Is there a reason to limit OTA size on ATMEGA1284p to 64kB ?

[Uncle Buzz]

I saw in the bootloader source than sketch is limited to 31k on ATMEGA328p (32k - 1k for bootloader), and  64k for ATMEGA1284p. I wonder why ATMEGA1284p can't have a limit to 127k (128k - 1k for bootloader) ?
Is there a technical reason or is it an arbitrary choice to limit size ?

[Felix]

It kind of pushes the limits of practicality.
With the standard protocol it can take a long time to transfer that much data, and also 64K is a very LARGE sketch for an AVR.
So it was just a decision made at that time to limit it to 64k for the MEGA. Nobody has ever reported a need to flash more than that over OTA.
Do you have such a case?

In any case, it is an easy code change to allow more address bytes. In fact for M0 I did allow this in HandleWirelessHEXData() (image signature ends at byte 10 leaving 3 bytes for address):

Code Select Expand

#if defined (MOTEINO_M0)
  flash.writeByte(10,':');
  uint32_t bytesFlashed=11;
#else
  flash.writeByte(9,':');
  uint32_t bytesFlashed=10;
#endif

And the bootloader will look for those 3 bytes instead of just 2. 3 bytes allows 16MBytes, I think that is more than enough ;-)

[Uncle Buzz]

I don't' have a need for the moment, but I have a project where the sketch is growing because of adding some libraries... I switched on MEGA because 31k was not enough. And since I want to be able to upload a new sketch over the air, I would like to know if I'm limited to 64k or if it could be increased if needed.

I saw this limit which is not the flash size, so I was wondering if there's another reason than a long upload process to restrict the size before trying to increase it and discovering some problem you already be aware of...

[Felix]

No problems that I'm aware of other than the usual potential interference during a long OTA upload. I saw that but very rarely and it can happen with shorter uploads as well.