Post by thread:[PIC:]18F452 A2D problem

Hi I recently started using the PIC micro (having previously used the 68HC11) mainly for hobby purposes. This week I tried to use the A2D converter on the 18F452 and hit a problem. I initialise the A2D as follows: InitializeAD movlw B'11111111' ; Make PortA all inputs movwf TRISA,0 movlw B'11000001' ; Select RC osc, AN0 selected, movwf ADCON0,0 ; A/D enabled movlw B'10001110' ; Make RA0 analog input movwf ADCON1,0 ;Use Vdd and Vss as reference for A2D ;Not using interrupts(polling GO/DONE bit) return In this configuration, the A2D result reads full scale (3F) when the input voltage is only about 3.52V. I first had a MPX5114A connected and couldn't get anything but '3F' so to test it I have a 5K pot connected between 5V and ground with the wiper connected to AN0. I have a 0.1uF cap to ground to prevent noise. If I vary the pot from 0V upwards, the reading starts at '00' and increases till the input voltage reaches 3.52V where it reads '3F'. I tried setting the ADCON1 register so that (PCFG0:3 = '1111') an external reference is used and physically connected Vdd and Vss to AN3 and AN2 respectively and in this configuration it works. I have read the documentation through a few times and can find no reference to Vref/2 or something like that and a friend who also uses the PIC has done the exact same thing but with a 16Fxxx chip and has no problem. Is there something that I am missing or do I just have a defective chip (I hope not as I've used an SMD device) I have searched the list for previous posts on this subject but haven't found any. Please could anyone shed some light on this. Regards Mark Capetown SA NOTE: This e-mail message is subject to the MTN Group disclaimer see http://www.mtn.co.za/email_disclaimer.asp -- 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: Jinx [joecolquittKILLspamCLEAR.NET.NZ] >Sent: 20 January 2004 07:20 >To: .....PICLISTKILLspam.....MITVMA.MIT.EDU >Subject: Re: [PIC:]18F452 A2D problem > > >> so to test it I have a 5K pot connected between 5V and ground > >The maximum recommended pot is 2k5 for the 18F452, not 10k >as on previous families. I can't say if anything else is wrong >with your set-up but you should correct this first > A 5k pot gives a 1.25k source impedance, so this is well in spec. A 10K pot would be just in spec at 2.5K. Mark, could the you post the code you are using to perform the conversion and read the A/D result? Is your PIC well decoupled with suitable capacitors physically near the Vss and Vdd pins? 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 EraseMEpostmasterspam_OUTTakeThisOuTbookham.com. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservspam_OUTmitvma.mit.edu with SET PICList DIGEST in the body

> > A 5k pot gives a 1.25k source impedance > > How did you arrive at that ? Highest impedance is whiper in middle position, norton (or was it thevenin?) equivalent is to parallel both 2.5k halves for a source impedance of 1.25k. Wouter van Ooijen -- ------------------------------------------- Van Ooijen Technische Informatica: http://www.voti.nl consultancy, development, PICmicro products -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email KILLspamlistservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

> > >> so to test it I have a 5K pot connected between 5V and ground > >The maximum recommended pot is 2k5 for the 18F452, not 10k >as on previous families. I can't say if anything else is wrong >with your set-up but you should correct this first > A 5k pot gives a 1.25k source impedance, so this is well in spec. A 10K pot would be just in spec at 2.5K. Mark, could the you post the code you are using to perform the conversion and read the A/D result? Is your PIC well decoupled with suitable capacitors physically near the Vss and Vdd pins? Hi Mike Thanks for the reply The board I made has the pic interfaced to a FTDI USB chip and I have routines to send to and receive from the board and a VB6 program on my pc to send commands and receive info and do the GUI. Both chips have a 0.1uF and 10uF tant decoupling. The whole board uses smd components with the caps within mm of the chips. The simple routine I use for doing the conversion is as follows: do_b bsf ADCON0,GO,0 ; Start A/D conversion adconvtst btfsc ADCON0,DONE,0 ;Is conversion complete goto adconvtst ;No, go test again movf ADRESH,w,0 ;get high byte call sendbyte ;send it to USB movf ADRESL,w,0 ;get low byte call sendbyte ;send it to USB goto again ;back to read USB for command loop NOTE: This e-mail message is subject to the MTN Group disclaimer see http://www.mtn.co.za/email_disclaimer.asp -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spamBeGonelistservspamBeGonemitvma.mit.edu with SET PICList DIGEST in the body

