ATtiny85 serial communication, bootloader and setting up fuses

Question

I have been reading some articles about ATtiny and how to burn the bootloader. And I have two questions which I can't figure out.

1. Do I need to manually setup fuses to run 8 MHz on my ATtiny when I'm using my Arduino board as my ISP (there we can select the internal clock and that confuses me)?

2. For the SoftwareSerial.h library, do I need to setup my ATtiny to 8 Mhz in order to work?

Answer 1

If you load this into the IDE https://github.com/SpenceKonde/ATTinyCore the fuses will be set correctly for the different selectable clock speeds.

Answer 2

1. If you search the internet, you mostly find statements, that the fuses will be set right, if you burn the bootloader from the IDE. However I had to set the fuses in an extra call to avr-dude, because the clock divider was still active, setting the clock effectivly to 1MHz, instead of the intended 8MHz. With a command like

   avrdude -P /dev/ttyUSB1 -b 19200 -c avrisp -p attiny85 -v

you can read the current fuses. Note that you have to set the parameters according to your setup (I used an ArduinoAsISP on the serial port ttyUSB1). You can write the fuses with a command like this:

avrdude -P /dev/ttyUSB1 -b 19200 -c avrisp -p attiny85 -v -U lfuse:w:0xE2:m

depending on which fuse you want to set, you have to replace lfuse with hfuse or efuse.

2. SoftwareSerial should also work with other clock frequencies, but I don't know the limitations. Keep in mind, that lower clock frequency means you can only reach lower baudrates. It is crucial, that the fuses are set correctly for the intended speed, because the library takes the speed defined in the IDE. In my case this was 8MHz, but the ATTiny still run at 1MHz, which confused the serial communication.