AVR-Chat

--- In AVR-Chat@yahoogroups.com, Dennis Clark <dlc@f...> wrote:
> 
> Tony,
> 1. Wait until EEWE becomes zero.
> 2. Write new EEPROM address to EEAR (optional).
> 3. Write new EEPROM data to EEDR (optional).
> 4. Write a logical one to the EEMWE bit in EECR.
> 5. Within four clock cycles after setting EEMWE, write a logical 
one to 
> EEWE.
> Caution: An interrupt between step 4 and step 5 will make the 
write 
> cycle fail, since the
> EEPROM Master Write Enable will time-out. If an interrupt routine 
> accessing the
> EEPROM is interrupting another EEPROM access, the EEAR or EEDR 
Register 
> will be
> modified, causing the interrupted EEPROM access to fail. It is 
> recommended to have
> the Global Interrupt Flag cleared during all the steps to avoid 
these 
> problems.
> "
> It is a good idea to disable interrupts before writing a byte to 
EEPROM.
> DLC
> -- 
> ---------------------------------------
> Dennis Clark    TTT Enterprises
> ---------------------------------------





OOOOPS.  That last part, 4 clock cyles, is pretty critical timing.  
I COULD disable interrupts for just step 4 and 5.

I can modify the code for the  Libc eeprom code to suit that.  This 
is a very brief suspension. of the interrupts.

What I do not want to do is to disable interupts for the entire 
duration of a multi-byte eeprom write.  That could be many many 
milliseconds.  But if the interrupts are suspended briefly, but 
repetetively, then that is not really a problem.

Do you think it is necessary to have interrupts suspended for all 
the steps?  Is there a limit to the duration of data sitting on the 
eeprom data registers?  Actually it does not seem as if all the 
steps between 2-5 would take that long.  While step 1 is waiting, 
interupts could be enabled.

I will check the atmega128 docs on eeprom reading and writing.  I 
printed the pertinent info out, but did not yet read it since the   
I had such luck with the built-in funtions.  Of course right now, I 
only have the millisecond timer interrupt occuring.  In real life I 
would have an external interrupt triggering at a regualr, but 
probably slow (10's of ms) rate.

I will see if I can find the C/ASM code for the eeprom so I can hand-
tune the interrupts.

-Tony