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Unused GPIO pins on ADS7951 and other ADS79xx

Brian Willoughby
Genius 4630 points
Other Parts Discussed in Thread: ADS7951

If the GPIO pins are not to be used in a given circuit with the ADS7951, is it better to leave them floating, or should they be tied to a fixed potential?  If the former, should they be programmed as outputs or inputs?  If the latter, should they be tied high, low, or would a mix be best depending upon the alternate function for each pin?  Finally, which of these options draws the least amount of power?

Other chips' spec sheets document the best practices in these situations, but I could not find anything in the ADS79xx data.

Note: There might be a slight advantage to leaving the pins floating, because I can leave traces on the board to access them and enable them in the future.  However, that is an incredibly unlikely situation, given that I plan on using the chip at its maximum 1 MHz bandwidth (leaving little bandwidth for reading or writing GPIO anyway), so I anticipate that the lowest power consumption option will have higher precedence than the most flexible option.

  • 0
    TI__Guru**** 180961 points

    Hi Brian,

    The ADS79xx devices power on default (and reset) to having the GP pins configured as inputs.  As a general rule, unused inputs are typically terminated in some way - to GND for instance.  If you were to terminate them via resistor to ground, they should not draw any power and you still have the option of lifting the resistor at some point down the road should you decide to use them. 

    If you don't want to actually install the resistor, you can pull a little trick that I use from time to time and make you own 'zero ohm' short.  I have placed resistor footprints and purposely shorted the pads together with a track of copper.  That gives my design rule checker fits but it saves me the cost of an actual component and I can simply cut the short later should I find out that I really do need to add some extra feature.

  • 0 in reply to Tom Hendrick
    Genius 4630 points

    A solution which should pass the Design Rule Check would be to create a "bowtie" package which is shaped like the SMD jumper of the desired size.  That way, you could still cut the short later, but it wouldn't appear to be a violation because you wouldn't need the track of copper in addition to what's already in the package.

    Thanks for the answer.