AVR-Chat

The stk500 and my breadboard are both driven from my bench supply so the ground should be good.

I am using the adc on port c pin 4. It is being driven directly by an LM34 temp sensor or a pot for testing so I think the impedence is low. the alternate function for this pin is sda but TWEN is set to 0.

I have set PUD to 1 in testing just to make sure all the pullups are off.

I do see some ringing on the input but its only 120mv peak.

could letting the adc input exceed aref cause damage that would manifest in  this manner?

any other ideas? thanks for all the suggestions.

--Aaron

----- Original Message ----
From: David VanHorn <microbrix@gmail.com>
To: AVR-Chat@yahoogroups.com
Sent: Monday, June 4, 2007 6:28:23 PM
Subject: Re: [AVR-Chat] adc wont read below 0.8v?

On 6/4/07, Jim Wagner <jim_d_wagner@ applelinks. net> wrote:
> Some devices allow interrnal pullup to remain on in ADC
> mode. Check that pullups, etc, are off on those pins.

That could well be it, or the sourcing impedance is too high on
whatever is driving the pins.
There is a certain amount of leakage current possible, that might be
giving you anomalously high voltages. IIRC they want the source
impedance below 10k.




       
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On 5 Jun 2007 at 11:01, A F wrote:

> could letting the adc input exceed aref cause damage that would manifest in  this manner?
> 


	My experience with the ATMega48 ADC tells me the following:

	- The ADC reading starts at zero again when the input voltage is 
higher than AREF voltage. 
	- So, if your input is higher than AREF, subtract AREF from the 
input and the difference will be your ADC reading.

	I wouldn't use that as a "feature"...

	Mark Jordan