I'm not tryin to offend anybody, but this is a legitimate question.
i got a DIO pin with rated capacity of 40mA, and i'm tryin to figure out if it's kosher to connect a 1uF cap straight from pin to ground and safely charge it without a series resistor.
i know mathematically and mechanically that there is a certain wattage limit, or more precisely, a certain "surge capacitance" limit, that it can comfortably handle if it only does it once on power up. And if it was a generally useful thing to know, then there would exist a standard measurement protocol and a clearly defined rating for digital output pins.
But it's not and there ain't. So does anyone have any experience with this?

 

i got a DIO pin with rated capacity of 40mA, and i'm tryin to figure out if it's kosher to connect a 1uF cap straight from pin to ground and safely charge it without a series resistor.

That's an absolute maximum specification that isn't to be exceeded. The realistic max current is 20mA and you need to consider overall power dissipation.

I wouldn't worry about charging a 1uF cap without a current limiting resistor.

 

What Device is it??
A Microchip powers up as inputs, and outputs have weak pull ups.
Max.

 

That's an absolute maximum specification that isn't to be exceeded. The realistic max current is 20mA and you need to consider overall power dissipation.

I wouldn't worry about charging a 1uF cap without a current limiting resistor.

i don't understand. if i connect a cap of any size to the pin then it will exceed the current at the moment of connection.

 

Voltage ratings, IMHO, are not to be violated. You can puncture a mos gate. Current ratings are related to the heat produced. One nanosecond is different than a microsecond and so forth. I do not ignore capacitor charging currents, but I am not a slave to them either.

 

What Device is it??
A Microchip powers up as inputs, and outputs have weak pull ups.
Max.

it's an ATMEGA16u2. And yeah it has optional pull ups. hmmm... in fact, that's a good idea, perhaps i could set it to input with pull up and charge the cap that way. gotta look up the PU resistors..... Ok with 50k resistor then a 1uF cap would charge in about 120mS. That might be my solution. THANX

 

i don't understand. if i connect a cap of any size to the pin then it will exceed the current at the moment of connection.

And I wouldn't worry about it. There all sorts of parasitic resistances that will limit current and the devices themselves will typically tolerate shorts for a limited amount of time.

 

And I wouldn't worry about it. There all sorts of parasitic resistances that will limit current and the devices themselves will typically tolerate shorts for a limited amount of time.

ok i get it. thanx

 

If really worried about the surge current, put in a small inductor in series with the capacitor. That will limit the surge pulse. However, if toggling that pin for some function, the inductance and capacitance will form some type of filter/resonance.

 

it's an ATMEGA16u2. And yeah it has optional pull ups. hmmm... in fact, that's a good idea, perhaps i could set it to input with pull up and charge the cap that way. gotta look up the PU resistors..... Ok with 50k resistor then a 1uF cap would charge in about 120mS. That might be my solution. THANX

no...
1T =50ms
5T= 250mS

also...

arduino gpio go tristate on reset/boot-up

 

1T =50ms
5T= 250mS

yeah that's true i gotta have full charge. But now i'm thinking that i'll just set the pin high and it will charge in <1mS