bootloader
I am designing a board with the ATmega328P and have decided to write my own bootloader rather than use Optiboot or Arduino, etc. I understand the logic of writing the boot code. Also that it resides in a specific area of the flash (max of about 1K). I am using the AVR Studio 6.
How do I target my code to precisely reside in that area? When I generate the hex file they always end up at 0x0000. How to locate them in 0x7000 or wherever the boot section is.
The ATmega32A Datasheet on page 245 shows the following table of where the bootloader area starts depending on how the BOOTSZ fuses are set:
So assuming you have BOOTSZ0 and BOOTSZ1 are both cleared for the largest bootloader section it will start at 0x3800. From the memory sections documentation you can see the code goes into the .text linker segment and the address may be changed by going under Project properties | Toolchain | AVR/GNU Linker | Memory Settings and adding .text=0x3800 under the FLASH segment section:
Normally if I want the bootloader combined with the application firmware for production programming I use the bootloader to load the main firmware and then read back the result from FLASH. It should be technically possible to combine them using custom linker segments but you'd have to be very careful about where things like standard library functions were placed so the former method is probably easier and safer.
How much memory is on the system?
If you have enough memory it should not be a problem to store the APN code in normal memory and call it from the bootloader (this is what normal OSs do anyway). You should be careful to never overwrite it using good memory management. (I would put it on one end of the system memory.) The point is if you want use more memory when you write applications you have to start writing memory with the bootloader above the APN code.
Further, you should be able to also use this code in your applications if you are careful enough.
Best Answer
First of all you will need to set the BOOTSZx fuses according to your bootloader size, the boot reset address is not fixed and changes with that. (page 282 of the datasheet , top). You will then need to tell the linker where to put your bootloader code, and this depends on what you are using to write code. In assembly what you need is the .org statement, with AVR studio you should mess around with the "memory settings" and put ".text=0x1234" under flash. Of course 0x1234 should be the right address as per page 282, i.e. 0x3800, 0x3c00, 0x3e00 or 0x3f00.