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GPIO

Former Member

So I recently got my rpi and I've been playing around with it a bit, nothing stupendous though, just playing with the os. I also saw the gert board is now available for preorder. I would like to get one but its a bit out of my price range. About a week ago I ordered the Breakout kit from adafruit, and have been looking at the different built designs for protecting the gpio pins. I'm fine with building one of those or maybe building a gert board on a breadboard, but what I'm trying to learn is what exactly or how exactly the boards protect the pi and process the data to provide it for the pi to use. I understand part of the protection is from preventing to much current and voltage thru the pins, but doesnt this limit the amount of things you can do through the pins. I'm sorry if my questions seem kind of vague. I recently started school for electrical engineering, and am to impatient to wait for when we get into programing, and the building of devices to do things.I'm fine with being taught the how's, whys and what's, and then building something to use my gpio's to do something. Looking forward to learning something through this thread, and maybe causing interest to learn for others.

  • Hi Chris,

     

    Sounds like you have the Pi Cobbler from Adafruit then?  I have it too and have found it very useful for breadboarding my own circuits that interface with the Pi.  If you've not already seen, then definitely checkout Adafruit's Raspberry Pi tutorials.

     

    In terms of protecting the Pi, you definitely need to be careful of the amount of current you draw when doing something like powering an LED.  You need to make sure a properly sized resistor is used in series (the Adafruit tutorials show this). 

     

    There's also the issue of logic level conversion.  Many devices and sensors from the Arduino world use 5V logic which the Pi's 3.3V GPIO pins can't handle.  This can be remedied by using a logic level converter.   I give an example of interfacing a 5V real-time clock in this blog post: What time is it?  How to add a RTC to the Raspberry Pi via I2C

     

    There's some more information on protection circuitry for Pi interfacing at: http://elinux.org/RPi_Tutorial_EGHS:GPIO_Protection_Circuits

     

    Cheers,

    Drew

  • For learning more about GPIOs, a good starting point is the RasPi Hardware Wiki: http://elinux.org/Rpi_Low-level_peripherals.

     

    You have to be careful playing with RasPi GPIOs because they're directly connected to RasPi's BCM2835 SoC and if you connect the GPIO to a voltage more than +3.3V or less than 0V you may damage the SoC.  Similarly, if a SoC GPIO draws too much current it could overheat and damage the SoC.  The SoC is designed to talk to other components in a cell phone or media box and not to the outside world.

     

    The GPIO pins can only source or sink a small amount of current.  It's enough to light a small LED, but not enough to drive a motor or speaker.  However, the GPIO can control an external transistor, and that transistor can drive your motor or speaker.  You have to be careful driving motors and speakers because they can generate nasty voltages: see http://en.wikipedia.org/wiki/Flyback_diode.

     

    The purpose of Gertboard and similar gadgets is to provide a set of robust circuits to protect RasPi's SoC from these hazards.  But a small solderless breadboard and suitable jumper wires is enough to get started.

  • in reply to fustini

    Yes it is the Pi Cobbler kit. Thanks for your quick response on that. And thanks for the links. I've actually read your rtc post before, very nicely written. I bookmarded the adafruit tutorial page,, and elinux page which I think I was looking at before. My kit arrives tomorrow, so hopefully I'll have something built and programmed soon.