ATTiny85 / 8MHz (internal) clock / seems to run 8x too slow

[Art-ut-Kia]

Hello,
I'm trying to develop an ATTiny85-based measurement tool. I'm using the Arduino-as-ISP to program the chip.
After running into some inconsistencies regarding timing, I performed the simplest test: loop{pin_HIGH/delay(5)/pin_LOW/delay(5)} and got a frequency of about 12.5Hz rather than the expected 100Hz.
After configuring the clock to 1MHz, all seems to get Ok.
I wonder whether I'm not doing something wrong with fuses.
It's not clear for me whether Arduino IDE + UNO-based programmer is able to configure everything properly or not.
I've read that some configurations require to send 12V on RST pin, what my programmer can't manage.
Help is welcome
Jean-Paul

[Art-ut-Kia]

Meanwhile, I understood that I had to execute the "Tools - Burn bootloader" command in Arduino IDE to configure properly the chip.
However I observed that I can set and reset the CKDIV8 bit with my UNO-based ISP programmer (no 12V onboard).

So I still cannot understand what bits (aka fuses, lockbits ?) require 12V to be changed ...

If anyone can clarify this for me, this would be great

[hmeijdam]

A High Voltage (12V) programmer is only needed if the RST pin has been disabled or the MCU has been locked for further programming.

If you can access the fuses you can set the clock source like you want, including the clock divider (CKDIV8 bit).

If you do not touch the RSTDISBL and the SPIEN bits you should be safe.

check for yourself at https://www.engbedded.com/fusecalc/