\ - - // "Yes it IS safe > >> > ( @ @ ) to switch on" > >> >+---oOOo-(_)-oOOo------------------+ > >> >| Mark Birks | > >> >| Hardware Section Leader | > >> >| OmniBus Systems, Stanford House, | > >> >| Stanford-on-Soar, | > >> >| Loughborough, Leicestershire. UK.| > >> >| | > >> >| Tel/Fax: +44 (0)990 004300/333 | > >> >| E-Mail: .....markbspam_OUTomnibus.co.uk | > >> >| ooo0 | > >> >| ( ) 0ooo | > >> >+---\ (----( )------------------+ > >> > \_) ) / > >> > (_/ > >> > > >> > > >> >> ---------- > >> >> From: Jason Wolfson[SMTP:TakeThisOuTjason.....TakeThisOuTMA.ULTRANET.COM] > >> >> Reply To: pic microcontroller discussion list > >> >> Sent: 29 October 1997 18:42 > >> >> To: TakeThisOuTPICLISTKILLspamspamMITVMA.MIT.EDU > >> >> Subject: Re: A stupid question but... > >> >> > >> >> Mark, > >> >> I did this with a 20x4 LCD and it worked great, > >> >> check out my code at http://www.ultranet.com/~jason > >> >> > >> >> I put the 4 buttons at each corner of the display and you > >> >> put up a message to their function depending on what you want > >> when... > >> >> Kinda like an ATM machine.... > >> >> > >> >> Jason > >> >> > >> >> > >> >> > >> >> {Original Message removed}

Alessandro Zummo wrote: > why don't we have a 4066 who activates the switches only when > the lcd isn't selected? (Nice diagram) Well, the long and short of it is keeping it simple. You use a whole 4066 to connect/ disconnect four switches, and you use an extra PIC pin function to perform the enable. If you are going to devote these resources, you could do it a lot better, such as using a shift register chip to read up to eight switches (needing pull-up/ pull-down resistors) with three connections to the PIC (latch data and shift may be commoned to the LCD data pins and serial data, being an output may either go to a separate PIC pin or be commoned to a third LCD data pin using a 33K resistor). Or, continuing with the theme of the analog switches, a 4051 could be connected to eight switches (no pull-ups here), a pull-up (4K7) on its common and the ubiquitous 33K resistor to one data line while the other three select the 1-of-8 on the 4051. The theory here is that an extra IC, be it 4066, 4051 or 74HC165 (shift register) COSTS - money, space and PCB holes (though if you are using SMD, the latter are trivial). However, the manufacturing cost for those three ICs is almost identical, so IF you are going to use an extra IC, you might as well get the maximum functionality. Otherwise, the discussion was about avoiding extra ICs. Here's a question to the professionals; am I correct here on costs? Board space: 1 16-pin IC = 14 resistors (through-hole) (& stuffing) 1 16-pin IC = 8 diodes (through-hole) 1 16-pin IC = 4 resistors (SMD) = 4 diodes Money: 1 16-pin IC = 16 resistors = 8 diodes Cheers, Paul B.