How to program an ATTiny85

I had the genius idea of stripping cheap ATTiny85 chips from their boards on a Digispark and integrating them onto my own boards because they are easier to program on a board, and when I'm happy with the result, I can just place it on another board, and voila! Everything should work. Unfortunately, things weren't as easy as I expected.

Figure 1. Digispark

The first issue was that the circuit did not work,

So now I am stuck with a SMD ATTiny85 with no means of reprogramming it. I thought about soldering wires to the traces, which frankly isn't too difficult since the traces were quite spaced apart, however I wouldn't be able to change other parts of the circuit on the board, and that was a problem, finally I broke down and bought a breakout board.

Figure 2. SMT Breakout Board

Of course if you started with a DIP version of the ATTiny85, you wouldn't have to do this.

Lesson 1: Always prototype your circuits on a board first!

I tried plugging in the power and LEDs but nothing worked, so I decided to use an Arduino as an ISP to program the blink sketch into the ATTiny85. It worked! The light started to blink! But one issue was that it was set at 8Mhz, and changing it to 16Mhz or 1Mhz caused the time between the blinking to react strangely. I read that it has some issue with Arduino assuming that the original frequency is 8Mhz. However, with the Digispark software, all timings are correct and I was able to set it to the frequencies that I required previously.

The reason for choosing a lower frequency was that I wanted to conserve power. A lower frequency results in a slightly lower power draw from a battery, quite important if you are using a battery powered project.

So I had to load the Digispark firmware onto the chip.

I tried using the littlewire tutorial on the Digispark forum to turn my other Digispark into an ISP, however it was never able to recognise the other ATTiny85 chip, even though it was registered as a USB device after I loaded the Littlewire program onto it.

So next I decided to use my Arduino as an ISP to load the Digispark firmware. I referred to an Adafruit tutorial for the specific command required to run it. Note that you need the baud rate for the Arduino ISP to work properly!

~$ avrdude -c arduino -p t85 -b 19200 -P /dev/ttyACM0 -U flash:w:'.../t85_default.hex'

On a new board you will also have to set the fuses (add to the original command):

 -U lfuse:w:0xe1:m -U efuse:w:0xfe:m -U hfuse:w:0xdd:m

I managed to load it successfully and tried to use Arduino to program the Digispark, however it reported that the firmware is too new for the tool! So now we have to replace the tool. From the micronucleus github, go into the commandline folder and run the command to make the micronucleus executable. Now you'll have to locate the old tool somewhere in ~/.arduino15/packages/digistump/tools/micronucleus/2.0a4/micronucleus and replace it with the new tool you just complied. Make sure to backup the old executable just in case you want to revert.

You can also upload the old micronucleus firmware, which is version 1.06.

Locate the old micronucleus program that is called by arduino replace it.

Wire the ATTiny85 to the Arduino ISP as follows:

VCC->5V  
PB5->10  
PB2->13  
PB1->12  
PB0->11  
GND->GND  

You should now be able to upload the micronucleus firmware to any ATTiny85 chip. The nice thing about desoldering the original chip is that if you do a 1:1 wiring of your ATTiny85 to the board you will be able to upload sketches as if the ATTiny85 were on the board itself! That way you can program and prototype many ATTiny85 chips.

Remember once you switch back to uploading code via micronucleus, you need to remove the wires from the ATTiny85 connecting them to the Arduino ISP.

Removing the 5 second delay from the Digispark ATTiny85

Now I wanted to remove the 5 second delay so I used the following release of micronucleus (v 1.11) and the upgrade file micronucleus-1.11-entry-jumper-pb0-upgrade.hex. I used the latest version of the micronucleus executable and ran it with the option of --run. So the code to run in the terminal looks like ~$ micronucleus --run '...upgrade.hex'. In this case, PB0 is pulled to ground to prepare the chip for programming. If it is not pulled to ground, then the chip will immediately run the program that is loaded on it.

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