{Quote hidden}>-----Original Message----- >From: Mark Beetge [ MTN - Harrington Street ] > >Hi Mike >Thanks for the reply >The board I made has the pic interfaced to a FTDI USB chip and >I have routines to send to and receive from the board and a >VB6 program on my pc to send commands and receive info and do >the GUI. Both chips have a 0.1uF and 10uF tant decoupling. The >whole board uses smd components with the caps within mm of the >chips. The simple routine I use for doing the conversion is as follows: > >do_b > bsf ADCON0,GO,0 ; Start A/D >conversion >adconvtst btfsc ADCON0,DONE,0 ;Is conversion complete > goto adconvtst ;No, go test >again > > movf ADRESH,w,0 ;get high byte > call sendbyte ;send >it to USB > movf ADRESL,w,0 ;get low byte > call sendbyte ;send it to USB > > goto again ;back to read >USB for command loop > That part looks fine. I'm guessing you have a suitable acquisition delay (around 15us) between setting the channel in ADCON0 and setting the GO bit? One thing to check is that both sets of Vdd/Vss pins are definitely connected to their respective supplies, this is just the sort of odd behaviour that can occur if only one set of pins is powered. 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 TakeThisOuTpostmasterEraseMEspam_OUTbookham.com. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email RemoveMElistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

> That part looks fine. I'm guessing you have a suitable acquisition delay >(around 15us) between setting the channel in ADCON0 and setting the GO bit? >One thing to check is that both sets of Vdd/Vss pins are definitely >connected to their respective supplies, this is just the sort of odd >behaviour that can occur if only one set of pins is powered. At the moment I am initiating a conversion by hitting a button on the PC and then looking at the result. The problem only occurs when I set the A2D to use its internal Vref. If I set it to use an external Vref and supply 5V and Gnd to the respective pins, I get the correct result. R Mark NOTE: This e-mail message is subject to the MTN Group disclaimer see http://www.mtn.co.za/email_disclaimer.asp -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservEraseME.....mitvma.mit.edu with SET PICList DIGEST in the body

> At the moment I am initiating a conversion by hitting a button on the PC > and then looking at the result. The problem only occurs when I set the > A2D to use its internal Vref. If I set it to use an external Vref and > supply 5V and Gnd to the respective pins, I get the correct result. > I could be totally missing the point here, but looking at the data sheet I think the internal reference in only 3v ? So inputs greater than this give f.s.d. (0x3f) I think. (Flame suit on) ;-) Peter > R > Mark > > > NOTE: This e-mail message is subject to the MTN Group disclaimer see http://www.mtn.co.za/email_disclaimer.asp > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > email EraseMElistservmitvma.mit.edu with SET PICList DIGEST in the body -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email RemoveMElistservEraseMEEraseMEmitvma.mit.edu with SET PICList DIGEST in the body

Mark Beetge [ MTN - Harrington Street ] wrote: > In this configuration, the A2D result reads full scale (3F) when the > input voltage is only about 3.52V. 3Fh is never full scale no matter how you set up the A/D. The 18F452 has a 10 bit A/D, the result of which can be either right or left justified into a 16 bit integer. Full scale is therefore either 3FFh or FFC0h. ***************************************************************** 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

Jinx wrote: >> so to test it I have a 5K pot connected between 5V and ground > > The maximum recommended pot is 2k5 for the 18F452, not 10k > as on previous families. I can't say if anything else is wrong with > your set-up but you should correct this first The maximum recommended input impedence is 2.5Kohms. A 5Kohm pot has its highest impedence when the wiper is centered. This makes a divider with two 2.5Kohm resistors, which has an impedence of 1.25Kohms. You can use up to a 10Kohm pot. ***************************************************************** 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 RemoveMElistservTakeThisOuTspammitvma.mit.edu with SET PICList DIGEST in the body

The data sheet on page 185 indicates that the internal reference is Vdd and Vss, but youv'e given me an idea: I use ICSP on the circuit which means that Vdd comes via a resistor (100ohm). Tonight I will check if maybe the voltage drop across it is too great thus giving 3.5 volts as Vdd Mark > A2D to use its internal Vref. If I set it to use an external Vref and > supply 5V and Gnd to the respective pins, I get the correct result. > I could be totally missing the point here, but looking at the data sheet I think the internal reference in only 3v ? So inputs greater than this give f.s.d. (0x3f) I think. (Flame suit on) ;-) Peter email EraseMElistservspamspamBeGonemitvma.mit.edu with SET PICList DIGEST in the body NOTE: This e-mail message is subject to the MTN Group disclaimer see http://www.mtn.co.za/email_disclaimer.asp -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email RemoveMElistservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

