[En-Nut-Discussion] EEPROM accessable via Bootloader

[PragmaLab]

Hello Jason,

> Is there a set of EEPROM Access functions (simple read and 
> write byte functions would suffice) with very little overhead 
> that I can use for the bootloader.  Obviously, I could use 
> the conventional method used in my main application, but 
> would prefer a less costly approach.
 
I used the 'avr-objdump' tool to extract the assembler code from the libc.a
to be sure to keep the code small and to avoid dependancies with other
routines (bootloader should be 100% self-containing in our case)

See below for the result for a Mega2561 (I used the method for the Mega128
as well). You can use it with GCC and ICC.... disadvantage is that it is
target-dependant.....

Best regards,

Rob van Lieshout

/*********************************************************/
/*                   EEPROM routines                     */
/*********************************************************/ 

/*!
 * \brief Write a byte in EEPROM
 *
 *       Extraction and disassembling was done by using the avr-objdump
tool:
 *
 *                  WinAVR\bin\avr-objdump -d  ..\avr\lib\avr5\libc.a >
disassembled.txt
 *
 *       Copy the ee_wb disassembled code from 'disassembled.txt' and paste
it in this source file,
 *       (don't copy the last 'ret' since the assembly code is encapsulated
in this 'C'-function)
 *
 * \note These routines work ONLY for the Mega256 (register addresses)
 */
void Beeprom_write_byte(void* ptAddress, unsigned char DataByte)
{
    asm ("\n\t"
         "sbic    0x1f, 1\n\t"
         "rjmp    .-4\n\t"
         "out     0x22, r27\n\t"    
         "out     0x21, r26\n\t"
         "out     0x20, r0\n\t"
         "adiw    r26, 0x01\n\t"
         "in      r0, 0x3f\n\t"
         "cli\n\t"
         "sbi     0x1f, 2\n\t"
         "sbi     0x1f, 1\n\t" 
         "out     0x3f, r0\n\t"
        );
}

/*!
 * \brief Read a byte from EEPROM
 *
 */
unsigned char Beeprom_read_byte(void* ptAddress)
{
    asm ("\n\t"
         "sbic    0x1f, 1\n\t"
         "rjmp    .-4\n\t" 
         "out     0x22, r27\n\t"
         "out     0x21, r26\n\t"
         "sbi     0x1f, 0\n\t"
         "adiw    r26, 0x01\n\t" 
         "in      r0, 0x20\n\t"
        );
}