Monday, October 6, 2014

Raspberry Pi and a brief primer on GPIO (still coming, a binary clock!)

I mentioned that I got my Raspberry Pi B+ last week. Sadly I've had fairly little free time to play with it. Still, a few days ago, about 24 hours after I received it, I put together a cute program I thought I'd share. It's a clock that counts out the time in binary. Nothing too earth-shaking, but a little more involved than the tutorials.

Before I get into that project, though, I want to lay out some information on how GPIO works.

My goal is to provide a tutorial and technical explanation, for those who already know what a Python comment is, but who need something more meaty than "here's a photo my my Pi, now copy-paste this Python code" The tutorials are cool, but they often lack background information and (for example, in my first project) I had to find a different site to explain how GPIO ports really work since the light wasn't blinking when it should have been.

First off, go find a copy of the Raspberry Pi GPIO diagram for your model. My personal favorite is this one for the B+. And I'll assume that you know a little bit about electricity, e.g. that GND means "ground" or "negative terminal".


What's GPIO? General Purpose Input and Output. It means that there's a bank of pins on the computer, and that these pins can be connected to things that generate input (buttons, thermometers, motion sensors) and to things that generate output (LEDs, motors and relays, radio transmitters). The GPIO bank also has several pins to act as grounds (- terminals) and power supplies (+ terminals at both 3.3V and 5V).

What decides whether a GPIO pin is input or output, and when it generates voltage? Software! That's the awesome part. The same GPIO pin may attach to a button (input) in one program or to a LED to light up (output) in another program, and that choice of behaviors is controlled by your Python program.

GPIO Output

In Output mode, what defines whether a GPIO output is on or off?

Well, that's a little more involved...

When a GPIO pin is defined as Output (or GPIO.OUT in Python) the pin can be in either a High state or a Low state (GPIO.HIGH and GPIO.LOW, or True/False, or 1/0).
  • When it's in a High state the pin is generating +3V DC If you connect that pin to the anode (+ side) of a LED and connect the LED's cathode (- side) to the ground, the LED would light.
  • When it's in a Low state, the pin is grounded and acts as a negative terminal. In this case, you could still connect an LED but the - would go to the GPIO pin and the + would go to a power source.
So it's not quite as simple as "on and off" or "the LED lights when the pin is High" as much as "the GPIO pin will go to +3V (High) or to 0V (Low), and the behavior of your circuit depends on the structure of your circuit"

Check out the following photo and diagram.

In my initial experiments, I simply didn't quite grok this simple relationship between voltage and tyhe High/Low state, and I had plugged in an LED backwards. As a result, High meant that the LED was off and Low turned it on -- the opposite of my intent! It all makes sense now that I know these basics.

GPIO Input

When a GPIO pin is set to receive input, what is it looking for? A button being pushed, or what?

When a GPIO pin is set to GPIO.IN mode, it listens for incoming voltage, e.g. +3V being applied to the GPIO pin. When there is voltage coming in, the pin reads High. And when there is no voltage... it reads randomly!

A pin in GPIO.IN state is floating, meaning that it is not clearly energized to +3V nor is it clearly grounded out. If you have a Python program doing While True: print input(12); you'll see that the input fluctuates between 0 and 1, even if there's nothing connected to the pin at all!

This is not a signal from your button, but simply line noise. To correct for this, you must pull down the line voltage to force it to clearly be Low. This means to connect the GPIO pin to ground so it is constantly draining voltage (and constantly reads 0 or Low), until such time as you hit the switch and connect +3V to the GPIO input.

It's not as convenient as having just the button that is clearly reading High or Low. On higher end hardware they have cleaner line voltage and built-in pull down mechanisms. But not here.

That resistor is important! When you engage the switch, you're connecting +3V directly to GND, and that's a short circuit. Putting in the resistor, reduces the voltage sufficiently so as not to reboot your Raspi with a short circuit, while also "encouraging" the power to flow up the GPIO pin.

So Now What ?

Well, those are the rudiments of GPIO, that took me an evening to figure out. Hopefully it saved you some time.

Coming shortly, the walkthrough of my binary clock.

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