Use of pull-ups in March 2013 Spin Zone circuit

Forum dedicated to discussions of articles by Jon Williams, columnist, author and contributing editor to Nuts and Volts magazine sine 1994. This is the place to ask questions and get errata on Spin Zone columns and a place to converse directly with Jon.

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rkidd7952
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Use of pull-ups in March 2013 Spin Zone circuit

Post by rkidd7952 » Wed Sep 18, 2013 6:03 am

Hi,

The March 2013 Spin Zone article gives a schematic to interface a 5V card reader to a 3.3V Propeller. The card reader has open collector outputs that are pulled up to 5V by the interface circuit. The circuit uses resistor values calculated to protect the 3.3V inputs on the Propeller.

If a 3.3V supply is available, is there a reason that the outputs should be pulled up to 5V instead of 3.3 for this circuit?

Thanks
Robert

cae2100
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Re: Use of pull-ups in March 2013 Spin Zone circuit

Post by cae2100 » Wed Sep 18, 2013 10:42 pm

if it's open collector, you'd need the pullup resistor to have the input on the MCU show up as a 1, otherwise it'd just float and you know how MCUs hate that :P

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jonnymac
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Re: Use of pull-ups in March 2013 Spin Zone circuit

Post by jonnymac » Mon Jan 20, 2014 1:45 pm

You can use 3.3v -- I used 5v to simplify connections. The only caveat is the size of the pull-up which limits the current into the pin when the transistors are not conducting. With 5v the should be at least 3.3K. As we tend to use 10K for pull-ups, we're well in the clear.
JonnyMac
Hollywood, CA

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