Thursday, June 13, 2013

Serial Communication on a ATtiny85 with the SoftwareSerial Library.

"Serial is giving me errors on my ATtiny! What can I do?" We have a solution.

Serial communication is not difficult on an ATtiny thanks to the SoftwareSerial Library. While the ATtiny85 does not have the hardware of a "real" Arduino, it can still function in similar fashion. If you're just getting started with using an ATtiny, here are some resources you might need.

  • Information on programming them from High-Low Tech. You can also look back at my previous posts.
  • My nifty programming adapter. A few minutes of soldering made my life much easier.
  • My USBtinyISP is discussed in THIS post. It also makes life easier.
  • Previous posts on Running Servos and the NewPing Library on an ATtiny.
  • THIS previous post mentioning my USB to UART converter cable
Now we are ready to go. First things first, I am using Arduino 1.0.4. The SoftwareSerial Library is included as a default library, so there is no reason to get a 3rd party library.

Step One: Wire it up. With my USB to UART cable it is as follows.
  • Black: Ground
  • Green: Tx (using Tx as pin 4)
  • White: Rx (using Rx as pin 3)
  • Red: 5v (this is optional if you have an external power supply)
Connect an LED with appropriate resistor to pin 1.

Step Two: Ensure that your ATtiny is burned to run at 8MHz. Now load THIS sketch onto the ATtiny. Note that parseInt() works with the SoftwareSerial Library.

Step Three: Open the Serial Monitor and set it to the correct Baud rate. To reset the ATtiny, bridge the reset pin to ground momentarily. When you see the connected message, enter an integer and count the flashes.

That's all there is to it. I you don't have a USB UART cable, this is easily adaptable to communication with another Arduino with a USB port. See THIS post for code. If you want to see a practical application of serial communication on an ATtiny85, check out my serial sonar controller HERE.

Hope this works for you all. As usual, if there are any questions, just let me know.


  1. Hi Matthew,

    Thank you for this post.

    I am having problems with the baud rate.

    In my serial monitor I am getting data (as 'yyyy')

    But i have both the attiny85 at 9600 and my monitor at the same.

    Any ideas in what to do to debug this?


  2. Hi William. Thanks for reading and commenting.

    I forgot to mention that the ATtiny needs to be running at 8MHz. If I had to guess a problem that would be it.

    Otherwise I would wiggle the wires, restart the ATtiny, and reinstall the UART drivers. If none of that works, comment again and we will see what we can do.

  3. I am getting this error: 'class SoftwareSerial' has no member named 'parseInt'
    I don not understand because I am running the attiny85 at 8MHz and I am using Arduino Uno as the ISP. Please help.

  4. I am getting this error: 'class SoftwareSerial' has no member named 'parseInt'

    I don not understand because I am running the attiny85 at 8MHz and I am using Arduino Uno as the ISP. Please help.

  5. Hi Tom,

    Check out this link

    You may just need a different core. Try this one

    Also, make sure you have a recent IDE version.

    Is this one of the examples? If not, check for typos and make sure your include is right. If you could, comment back with the results whatever they are. It helps to see and document what kind of problems people may encounter

  6. I'm having the same problem. Using IDE versions 1.0.3 and 1.0.5, and I would try the beta version 1.5.2 except that I can't figure out how to get the ATtiny chips to show up in that version. They show in the stable releases though, so that's not my problem. I'm also using the example code SoftwareSerialBlink.ino that I downloaded from this page.

    It's the same parseInt problem that Tom is/was having.

    Also thank you very much for writing up this how to and example code, I'm really looking forward to getting it working!!

  7. I'm not sure what the problem is. It compiles just fine for me on both 1.0.3 and 1.0.5. No chance the library has been changed or something, right? I am no expert, but the way I understand it SoftwareSerial inherits parseInt from Stream.h. Does parseInt work with your regular Serial library?

    Before you dig too much into that I would download a fresh copy of 1.0.5 and drop in a fresh copy of the ATtiny core from Google code.

    if all that fails, you could do a simple read() and then combine them into an int in code. I know there are various methods of doing that online.

  8. Hi Mathew.
    I just found your writeup.
    If I need serial communication and also need 2x digital (or analog) inputs, is it possible using this Attiny45 ?
    If so, which pins I can use as inputs ?


  9. Yes that is possible. See the pinout chart above. As far as I know, any of the IO pins can be used with SoftwareSerial. Then you can use whichever ones you don't need for other things. If you need serial and more than 3 IO pins, you could also use the reset pin as an IO.

    Hope that helps

  10. Does this require the Software SERVO library or is that a typo and you mean Software Serial?

  11. Yes that was a typo. Projects running together. I updated the post.
    Thanks for pointing that out.

  12. is attiny85 have minimum system ?

  13. I'm not sure I understand your question. Can you please rephrase it?

  14. yes "minimum system" , the minimum system to use attiny85.
    something like arduino uno (minimum system of atmega328), there is some component like crystal, capasitor, resistor, etc

  15. Ok. Well from your example the ATtiny would be like the atmega328 in an Arduino. I think it is the "minimum system" you are looking for. If you want to run it at 8MHz, you don't need a crystal. Just connect it to 5V and Ground (assuming the fuses are set correctly).

    That is all you really need. A decoupling capacitor might be nice if you are using a messy power supply. You might look at my ATtiny label if you want more information on using an ATtiny with the Arduino IDE.

  16. On a fresh 1.0.5 install with the core from the link provided (, I also get the ParseInt-error.. the person with the same problem on the arduino forum got his problem fixed, but didn't care to share the actual solution..

  17. So that didn't fix the problem. Well it could be that I actually used the high-low tech ATtiny core for this post. It's hard for me to keep up with it, but I don't think I used that one for very long. I would just suggest you triple check that you are running at 8MHz and have everything installed correctly. If you find a solution, let us know though.

    If it turns out to be really bothersome, I might just go with a and combine them myself.