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Re: [avr-gcc-list] Re: placing a variable in flash

From: David Kelly
Subject: Re: [avr-gcc-list] Re: placing a variable in flash
Date: Tue, 23 Nov 2004 08:24:36 -0600

On Nov 22, 2004, at 11:18 PM, Royce & Sharal Pereira wrote:
I first program the entire device. This automatically erases it, setting
0x1fff to 255.
Now, using feature (1), I read the calib. byte for 4MHz, & pipe SP12 output
to a stream editor, sed, which extracts the calib byte.
Using feature (2), I write the extracted calib byte to 0x1fff in
flash(program) memory.

My challenge was creating a variable in flash at 0x1fff so that the Mega8
could read this byte at reset & save it in OSCCAL.
I then realised that I really didn't need a 'label'.
So this works:


I'm only now in the reading phase at getting started with avr-gcc but in other toolsets for embedded CPUs one generally has to do a lot more work with the linker before ultimately able to create a project. And your need smells a lot like a job for the linker.

In /usr/local/avr/lib/ldscripts/ you will find the linker script used for your CPU. Copy one to your project and edit it. I don't know this linker well enough yet but its likely you can create your symbol totally within this file.

Another possible solution would be to create a single C source file containing nothing but your one byte constant definition. When linking in the Makefile follow one of the examples and load that one object file to a specific section and spell out the starting address (0x1fff) of that section on the command line. Might be able to force it into .text at that location, might not. Might need to create a .mytext section in the linker script and stuff it in there.

As I said, I'm still reading. Thinking if your byte is declared constant then avr-gcc will use the proper addressing to pluck it from FLASH.

One last solution, you could simply throw the byte in the normal constant "variable" pile and use your scripts to find it in the object table while you are determining the correct calibration value. If its referenced only in one file then the optimizer may remove its allocation. (Seems like that would require optimization in the linker.) If its created global in one file and declared constant there, but only "extern" elsewhere then it might fool the optimizer.

On the other hand as long as your program code never overflows to 0x1fff for storage your current solution works fine and is simple.

David Kelly N4HHE, address@hidden
Whom computers would destroy, they must first drive mad.

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