Re: [avr-gcc-list] Macro to read /write long to eeprom

[David Kelly]

On Wed, Aug 10, 2005 at 11:00:20AM +0200, Branislav Katreniak wrote:

> Another trick is not to clear the byte in eeprom before the write when
> it is sufficient to only clear bits on order to store the value into
> eeprom (you save half of the write). There is some datasheet on atmel
> site about this.

Which datasheet, please? Which AVR?

What you describe is what we used to do on 68HC11E1. When it came time
to write to EE on Atmega64 I found the control structure was different
in that one does not have separate clear and set steps when writing.

This is the eeprom write routine I use. Later compared to the hand
assembly in avr-libc and found the code size and steps performed were
almost identical. Given that, I'd rather use my own in C under my own
source control.

Notice how it reads first and skips the write if data is identical.

This is the only routine I use for writing EEPROM. If I only write one
byte then this just requires a little extra call setup. Don't see any
point in having another routine with only 2 args rather than 3.

//
//    write to EEPROM
//        addr is the address within the EEPROM
//        cp is where to read data in RAM
//        n is the number of 8 bit octets to copy
//
//    ee_writes( sram source, ee dest, count );
//
void
ee_writes(const void *cp, void *addr, uint16_t n)
{
    uint8_t sreg;        //  store SREG

    for( ; n ; n-- ) {
        wdt_reset();
        while( EECR & (1<<EEWE) )    //  wait until ready
            ;

        EEAR = (uint16_t)addr++;
        EECR |= (1<<EERE);        //  start a read

        //  Don't write unless the contents have changed:
        if( EEDR != *((uint8_t*)(cp)) ) {

            EEDR = *((uint8_t*)(cp));    //  looks frightening

            sreg = SREG;        //  save SREG to preserve global IRQ
            cli();            //  an IRQ would ruin this timing
            EECR |= (1<<EEMWE);    //  only stays set for 4 clocks
            EECR |= (1<<EEWE);    //  hit EEWE before EEMWE clears
            SREG = sreg;        //  restore prev global IRQ state
        }

        cp++;

    }
}

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