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Re: [avr-gcc-list] __checksum

From: David Kelly
Subject: Re: [avr-gcc-list] __checksum
Date: Thu, 29 May 2008 12:59:39 -0500
User-agent: Mutt/

On Thu, May 29, 2008 at 11:25:01AM -0600, Blake Leverett wrote:
> On Thursday 29 May 2008, trile58 wrote:
> >
> > I had flash changed to PROGMEM as instructed in the porting document.
> > However, I got an error "undefined reference to __checksum" .

> It looks like the problem is the "flash" keyword.  Avr-gcc handles
> flash differently from IAR - so more is needed than just a keyword
> change.  The __checksum name is just a variable name, so I think that
> the flash keyword is where the compiler is losing track.

Yes, but an undefined reference says that *nothing* is known about
__checksum. Would seem IAR automatically buries a __checksum in the code
image but avr-gcc does not.

> Be sure you are including <avr/pgmspace.h>, and you may want to place
> the PROGMEM keyword later in the line:
> extern unsigned short PROGMEM __checksum;

But somewhere one has to allocate this variable and somehow put
something in it.

> Also, in the slow_crc16() function, make sure that you call
> pgm_read_byte() or pgm_read_word() as necessary.  You cannot simply
> say
> uint16_t a = __checksum;
> you have to use
> uint16_t a = pgm_read_word(&__checksum);
> Or something like that.

Not only that but if the intent is to CRC the FLASH contents then
slow_crc16() needs to be coded accordingly. Is my understanding that
slow_crc16() is a standard IAR library function.

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

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