I'm using an ATtiny24A.
Is it still possible to enter serial programming mode if RSTDISBL fuse bit is programmed?
The data sheet 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."
So if I apply power to VCC while I hold RESET low it should enter serial programming mode?
Once you program RSTDISBL the chip is lost as far as ISP is concerned. You will need a different programmer and different programming technique once the bit is set. Some folks put a bootloader into such a chip before the link is broken to maintain a way to reprogram it without specialist equipment.
The usual advice is simply DO NOT PROGRAM RSTDISBL. Pick a micro with more pins if you are that tight on pins that you have no option but to use _Reset. If you are going into volume production (like 50,000+/annum) and the saving in $0.01's of using the smaller chip is justified then sure, get the code working completely (often in a bigger chip) or with a HV programmer or via a bootloader, then program the final code into all the devices with the RSTDISBL activated. But unless you are using volume production and economies of scale then RSTDISBL does not make a lot of sense.
EDIT: I wrote this a long time ago - it explains the perils of RSTDISBL: https://www.avrfreaks.net/forum/... Read on from where it starts:
Perhaps the most insidious of all the fuses...
Ok. But what about the statement in the datasheet?
If the RSTDISBL fuse is programmed, this start-up time will be increased to 14CK + 4 ms to ensure programming mode can be entered.
I believe the time is for you to enter High Voltage Serial Programming mode...
Yes, it has to be that. Once RSTDISBL is set you don't get a choice - there's no possibility of ISP being used any more (until HVSP has been used to switch it back).
Ok. But what about the statement in the datasheet?
If the RSTDISBL fuse is programmed, this start-up time will be increased to 14CK + 4 ms to ensure programming mode can be entered.
Please tell which AVR model has that quote, so it can be evaluated in context. I pulled up a couple of recent datasheets and cannot find a match.
[edit] I found it, in the "clock" section and not in the "programming" section. Indeed, I can see where the quote would throw off a datasheet reader. I agree with Cliff that if you program RSTDISBL you won't be able to do conventional ISP.
Now, what about debugWire and RSTDISBL combination?
which AVR model has that quote
EDIT: it is (and I think the battery in my mouse is dying as I could not draw a smooth ring around this!!!):
Ok. But what about the statement in the datasheet?
If the RSTDISBL fuse is programmed, this start-up time will be increased to 14CK + 4 ms to ensure programming mode can be entered.
Read it like this:
If the RSTDISBL fuse is programmed, this start-up time will be increased to 14CK + 4 ms to ensure high voltage programming mode can be entered.
...Because it potentially takes longer for the High Voltage power supply to come up, compared to the 5V supply.
