Author Topic: Input impedance  (Read 2790 times)

bioroids

Input impedance
« on: April 21, 2006, 07:48:18 PM »
Ok, I got a quick question:

What is the input impedance of an I/O pin set as input on the AVR family?

Ok, in the ATmega163 datasheet states 100Mohm for the analog input resistance (Rain) on the ADC, I don't see why there should be a difference when using the input pin without the ADC.

And what value are the internal pull-up resistors (I assume they set the input impedance when enabled)?
Ok I just found that value, it is between 35k and 120k on the same datasheet.

Thanks!

Miguel

Eramos tan pobres!

The Tone God

Re: Input impedance
« Reply #1 on: April 22, 2006, 04:14:32 PM »
What is the input impedance of an I/O pin set as input on the AVR family?

You know what, I don't know. When the pins are configured as an input they are are in Hi-Z mode which for logic purposes is high enough to not be an issue. This does not take into account pullups.

Ok, in the ATmega163 datasheet states 100Mohm for the analog input resistance (Rain) on the ADC, I don't see why there should be a difference when using the input pin without the ADC.

There is a reason. The pins when in standard logic I/O mode they have a digital buffer section in place. When the ADC function of the pin enabled this buffer would interfere with readings. AFAIK the mega163 is an older EOL'd part so it would automaticlly disable this buffer when you enable the ADC on the pin. Newer parts have a digital buffer disable control registry for the pins that use ADC so you do this manually.

Andrew

bioroids

Re: Input impedance
« Reply #2 on: April 22, 2006, 09:33:05 PM »
Thanks Andrew!

I don't remember why I asked, cause I'm using the pin for ADC anyway :P.

What precautions should be taken if the pin is expected to interact with an arbitrary external voltage?

I'm planning to run an op-amp as a buffer before the pin, and using a zener to prevent voltages higher than AREF. Should I avoid also negative voltages going to the pin?

Luck

Miguel
Eramos tan pobres!

bioroids

Re: Input impedance
« Reply #3 on: April 22, 2006, 09:43:17 PM »
OK, the datasheet is my friend is one of the commandments right?  :icon_redface:

Absolute maximum ratings:
Voltage on any Pin except RESET with respect to Ground ................................-1.0V to VCC+0.5V
Voltage on RESET with respect to Ground .....-1.0V to +13.0V

at least for the ATMega163.

So I'm gonna need a schottky diode to clamp negative voltages to ground, so nothing lower than, say, -0.25v gets to the pin.

Luck

Miguel
Eramos tan pobres!