Imagecraft c compiler and development environment for the atmel avr

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ICCV8 for AVR – C Compiler for Atmel AVRCRC GenerationYou can have ICCV8 for AVR compute the CRC of your flash image and thenprogrammatically verify it in your program. ICCV8 for AVR calculates a 16-bit CRCusing a common algorithm (as a reference, please check the Wikipedia entry for“CRC16”). The AVR routine uses a table of 512 bytes and takes about 80 bytes ofcode. You may possibly reduce the total memory consumption by replacing the codethat does not use a table, but this will result in a slower running time.To use, specify in the Build Options - Target CRC edit box. The linkerstores the 4-byte CRC structure in :unsigned address;unsigned crc16; must not be used for other purpose. Typically you would specify theaddress 4 bytes before the end of the flash memory.CRC is computed from location 0 to .In your code, write#include ...__flash unsigned char *p = (__flash unsigned char *)0x????;unsigned int crc16;...crc16 = calc_crc16((__flash unsigned char *)0, p[0]);if (crc16 != p[1])// CRC mismatched0x???? is the same as in the -crc switch. The header file crc.hcontains the declaration for the calc_crc16 routine. Thus p[0] contains the addressand p[1] contains the crc value calculated by the linker.Obviously CRC is not foolproof since any flash memory corruption means that theCRC checking code may not operate correctly. Nevertheless, CRC can be part of aquality assurance workflow.116
ICCV8 for AVR – C Compiler for Atmel AVRProgram Data and Constant MemoryThe AVR is a Harvard architecture machine, separating program memory from datamemory. There are several advantages to such a design. One of them is that theseparate address space allows an AVR device to access more total memory than aconventional architecture. In an 8-bit CPU with a non-Harvard architecture, themaixmum amount of memory it can address is usually 64K bytes. To access morethan 64K bytes on such devices, usually some type of paging scheme needs to beused. With the Harvard architecture, the Atmel AVR devices have several variants thathave more than 64K bytes of total address space without using a paging scheme.Unfortunately, C was not invented on such a machine. C pointers are either datapointers or function pointers, and C rules already specify that you cannot assume thatdata and function pointers can be converted back and forth. On the other hand, with aHarvard architecture machine like the AVR, a data pointer might point to either datamemory or to a constant data located in the flash (program memory).There are no standard rules for handling this. The ImageCraft AVR compiler uses the__flash qualifier to signify that an item is in the program memory. Note that in apointer declaration, the __flash qualifier may appear in different places, dependingupon whether it is qualifying the pointer variable itself or the items to which it points.For example:__flash int table[] = { 1, 2, 3 };__flash char *ptr1;char * __flash ptr2;__flash char * __flash ptr3;Here table is a table allocated in the program memory, ptr1 is an item in the datamemory that points to data in the program memory, ptr2 is an item in the programmemory that points to data in the data memory, and ptr3 is an item in the programmemory that points to data in the program memory. In most cases, items such astable and ptr1 are most typical. The C compiler generates the LPM or ELPMinstructions to access the program memory.If a constant data is allocated above the 64K byte boundary, you will need to takespecial measures to access it. See Accessing Memory Outside of the 64K Range.117
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Imagecraft c compiler and development environment for the atmel avr

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