What is the meaning of Microchip's acronym TRIS for data direction registers?

Question

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.

Answer 1

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)

Answer 2

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).