EEPROM Xicor X28256D, won't remove write protect

[Staffan321]

Hello,

I have a board with 2x10 Xicor X28256 32x8kb EEPROM (https://www.digchip.com/datasheets/parts/datasheet/000/28256.php).
It is for a test instrument and I would like to update the firmware stored on it. For some reason, a few (at least one) ICs can be written to, but most of them will not accept the data.

The board is an HP design so I doubt voltages are bad or signal integrity is poor. Noticed a 20 mV voltage drop on its VCC pin when accessing an IC.

With a logic analyzer, I can verify that the correct sequence is sent to the IC for a write sequence, but when reading back from most ICs it is not the correct data. I suspect the software data protection is activated on the ICs and this prevents me from actually writing data.

According to the datasheet, there are two ways to unlock the write protect.
One method requires a short sequence of data to be written to specific positions within the IC before actually writing the correct data.
A second method is to 'write' a longer sequence to the chip in order to deactivate write protection all over.

Due to the design of the board and my implementation of interface to the board (Arduino), I can't use the first method as it has -fairly- stringent timing requirements (100 microseconds between sending a sequence and starting the actual data write).

The second method (outlined on p 3-80 in the datasheet) does not seem to have any timing requirements and I have verified that the chip actually receives the correct sequence of addresses, data and control signals. I have even cycled power to the board in order to make sure the write protect is deactivated. Still, the chips won't accept data.

Anyone with experience of this write protect or writing to this EEPROM?

Any suggestinos welcome.

Regards,
  Staffan

[wraper]

Why do you link wrong datasheet that is not even for EEPROM device but NOR FLASH with different pinout?

[Staffan321]

Hello,

Why do you link wrong datasheet that is not even for EEPROM device but NOR FLASH with different pinout?

I try to link in order to save space, but this time it went wrong. You seem to have found the document so now it's here. Thanks.

Regards,
  Staffan

[coromonadalix]

can you get your hand on a tl866 1st generation,  they can normally do them ?

[Staffan321]

Hello,

can you get your hand on a tl866 1st generation,  they can normally do them ?

Problem is that the ICs are soldered to a board. I really need to communicate via a fairly complicated interface. The original design was for bubble memories - happy I don't need to mess with those...

Regards.
  Staffan

[Staffan321]

Now this is odd. Pulling the board out of the system and reinserting it makes writing work.

I would expect a bad connector somewhere, only I checked all communication on the pins of the ICs themselves and the sequence was correct. Time to think about this, but for the moment I'm happy to be able to upload new firmware.

Regards,
  Staffan