I did send a correction email immediately after that one correcting my typo(within 5 minutes) -----Original Message----- From: Olin Lathrop [olin_piclistSTOPspamspam_OUTEMBEDINC.COM] Sent: 20 January 2004 14:33 To: spamBeGonePICLISTSTOPspamEraseMEMITVMA.MIT.EDU Subject: Re: [PIC:]18F452 A2D problem Mark Beetge [ MTN - Harrington Street ] wrote: > In this configuration, the A2D result reads full scale (3F) when the > input voltage is only about 3.52V. 3Fh is never full scale no matter how you set up the A/D. The 18F452 has a 10 bit A/D, the result of which can be either right or left justified into a 16 bit integer. Full scale is therefore either 3FFh or FFC0h. ***************************************************************** 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 KILLspamlistservspamBeGonemitvma.mit.edu with SET PICList DIGEST in the body NOTE: This e-mail message is subject to the MTN Group disclaimer see http://www.mtn.co.za/email_disclaimer.asp -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email EraseMElistservEraseMEmitvma.mit.edu with SET PICList DIGEST in the body

{Quote hidden}>-----Original Message----- >From: Peter Onion [@spam@ponion@spam@spam_OUTALIEN.BT.CO.UK] > >> At the moment I am initiating a conversion by hitting a >button on the >> PC and then looking at the result. The problem only occurs >when I set >> the A2D to use its internal Vref. If I set it to use an >external Vref >> and supply 5V and Gnd to the respective pins, I get the correct >> result. >> > >I could be totally missing the point here, but looking at the >data sheet I think the internal reference in only 3v ? So >inputs greater than this give f.s.d. (0x3f) I think. > >(Flame suit on) ;-) > >Peter There is no internal reference per se. The A/D converter either uses the supply rails as the reference, or external voltages applied to the Vref+ and Vref- pins. 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 spamBeGonepostmasterKILLspambookham.com. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email .....listservspam_OUTmitvma.mit.edu with SET PICList DIGEST in the body

Mark Beetge [ MTN - Harrington Street ] wrote: > The problem only occurs when I set the > A2D to use its internal Vref. If I set it to use an external Vref and > supply 5V and Gnd to the respective pins, I get the correct result. Are all the other PIC pins between 0 and 5V? I've seen strange things happen to A/D readings when even a little current goes thru a protection diode, particularly if it's another analog input. ***************************************************************** 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 TakeThisOuTlistserv.....TakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

Mark Beetge [ MTN - Harrington Street ] wrote: > I did send a correction email immediately after that one correcting my > typo(within 5 minutes) Which I saw after sending my response to your first post, of course. ***************************************************************** 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 TakeThisOuTlistservKILLspamspammitvma.mit.edu with SET PICList DIGEST in the body

On Tue, 2004-01-20 at 12:32, Mark Beetge [ MTN - Harrington Street ] wrote: > The data sheet on page 185 indicates that the internal reference is Vdd > and Vss, but youv'e given me an idea: I use ICSP on the circuit which > means that Vdd comes via a resistor (100ohm). Tonight I will check if > maybe the voltage drop across it is too great thus giving 3.5 volts as > Vdd > Mark I was looking at Page 289 of 39564b, table 22-21, but I think I mis-understood what it means by "vref" ... D'OH ! Peter -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email .....listservRemoveMEmitvma.mit.edu with SET PICList DIGEST in the body

On Tue, 2004-01-20 at 12:43, Michael Rigby-Jones wrote: > >Peter > > There is no internal reference per se. The A/D converter either uses the > supply rails as the reference, or external voltages applied to the Vref+ and > Vref- pins. Yes I see that now, I was misunderstanding the figure given for the minimum difference between the two reference voltages. Sorry ! D'OH ! Peter -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email RemoveMElistservspamBeGonemitvma.mit.edu with SET PICList DIGEST in the body

Peter Onion wrote : > I could be totally missing the point here, but looking at the > data sheet I think the internal reference in only 3v ? So > inputs greater than this give f.s.d. (0x3f) I think. I don't think there even *is* an internal V-ref for the ADC. It can use Vdd or an *external* V-ref. Are you sure you aren't thinking of the V-ref for the LVD ? Jan-Erik. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spamBeGonelistserv@spam@spam_OUTmitvma.mit.edu with SET PICList DIGEST in the body

Jan-Erik Soderholm: I believe you are correct - as far as I can tell from the docs (DS39564A page 181) the A/D uses either an external Vref or the Vdd / Vss voltage. The Block Diagram on this page clearly shows this connection via PCFG0. Unless the input pin is not correctly defined as ANALOG IN in the config. register, I can't see any reason for full scale being other than Vdd. I just tried the experiment on my test card and I do read $3ff for an input from Vdd. Roy J. Gromlich Jan-Erik Soderholm XA (TN/PAC) <TakeThisOuTjan-erik.xa.soderholmspamERICSSON.COM> wrote : {Quote hidden}> Peter Onion wrote : > > > I could be totally missing the point here, but looking at the > > data sheet I think the internal reference in only 3v ? So > > inputs greater than this give f.s.d. (0x3f) I think. > > > I don't think there even *is* an internal V-ref for the ADC. > It can use Vdd or an *external* V-ref. > > Are you sure you aren't thinking of the V-ref for the LVD ? > > Jan-Erik. > > -- > http://www.piclist.com#nomail Going offline? Don't > AutoReply us! > email listservEraseMEmitvma.mit.edu > with SET PICList DIGEST in the body -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email RemoveMElistservEraseMEspam_OUTmitvma.mit.edu with SET PICList DIGEST in the body

