>
> Hi All,
>
> Thanks in advance for any help you can give me with a problem I am having:
>
> I am trying to use the ADC on a 16C71 and I am getting an error of as much
> as 2 LSB (when the datasheet says +/- 1 LSB max). I am measuring the
> voltage directly between the ADC pin and the PIC's Vss pin and the output
> code seems to be off. I am waiting about 800 uS from the time I enable the
> ADC until I begin the conversion, and the output impedance of my source is
> 2.2K
>
> Now as to the reason why this is entitled "SLEEP": AS recommended in the
> datasheet, I am placing the pic in sleep mode during the ADC,using the
> internal RC oscillator for the ADC. My normal oscillator element is a 4MHz
> xtal with 20 pF load caps. When I observe the pic's oscillator pins with my
> scope (about 20pF loading with my x10 probe),instead of seeing the
> oscillator exponentially decay to zero during sleep (as I would expect), I
> see the DC level increase slightly and the the amplitude remain constant.
> This region of the waveform is about 50 uS long after which there is a
> small glitch and then the DC level returns back to the same as before
> sleep. Is this normal for sleep mode? I thought the whole idea was to
> disable the oscillator (and other internal PIC circuits) to prevent digital
> switching noise! Perhaps the xtal's Q is so high that it won't even begin
> to decay in only 50uS? (This is my guess, since 50uS is only a few 100
> cycles at 4MHz,BUT if this is the case,are we loosing some of the benefit
> of sleep mode?)
>
> I know that 2 LSB error isn't that bad,but in my case,it is really annoying
> because the actual ADC count gets multiplied by 7 before being displayed on
> an LCD display. SO, if I calibrate it to be correct in one portion of the
> scale, I get as much as 14 or sometimes even 21 counts (after being
> multiplied by 7). Yeah, I know,an 8 bit ADC on a ucontroller is not a
> really precision device and multiplying by 7 is making things worse,but I
> think I could live with 1 LSB of error more easily. MORE IMPORTANTLY, I
> want to make sure I am not doing something dumb with the sleep mode.
>
> Thanks,
>
> Sean
>
> |
> | Sean Breheny
> | Amateur Radio Callsign: KA3YXM
> | Electrical Engineering Student
> \--------------=----------------
> Save lives, please look at http://www.all.org
> Personal page: http://www.people.cornell.edu/pages/shb7
> shb7KILLspamcornell.edu ICQ #: 3329174
> ________________________________________________________
> NetZero - We believe in a FREE Internet.  Shouldn't you?
> Get your FREE Internet Access and Email at
> http://www.netzero.net/download/index.html

>Hi All,
>
>Thanks in advance for any help you can give me with a problem I am having:
>
>I am trying to use the ADC on a 16C71 and I am getting an error of as much
>as 2 LSB (when the datasheet says +/- 1 LSB max). I am measuring the
>voltage directly between the ADC pin and the PIC's Vss pin and the output
>code seems to be off. I am waiting about 800 uS from the time I enable the
>ADC until I begin the conversion, and the output impedance of my source is
>2.2K
>
>Now as to the reason why this is entitled "SLEEP": AS recommended in the
>datasheet, I am placing the pic in sleep mode during the ADC,using the
>internal RC oscillator for the ADC. My normal oscillator element is a 4MHz
>xtal with 20 pF load caps. When I observe the pic's oscillator pins with my
>scope (about 20pF loading with my x10 probe),instead of seeing the
>oscillator exponentially decay to zero during sleep (as I would expect), I
>see the DC level increase slightly and the the amplitude remain constant.
>This region of the waveform is about 50 uS long after which there is a
>small glitch and then the DC level returns back to the same as before
>sleep. Is this normal for sleep mode? I thought the whole idea was to
>disable the oscillator (and other internal PIC circuits) to prevent digital
>switching noise! Perhaps the xtal's Q is so high that it won't even begin
>to decay in only 50uS? (This is my guess, since 50uS is only a few 100
>cycles at 4MHz,BUT if this is the case,are we loosing some of the benefit
>of sleep mode?)
