Is it still possible to enter serial programming mode if RSTDISBL fuse bit is programmed?

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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?

 

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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...

Last Edited: Mon. Oct 1, 2018 - 09:04 AM
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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.

 

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I believe the time is for you to enter High Voltage Serial Programming mode...

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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).

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MASIP wrote:

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?

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

Last Edited: Mon. Oct 1, 2018 - 01:47 PM
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theusch wrote:
which AVR model has that quote
he mentions tiny24A in the first post so you'd kind of hope it might be in the t24A data ?

 

EDIT: it is (and I think the battery in my mouse is dying as I could not draw a smooth ring around this!!!):

 

 

Last Edited: Mon. Oct 1, 2018 - 01:51 PM
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MASIP wrote:

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.

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...Because it potentially takes longer for the High Voltage power supply to come up, compared to the 5V supply.