Found the problem, the voltdrop across the 100ohm series resistor I use with the ICSP caused the problem. I've shorted it out but now I have to unplug the USB cable every time I program it so as not to have power go back into the PC and then unplug the programmer every time I connect the circuit to the PC (Using power from the pc USB ) Has anyone got any suggestions as to how to get around this inconvenience? Thanks to all for their input. Regards Mark {Original Message removed}

How about series diodes in the power supplies ? 0.6V drop may be OK (or less if you use schottys) Or run off the USB supply only? RP Found the problem, the voltdrop across the 100ohm series resistor I use with the ICSP caused the problem. I've shorted it out but now I have to unplug the USB cable every time I program it so as not to have power go back into the PC and then unplug the programmer every time I connect the circuit to the PC (Using power from the pc USB ) Has anyone got any suggestions as to how to get around this inconvenience? Thanks to all for their input. Regards Mark -- http://www.piclist.com hint: To leave the PICList @spam@piclist-unsubscribe-requestRemoveMEEraseMEmitvma.mit.edu

-- Original Message -- From: Jinx <EraseMEjoecolquitt@spam@CLEAR.NET.NZ> THe bottom of the 5K pot is connected to circuit Ground. The top is connected to some voltage source, which is considered to be a Low impedance (if it IS a voltage source). The pot wiper is centered. Now assume you are the A/D input looking OUT toward the pot. You see 2.5K to Ground (bottom half) and 2.5K to the Voltage Source (top half). Because the V.S. (top) is Low-Z it can also be considered to be connected to ground (from the viewpoint of the A/D input). So you have 2.5K in parallel with 2.5K = 1.25K. To: @spam@PICLISTspam_OUT.....MITVMA.MIT.EDU Send: 2004-01-20 Subject: Re: [PIC:]18F452 A2D problem > A 5k pot gives a 1.25k source impedance How did you arrive at that ? -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spamBeGonelistservEraseMEmitvma.mit.edu with SET PICList DIGEST in the body -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

At 09:53 AM 1/23/2004 -0500, you wrote: >-- Original Message -- >From: Jinx <joecolquittspamBeGoneCLEAR.NET.NZ> >THe bottom of the 5K pot is connected to circuit Ground. >The top is connected to some voltage source, which is considered to be a = >Low impedance (if it IS a voltage source). =20 >The pot wiper is centered. =20 > >Now assume you are the A/D input looking OUT toward the pot. >You see 2.5K to Ground (bottom half) and 2.5K to the Voltage Source (top = >half). >Because the V.S. (top) is Low-Z it can also be considered to be connected= > to ground (from the viewpoint of the A/D input). > >So you have 2.5K in parallel with 2.5K =3D 1.25K. This assumes that the voltage source (Vdd, below) is regulated. If the voltage on the pot is established by a voltage divider it's a bit more complex (see below). For the situation Roy describes: Vdd Vdd o o | | .-. .-. | | R*(1-a) Rotation 0..1 | |<--o | | | | '-' R '-' | | +-------o o | 0V .-. | | R*a | | '-' | o 0V Note that the impedance for R=5K varies from 0 to 1.25K (ideal pot), with the maximum when the wiper is centered. Rp = a*R || (1-a)*R where R is the element resistance, a is the shaft electrical rotation from 0..1 = R * (a - a^2) You can find the min/max by setting dRp/da = 0, so 0 = 1 - 2*a => a = 0.50. If the voltage on the pot is established by a voltage divider the maximum impedance will always be higher, and the maximum will occur at a pot rotation closer to maximum rotation. Limiting case (a current source) will give an impedance maximum of R, not R/4, occurring at the maximum position of the pot rotation, not halfway. Or, suppose we have a 5K pot with 5K in series from the top to Vdd (bad design, but let that slide..) Then the maximum will occur at 100% rotation (as it will for any series resistance >= R) and will be Rp = 2.5K. Or if there was 1K in series with the top of the element, the maximum would occur at 0.60 rotation and would be 1.5K. Etc. Best regards, Spehro Pefhany --"it's the network..." "The Journey is the reward" RemoveMEspeff@spam@spamBeGoneinterlog.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 list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics