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That unused RTCC pin — Parallax Forums

That unused RTCC pin

LoopyBytelooseLoopyByteloose Posts: 12,537
edited 2009-07-28 19:13 in General Discussion
Mostly I have seen schematics just ignoring an unused RTCC pin, but in the Nuts & Volts Article 153, it is tied to +5volts. So now, I am wondering if it should always be tied to +5, if it can be tied to +5 or ground, or if it doesn't really matter. Reading the SX28 PDF really doesn't offer any insight.

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aka G. Herzog [noparse][[/noparse] 黃鶴 ] in Taiwan

Comments

  • ZootZoot Posts: 2,227
    edited 2009-07-19 17:31
    Pins, especially input pins, should *never* be left floating. RTCC should be tied high if it is not being used. Unused I/O pins should either be pulled high using internal weak pullups or set to outputs and forced low (for minimum power consumption).

    I do not usually come across schematics posted by knowledgeable users where RTCC is left floating.

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  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2009-07-19 18:25
    Thanks Zoot. That's the common sense approach. Am I to take your text literally and interpret that the RTCC is just pulled directly high to +5 without a pullup/current limiting resistor?

    To the best of my knowledge, the RTCC pin does not document whether it has internal weak pullups or not. And there is NO OUTPUT mode. Configuration is via the Option command, not the i/o port configuration registers.

    In this schematic, it is pulled high. But there is no pullup resistor included. I am wondering if I should insert a 10K resistor for safety's sake. Due to being unused, pulling the RTCC to ground or to +5 seems arbitrary.

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    Ain't gadetry a wonderful thing?

    aka G. Herzog [noparse][[/noparse] 黃鶴 ] in Taiwan

    Post Edited (Loopy Byteloose) : 7/19/2009 7:22:32 PM GMT
  • ZootZoot Posts: 2,227
    edited 2009-07-19 19:48
    No, you wouldn't need series protection on RTCC since it can never be an output. Presuming your power supply is clean and filtered, there's no reason not to just tie it to the Vdd pin which is adjacent.

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  • PJAllenPJAllen Banned Posts: 5,065
    edited 2009-07-19 20:24
    RTCC is shown not connected in the examples in the datasheet/s.

    Post Edit -- If you are using the RTCC input then you might want to set it high or low, as nec.,·because its status is "undefined" (as noted in the datasheet/s)·at power-on.

    Post Edited (PJ Allen) : 7/19/2009 8:39:31 PM GMT
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-07-20 00:38
    It's closer to the Vss pins than it is to Vdd. From a layout standpoint, it's usually just easier to ground it — that is, unless you need it as a convenient way to route Vdd across to the left side of the SX.

    -Phil
  • ZootZoot Posts: 2,227
    edited 2009-07-20 01:59
    Phil -- you're right for some SXes. I was thinking about the SX28 (DIP or SSOP), which was presumptuous, I guess.

    attachment.php?attachmentid=62380

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    When the going gets weird, the weird turn pro. -- HST

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    Picture 1.png
    673 x 309 - 77K
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-07-20 02:19
    Zoot,

    Right you are. I've rarely used the SX28 and don't have its pinout committed to memory. It looks like, with that one, you have two good choices! smile.gif

    -Phil
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2009-07-20 08:52
    Thanks everyone - it is a small detail that is not well documented.

    Nearly all the Parallax general examples show nothing attached to any I/O or to the RTCC. There is clear discussion about the the regular I/O pins. But the RTCC pin seems entirely ignored in documents about construction.

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    Ain't gadetry a wonderful thing?

    aka G. Herzog [noparse][[/noparse] 黃鶴 ] in Taiwan
  • BeanBean Posts: 8,129
    edited 2009-07-20 14:09
    According to the datasheet figure 2.2 the RTCC pin is always a schmitt trigger input.
    Also I remember that someone (I think geunther) had a way to use the RTCC input to sense if a jumper was installed or not.
    I think you changed the RTCC trigger direction or something....

    Bean.

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  • RobotWorkshopRobotWorkshop Posts: 2,307
    edited 2009-07-28 19:13
    I've always connected it to ground when it isn't needed. On my SX48 OEM module I just solder bridge that pin (48) to the Vss pin (47) next to it. By leaving the solder bridge off, that pin is accessible from a pad near the pin.

    Robert
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