How to prevent overwriting data in external EEPROM after msp430 restarts urgent plz

Record in the EEPROM the last address you wrote to at a known location (like 0x0000 or 0xFFFF).  On power up, read that address from the EEPROM and that becomes the new starting address.  When you get ready to power down, write the current address to the EEPROM so you can resume it on the next power cycle.

If your power state is unpredictable, then you might need to write the address to EEPROM more frequently.

Faisal khan2 said:

How can I read the last accessed address from EEPROM after msp430 restarts ,because it wil continue to execute its code from the beginning 

You change your code.

At startup you check the magic location to see where you should start storing your data.

   Thanks for your replies,I m new to msp430 .I didnot get it properly .For example First time I start msp430 so it starts to write temp and  pressure values of sensors to eeprom from 0000h and so on and after some time the battery is off and then the battery is turned on again so it always starts to write it in eeprom from 0000 and then further.How can I figure out the last accessed location of eeprom upon restart of msp430.I did not get your point properly

If you have some code or any reference link then please share with me  

As Brian stated, the magic location is a known place in the EEPROM. Pick an address.  0x0000 is a good address.  At that address, you constantly record the last address that you wrote to:

InitializeBoard();
InitializeI2C();
currentWriteAddress = ReadEEPROMAddress(0x0000);
if (currentWriteAddress  == 0x0000)
{
   currentWriteAddress  = 0x0001;
}
while(true)
{
   /* Read Sensor */
   writeEEPROMAddress(currentWriteAddress , newSensorValue);
   currentWriteAddress++;
   writeEEPROMAddress(0x0000, currentWriteAddress);
}

You still need to account for failed I2C operations, address rollover, etc.

EDIT: edits for forgetting things...

Hi James,

There’s another way to do it without the magic location.
You can simply start reading the eeprom to the first ‘unwritten address’. When you start with an eeprom that is all 0xff or 0x00 (or any other value that can’t be valid), you simply start reading from 0x0000 until you get the first location with an invalid value. This is your first write position.
The advantage is that you don’t need to write the address to the magic address on each write. Since EEPROM write is slow, this may speed-up things and also reduces the risk of spoiling the address if power fails during the address write.
Of course it results in a slower startup (you may use an approximation algorithm to increase he search speed). Also, it means the eeprom needs to be initialized. Or you write not only the data, but also an invalid entry to the next address. This voids the speed advantage when writing but still reduces the risk of a spoiled pointer on power failure. It is a preferable approach when using flash, as flash can be erased (= initialized) block-wise. I use it for storing data in the MSP’s internal flash.

Dear Michael,

                   Thanks a lot for your reply.Yes you are right too in this sense .I do not have to write again and again to store my current address in Memory .

Regards 

Faisal Ali Khan