ATMEL ATTINY85 - How to switch clock to 128 kHz or less?

[mpcz]

Hello,
upload to TINY85:

avrdude -B250 -c usbasp -p t85 -U lfuse:w:0x04:m -U hfuse:w:0xdf:m -U efuse:w:0xff:m ....

The base frequency is reduced. 8x division does not work.
Where is the error?
Thanks, mpcz, 16.oct.2017

[Smajdalf]

Maybe you chande main clock prescaler in your code?

[mpcz]

Yes, the change in code "clock/8" works OK.
I need a change of "frequency and divider" via FUSE.
mpcz, 16.oct.2017

[Smajdalf]

The fuse sets prescaler to divide by 8. If you change it via code later the value from fuse is overwritten. I.e. if it is set by fuse to 8 and you set it to 8 too you get total prescaling 8 (~16kHz), not 64 (~2kHz). Isn't this your problem?

[mpcz]

Sorry, I do not understand.
I set fuses:
avrdude -B250 -c usbasp -p t85 -U lfuse:w:0x04:m -U hfuse:w:0xdf:m -U efuse:w:0xff:m
The code is empty. Frequency is 128k, not 16kHz.
What is wrong?
Thank, mpcz, 16.oct.2017

[Smajdalf]

Are you sure the code is empty? Are you sure no core/bootloader is present that sests the prescaler to 1? Try to add
-e
command line option to the avrdude command to erase possible remnants from the chip.

[DrAzzy]

How do you know that it's running at 128khz?

[mpcz]

I have an ocsiloscope attached to pin # 3, there is an output clock.
mpcz, 16.oct.2017

[Smajdalf]

How do you know the code is empty? Did you erase the chip? Try to read flash to be sure it is empty.

[DrAzzy]

Sanity check...

Upload blink to it via ICSP, compiled for 128khz. Does it blink for 1 second or for 8 seconds?

I agree with your assessment though, it should be outputting - and running at - 16khz!

I would also consider trying to set the prescaler from the application (or at least read it out and blink an LED that many times, or something...)