>
>I know that 2 LSB error isn't that bad,but in my case,it is really annoying
>because the actual ADC count gets multiplied by 7 before being displayed on
>an LCD display. SO, if I calibrate it to be correct in one portion of the
>scale, I get as much as 14 or sometimes even 21 counts (after being
>multiplied by 7). Yeah, I know,an 8 bit ADC on a ucontroller is not a
>really precision device and multiplying by 7 is making things worse,but I
>think I could live with 1 LSB of error more easily. MORE IMPORTANTLY, I
>want to make sure I am not doing something dumb with the sleep mode.
>
>Thanks,
>
>Sean
>
>
>|
>| Sean Breheny
>| Amateur Radio Callsign: KA3YXM
>| Electrical Engineering Student
>\--------------=----------------
>Save lives, please look at http://www.all.org
>Personal page: http://www.people.cornell.edu/pages/shb7
>@spam@shb7KILLspamcornell.edu ICQ #: 3329174
>________________________________________________________
>NetZero - We believe in a FREE Internet.  Shouldn't you?
>Get your FREE Internet Access and Email at
>http://www.netzero.net/download/index.html
>

>
>Sean Breheny wrote:
>>
>> Hi All,
>>
>> Thanks in advance for any help you can give me with a problem I am having:
>>
>> I am trying to use the ADC on a 16C71 and I am getting an error of as much
>> as 2 LSB (when the datasheet says +/- 1 LSB max). I am measuring the
>> voltage directly between the ADC pin and the PIC's Vss pin and the output
>> code seems to be off. I am waiting about 800 uS from the time I enable the
>> ADC until I begin the conversion, and the output impedance of my source is
>> 2.2K
>>
>> Now as to the reason why this is entitled "SLEEP": AS recommended in the
>> datasheet, I am placing the pic in sleep mode during the ADC,using the
>> internal RC oscillator for the ADC. My normal oscillator element is a 4MHz
>> xtal with 20 pF load caps. When I observe the pic's oscillator pins with my
>> scope (about 20pF loading with my x10 probe),instead of seeing the
>> oscillator exponentially decay to zero during sleep (as I would expect), I
>> see the DC level increase slightly and the the amplitude remain constant.
>> This region of the waveform is about 50 uS long after which there is a
>> small glitch and then the DC level returns back to the same as before
>> sleep. Is this normal for sleep mode? I thought the whole idea was to
>> disable the oscillator (and other internal PIC circuits) to prevent digital
>> switching noise! Perhaps the xtal's Q is so high that it won't even begin
>> to decay in only 50uS? (This is my guess, since 50uS is only a few 100
>> cycles at 4MHz,BUT if this is the case,are we loosing some of the benefit
>> of sleep mode?)
>>
>> I know that 2 LSB error isn't that bad,but in my case,it is really annoying
>> because the actual ADC count gets multiplied by 7 before being displayed on
>> an LCD display. SO, if I calibrate it to be correct in one portion of the
>> scale, I get as much as 14 or sometimes even 21 counts (after being
>> multiplied by 7). Yeah, I know,an 8 bit ADC on a ucontroller is not a
>> really precision device and multiplying by 7 is making things worse,but I
>> think I could live with 1 LSB of error more easily. MORE IMPORTANTLY, I
>> want to make sure I am not doing something dumb with the sleep mode.
>>
>> Thanks,
>>
>> Sean
>>
>> |
>> | Sean Breheny
>> | Amateur Radio Callsign: KA3YXM
>> | Electrical Engineering Student
>> \--------------=----------------
>> Save lives, please look at http://www.all.org
>> Personal page: http://www.people.cornell.edu/pages/shb7
>> KILLspamshb7KILLspamcornell.edu ICQ #: 3329174
>> ________________________________________________________
>> NetZero - We believe in a FREE Internet.  Shouldn't you?
>> Get your FREE Internet Access and Email at
>> http://www.netzero.net/download/index.html
>
>--
>Steam engines may be out of fashion, but when you consider that an
>internal combustion engine would require recovery of waste heat by
>transfer just before top dead centre then fashion becomes rather
>redundant, USE STRATIFIED HEAT EXCHANGERS ! and external combustion.
