just wondering what's the purpose of external pull-up when there is internal pull-up available on GPIO?
In what context? Any specific external pull-up you're looking at or is this a general question?
For a push-button the internal pull-up might do just well, but IIRC older AVRs did not have pull-ups on all pins. In other cases you need a stronger pull-up than the internal one (eg. if something external to the AVR is pulling down weakly you need something that pulls up stronger than that).
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ok, got it.
But what's value of UC3B's internal pull up? 20K?
Oooops. Didn't realise that I was responding in an AVR32 forum. Those are off limits for me...
But what's value of UC3B's internal pull up?
Provided it pulls up internally to Vcc, just configure it with a pull-up enabled, and ground it with a ~30Kb resistor. Measure the voltage on the pin and calculate the built-in pull-up value out of that and the pull-down you used.
yes, right, thanks.
Perhaps this is a design paradigm but--
IMHO, it is always beneficial to include a pull-up or pull-down on a pin if you need to ensure that the pin has a specific state on system startup. It may not be necessary, but it ensures that the system remains stable (i.e. pull-up resistor on #CS of SPI device).
yes, i understand this, my main consideration is the extra power consumption these resistors introduced in my low power design.
i got this from ATMEL for UC3B:
the pull-up value :
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