*unofficial* FIRST IR module driver released

[Kevin Sevcik]

Originally Posted by JBotAlan

Curiosity drives me to ask this: What exactly is High-Z? Does that mean the pins would go to high resistance (seen it called impedance) instead of driving them to ground?

Yes, High-Z means exactly that.

(Gross oversimplification to follow)

Typically, digital outputs come in two general flavors, two-state and tri-state. Two state outputs either drive a pin to high voltage, or drive a pin two low voltage as you seem familiar with. Tri-state outputs have this third additional state called High-Z where the output is at a high resistance. This only becomes important or even necessary if you have two outputs both wired to the same input. If you want them both to reliably drive the input at different times, you can't just assume that driving output A low will let output B signal high and low reliably. That would depend entirely on the resistances of the traces an a host of other factors that might not stand still over time. In most cases it wouldn't ever work, period, and the input would always see a low voltage. So, instead, you put output A into a High-Z state and it no longer affects the signal because no current is passing through it. (To a first order approximation)

(End oversimplification)

That's all ignoring the host of other output styles available and a lot of other issues, but that's the gist of the purpose of a High-Z state. It's simply to more or less remove that particular output from the circuit that's driving the input.

Pull-up resistors serve a slightly different function. Without a pull-up resistor, an input with nothing connected to it isn't in any well defined state and might, in fact, fluctuate wildly between high and low states. With the pull-up (or down in some cases) resistor, if there isn't an active device sourcing or sinking current on the other end of the circuit, a trickle of current flows through the pull-up resistor giving more or less 0 volts of drop across the resistor, meaning the input is at (high) volts. When the circuit is sinking current to ground either through a dead short like a limit switch or via a digital output like on the IR module, the pull-up resistor ends up making a voltage divider with the short or digital output. Since the resistor is such a high value (100k) the 5V contributes little to the final voltage at the input, and you end up with ~(low) volts at the input. The main reason for turning off pull-up resistors is that they can slightly slow down logic transitions and introduce noise from the power supply into the digital input.

For all this and more, I heartily recommend "The Art of Electronics" by Horowitz and Hill. It's a bit thick and expensive and the '89 2nd edition is a little dated... but a new edition is set for March and the do a rather good job of introducing things with as little math as possible at first to help you get a good feel for it.