>
>You heard it first from: Graham Daniel, managing director of Electronic
>Product Enhancements.
>Phone NZ 04 387 4347, Fax NZ 04 3874348, Cellular NZ 021 954 196.
>
|

>
>Hope I contributed
>
>
>Sven
>-----Ursprungligt meddelande-----
>FrŒn: Sean Breheny <spamBeGoneshb7spamBeGoneCORNELL.EDU>
>Till: TakeThisOuTPICLISTEraseMEspam_OUTMITVMA.MIT.EDU <RemoveMEPICLISTTakeThisOuTMITVMA.MIT.EDU>
>Datum: den 30 maj 1999 07:19
>€mne: Sleep
>
>
>>Hi All,
>>
>>Thanks in advance for any help you can give me with a problem I am having:
>>
>>I am trying to use the ADC on a 16C71 and I am getting an error of as much
>>as 2 LSB (when the datasheet says +/- 1 LSB max). I am measuring the
>>voltage directly between the ADC pin and the PIC's Vss pin and the output
>>code seems to be off. I am waiting about 800 uS from the time I enable the
>>ADC until I begin the conversion, and the output impedance of my source is
>>2.2K
>>
>>Now as to the reason why this is entitled "SLEEP": AS recommended in the
>>datasheet, I am placing the pic in sleep mode during the ADC,using the
>>internal RC oscillator for the ADC. My normal oscillator element is a 4MHz
>>xtal with 20 pF load caps. When I observe the pic's oscillator pins with my
>>scope (about 20pF loading with my x10 probe),instead of seeing the
>>oscillator exponentially decay to zero during sleep (as I would expect), I
>>see the DC level increase slightly and the the amplitude remain constant.
>>This region of the waveform is about 50 uS long after which there is a
>>small glitch and then the DC level returns back to the same as before
>>sleep. Is this normal for sleep mode? I thought the whole idea was to
>>disable the oscillator (and other internal PIC circuits) to prevent digital
>>switching noise! Perhaps the xtal's Q is so high that it won't even begin
>>to decay in only 50uS? (This is my guess, since 50uS is only a few 100
>>cycles at 4MHz,BUT if this is the case,are we loosing some of the benefit
>>of sleep mode?)
>>
>>I know that 2 LSB error isn't that bad,but in my case,it is really annoying
>>because the actual ADC count gets multiplied by 7 before being displayed on
>>an LCD display. SO, if I calibrate it to be correct in one portion of the
>>scale, I get as much as 14 or sometimes even 21 counts (after being
>>multiplied by 7). Yeah, I know,an 8 bit ADC on a ucontroller is not a
>>really precision device and multiplying by 7 is making things worse,but I
>>think I could live with 1 LSB of error more easily. MORE IMPORTANTLY, I
>>want to make sure I am not doing something dumb with the sleep mode.
>>
>>Thanks,
>>
>>Sean
>>
>>
>>|
>>| Sean Breheny
>>| Amateur Radio Callsign: KA3YXM
>>| Electrical Engineering Student
>>\--------------=----------------
>>Save lives, please look at http://www.all.org
>>Personal page: http://www.people.cornell.edu/pages/shb7
>>shb7EraseME.....cornell.edu ICQ #: 3329174
>>________________________________________________________
>>NetZero - We believe in a FREE Internet.  Shouldn't you?
>>Get your FREE Internet Access and Email at
>>http://www.netzero.net/download/index.html
>>
>
|

> -----Original Message-----
> From: Sean Breheny [RemoveMEshb7spam_OUTKILLspamCORNELL.EDU]
> Sent: Monday, May 31, 1999 5:33 PM
> To: RemoveMEPICLISTTakeThisOuTspamMITVMA.MIT.EDU
> Subject: Re: Sleep
>
>
> Hi Graham,
>
> Thanks for your reply. Please see my comments below:
>
> At 08:35 PM 5/30/99 +1200, you wrote:
> >Sean,
> >1)      if another interupt (WDT, external pin change, eeprom write)
> >completes then the PIC will wake again. (this would include pending
> .... and more

>Sean Breheny wrote:
>
>> I am seeing an ADC behavior as follows: if I apply a voltage of, say,
>> 2.00 volts to the ADC, I might get a code of 102, as expected.
