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I am working on a project that involves using an ATtiny85 to deliver a PWM signal to an RC filter. On the other side of the filter is an OP AMP that does not draw any current.

My question is: is it safe to allow a capacitor to discharge back into the ATtiny85 when the output signal is held low? How would I find the input impedance of the output pin when held low?

Is this recommended against? Any help is appreciated!

EDIT

An image of the application (in a nutshell) is attached below:

Application

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My question is: is it safe to allow a capacitor to discharge back into the ATtiny85 when the output signal is held low?

enter image description here

Whatever current is delivered to the capacitor is limited by R and whatever current is taken back from the capacitor is, again, limited by R.

Digital IO pins when used as outputs are cable of delivering (sourcing) current and taking (sinking) current. Find the data sheet and locate the section called electrical characteristics and there you will find the information you need.

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the R component of the RC filter restrict the current sink by the I/O pin, if the R value are high nothing to worry about directly driving the filter.

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  • \$\begingroup\$ So the ATtiny85 is able to handle current being input into it even while configured as an output? Is there a way to know how much is too much? \$\endgroup\$ – Joshua Granger Feb 2 '17 at 14:41
  • \$\begingroup\$ IIRC the datasheet says maximum 40 mA. \$\endgroup\$ – Dampmaskin Feb 2 '17 at 14:45
  • \$\begingroup\$ when configured as output the i/o pin can source and sink the rated current, usually sink current will be higher than source current, the direct driving is decided by the interface requirement, can you share the RC filter circuit \$\endgroup\$ – Raj Feb 2 '17 at 14:46
  • \$\begingroup\$ The RC filter circuit is approximately as follows: i.imgur.com/WS6oJgx.png \$\endgroup\$ – Joshua Granger Feb 2 '17 at 19:56
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is it safe to allow a capacitor to discharge back into the ATtiny85 when the output signal is held low?

yes and no, depending on the current into the output pin - max 20ma.

How would I find the input impedance of the output pin when held low?

most of them are spec'd for a max of 20ma @ 5v. or 250R. In reality, they can deliver more. I typically design to a 220R.

The "dynamic" resistance, how much the voltage will drop when a given load is attached, is much lower.

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