ATtiny not working – PB5/RESET controlled by RSTDISBL fuse
While working on a ATtiny13 development board I noticed that my board only worked when the PB5 pin was high.
If you have a non-functional or erratic ATtiny13 or similar Atmel MCU that shares RESET with a IO pin, try setting the IO pin that is shared with RESET to a logic high. It is possible that the RESET pin is being pulled low and this is keeping your code from running. This happens because instead of allowing for a software switch, Atmel uses the RSTDISBL fuse to control the switching between RESET and IO pin use.
One caveat in setting the RSTDISBL fuse is that the AVR can no longer be programmed via ISP mode, the fuse can only be reset with a high voltage programmer. I don’t understand why Atmel chose to do this, Atmel should have allowed users to toggle the RESET pin via software – maybe they wanted to give a large customer
the ability to lock out the ability to reprogram the microcontroller!