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PICList Thread
'Vref gaffe'
2000\04\07@213905 by Paul Howell

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Some of you may have seen my earlier post concerning the apparent failure of
a PICC74A to work properly with it's A/D Vref set to reference the voltage
on AN3 instead of chip Vdd. It was behaving as if Vref was set to Vdd no
matter what software selection was made.  Well, I finally solved it, and the
answer is pretty embarrassing so I though I might save someone else's face
by exposing my gaffe.

For convenience (read laziness) in my breadboard circuit I only hooked up
pin 32 to Vdd and pin 12 to Vss leaving pins 31 and 11 floating because I
assumed they were redundant. Well, they're not. Leaving those 'redundant'
pins floating makes the AN3 ineffective as a Vref for some reason I do not
understand. It is odd that other than the A/D, the rest of the chip worked
fine with those pins left unconnected. I am going to speculate that Vdd was
making it's way through a forward biased input protection diode or something
thus allowing the digital portion to function, but wreaking havoc with the
analog section....whatever, at least now things behave correctly.

Moral - connect all your power pins please and thanks to everyone who
patiently tried to 'debug' my problem.

-Paul Howell

2000\04\07@214304 by David VanHorn

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>Moral - connect all your power pins please and thanks to everyone who
>patiently tried to 'debug' my problem.


You ain't the first one to get bitten by this :)

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2000\04\10@055830 by Alan B Pearce

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>For convenience (read laziness) in my breadboard circuit I only hooked up
>pin 32 to Vdd and pin 12 to Vss leaving pins 31 and 11 floating because I
>assumed they were redundant. Well, they're not. Leaving those 'redundant'
>pins floating makes the AN3 ineffective as a Vref for some reason I do not
>understand. It is odd that other than the A/D, the rest of the chip worked
>fine with those pins left unconnected. I am going to speculate that Vdd was
>making it's way through a forward biased input protection diode or something
>thus allowing the digital portion to function, but wreaking havoc with the
>analog section....whatever, at least now things behave correctly.

does this mean that someone at Microchip was reading the recent discussion on
Power Supply problems between analog and Digital circuits, and separated out the
supply pins for noise and ground bounce reasons?

2000\04\10@073104 by Andrew Kunz

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Check the pinout on the 17C756

Andy









Alan B Pearce <spam_OUTA.B.PearceTakeThisOuTspamRL.AC.UK> on 04/10/2000 05:57:17 AM

Please respond to pic microcontroller discussion list <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>








To:      PICLISTspamKILLspamMITVMA.MIT.EDU

cc:      (bcc: Andrew Kunz/TDI_NOTES)



Subject: Re: Vref gaffe








>For convenience (read laziness) in my breadboard circuit I only hooked up
>pin 32 to Vdd and pin 12 to Vss leaving pins 31 and 11 floating because I
>assumed they were redundant. Well, they're not. Leaving those 'redundant'
>pins floating makes the AN3 ineffective as a Vref for some reason I do not
>understand. It is odd that other than the A/D, the rest of the chip worked
>fine with those pins left unconnected. I am going to speculate that Vdd was
>making it's way through a forward biased input protection diode or something
>thus allowing the digital portion to function, but wreaking havoc with the
>analog section....whatever, at least now things behave correctly.

does this mean that someone at Microchip was reading the recent discussion on
Power Supply problems between analog and Digital circuits, and separated out the
supply pins for noise and ground bounce reasons?

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