>> HOWEVER, if I apply 4.00 volts, I might get 206 instead of 204.
>
>  OK, I don't see the problem!  102 means 102 +/- 1 which means it could
>easily be 103.  Twice 103 is 206, which is what you get.  No problem!
>
>  If you want to make the FSD match, you need a tweakable reference.  If
>you want zero to match, you need a tweakable ground.  If the results you
>get are stable and repeatable, the ADC has fulfilled its mission.
>Absolute accuracy is another thing again.
>
>  I think you've got enough answers regarding the crystal.
>--
>  Cheers,
>        Paul B.
>
|

>  What I was pointing out was that to do this, you made two measurements
>*each* of which is +/- 1 LSB.  If then your lower measurement is 1 LSB
>lower than actual, the "actual" is in fact 1 LSB *higher* than the
>reading.  You double the "actual" so this is now 2 LSB high and read it.
>If in the process, it reads 1 LSB high, then your reading is now *three*
>counts higher than you expected, all due to measurement error.
>
>  That's one error count from *each* measurement and one from the
>doubling process.
>
>  You were looking from the wrong end.  Calibration is always performed
>as close as possible to full scale.  Accordingly, what you must look at
>is say, the measurement at 4.00V, then halve it and see how close to the
>reading for 2.00V you are.  Allow 1 LSB for each measurement, and your
>original examples (206/2 = 103 ~= 102) are dead on!
>
>> In other words, the ADC's transfer function should be a straight line
>> which crosses at the origin. INSTEAD, it appears to deviate from this
>> such that doubling the input doesn't exactly double the output.
>
>  Have you actually plotted it?  I suspect if you do, it'll look quite
>good, at least in terms of linearity...
>
>> This "system" is supposed to be a voltmeter to measure a voltage in
>> the range of 2v to 17.85v, built into a power supply.  The input is
>> electrically divided by about 3.5 and then digitally multiplied by
>> 7, to produce a 16-bit number in the range 200 to 1785.
>
>  OK, resolution of the ADC is 20mV per bit, multiply by 3.5, you get
>70mV.
>
>> If I set the meter to read correctly at mid range (9.75v),
>
>  But by doing so, you only get half accuracy.  Calibration is always
>performed at full-scale, at least if you want to utilize the full
>resolution.
>
>> by the time I get up to 17v I might read 16.87
>
>  That's 130mV error, within the statutory two bits of error (one for
>the calibration measurement, one for the final).
>
>> and if I go down to 2v, I might read 2.17v.
>
>  You haven't mentioned a zero null on your divider yet.  You've just
>said it reads low at high output, high at low output.  That error is
>linear, telling you your zero is not nulled.  If it read high in the
>middle, low at each extreme, *then* you'd have non-linearity.
>
>> 70mV off is OK, but by the time you get to 140mV or 210mV, you could
>> easily exceede the 5% power supply tolerance of many components.
>
>  200mV is 5% of 4 volts.  What component has a 5% power supply
>tolerance at 4V praytell?
>
>> If I set it to be correct at 2v, I am way off at 17v
>
>  *Not* smart.  See above.
>
>> YET, when I measure the actual input the the PIC's ADC, that is
>> exactly what it should be, so the gain error is not in the pot
>
>  Fine.
>
>> or op-amp,
>
>  Maybe not...  All analogue returns are made directly to the PIC's
>ground pin, aren't they?
>
>> so I concluded it must be in the PIC's ADC.
>
>  May well be.
>
>> Thanks for your help and please tell me where I am going wrong,
>
>  Fit an offset adjustment to the op amp.  Recursively calibrate the
>offset at 2.03V and the gain at 17.78V until satisfaction is achieved!
>--
>  Cheers,
>        Paul B.
>
|