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'[SX] Internal pullup resistor for pushbutton input'
2008\07\03@122547 by TheBanditn/a

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In SX Microcontrollers, SX/B Compiler and SX-Key Tool, TheBandit wrote:

Is it acceptable to have a pushbutton connected between an input and ground/Vss without any external resistors? I would enable the internal pullup resistor for that input to keep the input high when the pushbutton is not pressed.

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2008\07\03@122631 by Chris Savagen/a

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In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Chris Savage wrote:

Clint, just be sure the I/O pin is never made an output and you will be fine.  Take care.

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2008\07\03@125915 by TheBanditn/a

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In SX Microcontrollers, SX/B Compiler and SX-Key Tool, TheBandit wrote:

Thank you Chris. I was reading "Programming the SX Microcontroller" by Gunther Daubach and it seemed to be describing what I wrote above (section 2.2.4.7 pg 213). I just wasn't sure because the SX/B help example for "Digital Dice" showed external resistors being used for a pushbutton.

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2008\07\03@132850 by Chris Savagen/a

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In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Chris Savage wrote:

Clint, the resistor is for safety in case the I/O pin ever becomes an output.  In an ideal situation this will never happen.  ;)  Of course, the price of insurance is cheap in this case.  A 220 ohm resistor is mere pennies for peace of mind.  Take care.

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2008\07\03@144856 by TheBanditn/a

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In SX Microcontrollers, SX/B Compiler and SX-Key Tool, TheBandit wrote:

Other than a programming error, is there any other reason that pin might become an output?

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2008\07\03@163345 by Zootn/a

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In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Zoot wrote:

No. Unless you have turned off the initialization routine in your program (that is generated by SX/B) which clears registers and "defaults" the I/O pins to input. But that would be a programming error also :-) since it would be you who did it :-)
I will add that the protecting resistor also protects agains over-voltage, over-current, not just the short-circuit that might happen if the pin is made an output. Remember that the SX has both pin source/sink and whole-chip source/sink limits. You don't want to overdraw through the pin(s).

On a Stamp I always use them, on SX projects it depends on the project, my comfort level, space trade-offs, etc. (big difference between frying a (2.79 SX and a 79.00 Stamp).

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