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Looking at the datasheet for the attiny family, you find something called 'reference input resistance'. (Datasheet: http://www.atmel.com/images/Atmel-7669-ATtiny25-45-85-Appendix-B-Automotive-Specification-at-1.8V_Datasheet.pdf)

Is this a series resitance at the IC input limiting the input current?

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All of the inputs of the ATTiny devices feed CMOS gates, and the output of a tristateable output driver. The exception is ADC inputs which also feed an RC circuit with about 100kΩ resistance in series with a 14pF capacitor.

In all cases, when configured as an input, the impedance will be very high - on the order of a few hundred MΩ. This means that very little current will flow by virtue of the design of CMOS inputs. The current might be on the order of a few nA.

When configured as an output, the current flowing out is limited by the internal resistance which from experience is in the order of 60Ω. If you configure a pin as an output and then try to drive it from another output (e.g. you set the pin high, and then short it to ground), the current will be limited by only this internal resistance - about 80mA flows which is way outside the spec of the ATTiny output drivers. Basically this is a bad thing to be doing.

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  • \$\begingroup\$ Thanks this is what I was looking for. Do you have a reference for the 100kOhm / 10pF values? \$\endgroup\$ – Kafros Sep 19 '16 at 11:27
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    \$\begingroup\$ Page 129. I remembered incorrectly, it is 14pF. \$\endgroup\$ – Tom Carpenter Sep 19 '16 at 11:53
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Nearly, the reference input resistance is talking about specifically the ADC external reference for the chip or the Aref pin.

This is useful info if you have any other resistance from your source to expect a voltage drop at the pin.

I don't think its talking about the internal reference only the external input.

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The specification is there to make the user aware that the ADC's reference input is not a high-impedance input. The impedance is not a resistor that is just there there to limit the current. It is a property of the circuit which is connected to this reference input.

As a user what you need to do is make sure that the impedance level of your external reference is significantly lower than this value, at least if you want an accurate reference voltage.

So in practice you would us a voltage buffer (opamps as 1x voltage amplifier) to supply a reference voltage to the ADC reference input. Then the input impedance becomes irrelevant.

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