Electronic – Can an ATtiny with RSTDISBL programmed be reprogrammed over ISP


The ATtiny85 datasheet, on page 152, seems pretty clear on this:

After this fuse has been programmed device can be programmed via
high-voltage serial mode, only.

But elsewhere, on page 28, it says:

If the RSTDISBL fuse is programmed, this start-up time will be
increased to 14CK + 4 ms to ensure programming mode can be entered.

Can an ATtiny be reprogrammed over ISP by entering programming mode within a short interval after powering it on? If not, why is the startup delay increased as described in the second quote?

Best Answer

I don't think there's any way to reprogram the AVR via ISP if the RSTDISBL fuse is programmed; perhaps the second sentence is itself referring to the high voltage serial mode? The only way I know of to low-voltage program the ATTiny with the reset pin used as an output is to use a bootloader - on startup the bootloader will check for a serial signal on its configured pins, and if it's there it will download the code via serial and then flash itself.