Single pin as input and output

Question

Is there a way to set an Arduino pin as being both an input, and an output?

I was looking at tristates, I'm not sure if that is related, or if there is some other method.

Perhaps I can try switching states rapidly. on one pin. I'm just not familiar with how to do it, and I don't see anything about it on the Arduino site.

This is a function of other microcontrollers, such as PICs, but I'm not sure about AVRs and Arduino types.

Answer 1

The source code for the 1-Wire protocol is a great place to start when learning how to use a single pin for both input and output.

See also chap. 18. I/O-Ports in the ATmega328p datasheet.

Cheers!

• j0h: Does it mater that the device isnt a 1-wire protocol item? Its NeoPixel Rings. such as adafruit.com/products/1643
• Ignacio Vazquez-Abrams: @j0h: Why do you need to connect that as an input? The output from those is usually only useful for chaining them together.

Answer 2

A tri-state pin is not both an input and an output, rather each pin can be in 1 of 3 states (High, Low, High Impedance). These are typically used on a bus, where many similar devices are connected.

AFAIK the Arduino does not support tri-state, but it is possible to implement a bus. This was done by using open collector/drain devices with a resistor pull-up.

This was a common method of implementing computer busses (more than 40 years ago with RTL logic), but suffers from slow speed, and low fan-out, due to capacitance.

It is still used for low speed short busses such as I2C and 1-wire.

In practice all devices are set as Input. When a device wants to communicate it is configured as Output.

• Ignacio Vazquez-Abrams: The three states are "High", "Low", and "High-Impedance"; a tri-state pin is always an output.
• Milliways: @IgnacioVazquez-Abrams True - brain fade on my part.
• Edgar Bonet: All the I/O pins of an AVR-based Arduino are four-state: not only do they support the three states you mention, they also have a fourth INPUT_PULLUP state, which is a weak pull to Vcc. You can connect a pin to an open-collector bus directly and then just switch between INPUT (i.e. HiZ, to either read or write 1) and OUTPUT LOW (to write 0).