Changing the I/O Default State during Reset or Power up

[brianvenus9]

Posted on September 06, 2010 at 00:52

Changing the I/O Default State during Reset or Power up

[Andrew Neil]

Posted on May 17, 2011 at 14:05

No, there isn't.

The ''default'' state is the state of the

hardware before it has been reprogrammed into any other state by the running application.

Clearly, there will always be some delay between power being applied and any software getting to the point where it can re-configure pins - hence there always has to be a

hardware default state during that period!

Your external hardware has to be designed to be ''safe'' during this state.

As

says, it's the same on all micros!

[ashley23]

Posted on May 17, 2011 at 14:05

Not to my knowledge.  Most micros I have ever worked with float the io pins during reset.  You need working logic to turn on the pullups, and supply voltage to work the logic.  Chicken or egg?  

I think this is where you need correct power up and reset sequencing.

Any external device that could be affected by unknown states should have a pullup/down on its reset or chip select line.  As long as that line is controlled the state of its data lines probably doesn't matter.

[domen23]

Posted on May 17, 2011 at 14:05

Just a FYI, it's not the same on all MCUs, lpc13xx are input pull-up (and not to 3.3, but 2.3V or so) by default.

It's handy for quick pin connectedness tests :)

[Andrew Neil]

Posted on May 17, 2011 at 14:05

Aha!  When I said, ''it's the same on all micros'' I meant the fact that they all have a default state that is defined by the hardware and cannot be altered - I didn't mean to say that all micros have the same default state!

Sorry for the confusion!