When reading/writing/talking about electronics, I like to understand the acronyms and mneumonics for abbreviations used for registers, functions, filenames, pin names, etc. Usually, the first time the abbreviation is used, either the context or a parenthetical note sets it off, or it's blatantly obvious.

On the Microchip dsPIC parts, the TRIS register controls the data direction. I can't find a note which uses the full word/phrase that would be abbreviated or have the acronym TRIS. ('The Register Input/output Settting' is about the best I can come up with, although 'TRIS Really Is a Silly abbreviation' is a close second guess).

How do you remember this? I've heard it pronounced as a word, but I'd like to know what it means to make it easier to remember, read, and write.


2 Answers 2


TRIS stands for TRIState. It means the port is waiting for an input rather than output a high or low signal.

It's named as such because a port can have 3 states:

  • Output High
  • Output Low
  • Input (High Impedance)
  • \$\begingroup\$ That was quick! Do you have a source for this name (and, ideally, others I'll come across), or did it just 'click' for you? \$\endgroup\$ Mar 31, 2011 at 15:07
  • \$\begingroup\$ @reemrevnivek: It's just something that I already knew... I'm not sure where I learnt it though! Probably a microchip datasheet or something. \$\endgroup\$
    – BG100
    Mar 31, 2011 at 15:09
  • 2
    \$\begingroup\$ The reason that Microchip don't spell-out the words TRI-STATE is that TRI-STATE® is a registered trademark of National Semiconductor. \$\endgroup\$
    – MikeJ-UK
    Mar 31, 2011 at 15:14
  • 1
    \$\begingroup\$ @reemrevnivek: I think I learnt this when I first started learning pic assembler from this page: mstracey.btinternet.co.uk/pictutorial/progtut3.htm The code snippet second from the bottom has this comment: "which is the address of the Tri-State register for PortA" \$\endgroup\$
    – BG100
    Mar 31, 2011 at 15:18

The term "tri-state" is often used informally to refer to the act of floating a pin; I know that National Semiconductor used to have a trademark on the term, though its widespread usage as a common term may have rendered it unenforceable.

Incidentally, in case anyone's curious why the OPTION instruction is so called, and the settings aren't controlled via a register, on the original General Instruments PIC devices, there were no TRIS or OPTION instructions; instead, the selection of which pins should be floating, as well as the settings for the timer/watchdog prescalar and modes, were part of the factory specification (the devices used mask ROM, so the whole program would be a factory spec).

  • \$\begingroup\$ National used to have a habit of registering just about every term in the English language. Some of the more dubious ones like 'National Anthem' and 'Damn Fast' have disappeared (unsurprisingly) from their list but TRI-STATE is still there! \$\endgroup\$
    – MikeJ-UK
    Apr 1, 2011 at 15:22
  • \$\begingroup\$ @MikeJ-UK: Tri-state is listed as a live trademark according to the USPTO web site, so I would certainly not recommend anyone selling chips describe the outputs as "tri-state"; the trademark may or may not be enforceable given the common generic usage, but that doesn't mean I'd want to spend the legal fees to find out. \$\endgroup\$
    – supercat
    Apr 1, 2011 at 18:52

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