Pointer to memory location in Atmega128

Apr 20, 2005 3 Replies
T

hi good day, We are using codevision AVR as compiler for atmega128 , and



bootloader(upper 2K) for loading application firmware(start from 0x0000



, size nearly 70K) and a separate data structure(start from 0xB400, size nearly 20K). Now we want to access that data structure from our application firmware.Actually we failed to access the upper 64k area by



declaring a pointer to that memory address.But we can access whatever location which is below than 64K like as follow,


int flash *fp;


fp = 0x6800; (this pointer point to 0x68 th page(104 th page), that means the actual



memory location is 0x3400)


what should we do to access the actual memory location 0xB400( that means 360 th page (0x168 th page)).In atmega128 the address line is 16 bit. Can anybody point out me the correct direction to solve this problem. thank you


with regards marans



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L

I suspect your problem lies in the use (somewhere buried deep in a macro) of the LPM instruction which only accesses the lower 64kB. you'll need to use the ELPM which avoids that restriction. Sorry I can't help more with the specifics - I don't know the Codevision tools.

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R

structure from our

What size (in bit) is your pointer variable? Typically it is 16 bit and does not hold the extra bit to address the upper 128k.

/Roland

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D

[...]

You can't do that in CodeVisionAVR. It's a stated limitation of the compiler that pointers to flash can only access the first 64k.

A new version of CodeVisionAVR is in development that _will_ allow access to the upper 64k (and presumably upper 128k on the mega256, and beyond on larger chips). But it's not yet available AFAIK. And any such pointer will require more than two bytes of storage.

Until then, you'll have to drop into assembly. CVAVR makes this relatively painless. Something like (WARNING: untested, probably non-optimal code)

unsigned int read_flash_word(unsigned int adr) { #asm ld r30,Y ldd r31,Y+1 clr r0 sbrc r31,7 inc r0 out RAMPZ,r0 clc rol r30 rol r31 elpm r0,z+ elpm r31,z mov r30,r0 #endasm }

unsigned int fp; unsigned int result;

fp = 0x6800; result = read_flash_word(fp);

fp = 0xB400; result = read_flash_word(fp);

Please RTFM for more details. Good luck, -=Dave

Change is inevitable, progress is not.
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