According to the data sheet for the SLE4418, which you can see at the this link, the data in this part can only be written taking three considerations into account.

Firstly note that these chips require the entry and verification of a 16 bit programmable security code (PSC). The PSC must be correctly entered in order to arm the chip for write and erase operations. Once the 16 bit PSC has been entered the chip will remain in the armed state as long as power continues to be applied to the chip. When the chips are first manufactured they come from the factory pre-programmed with a PSC code that is called the "transport PSC". This code is preagreed upon between the manufacturer and the customer. After initial incorporation of the SLE4428 into the customers application the PSC is generally changed by the customer to some other 16 bit value that is unique to each instance of the SLE4428.

Secondly, note that there is an error counter mechanism that will prevent chip access after a certain number of attempts to access the chip with entry of an invalid PSC. Once the error counter has reached its max count and 'disallow' is in force it is only possible to access the chip with the proper PSC. If the chip was in the state where the error counter has reached its max count and the disallow mode is active it is necessary to properly clear the error counter bits, after successful PSC entry, but before power off in order to allow future access to the part. This error counter is designed to keep unauthorized persons from trying to 'discover' the PSC code by trying all 65536 combinations of the PSC. After the certin number of attempts have been tried the chip effectively freezes up until the correct PSC is entered.

Lastly note that each byte of storage in the chip has a protect bit associated with it. The protect bit for a given location, if set, will prevent writing new data to that location. The protect bit can only be written ONCE and once it is set the corresponding location becomes a read-only location. It is fully posssible to read out each byte along with its protect bit to see whch locations have been made read only. Thus it is possible to change the data in only those locations that you note do not have the protect bits set.

These security measures are designed to prohibit access to and entry of data into the SLE4428 by people who do not know the appropriate PSC. If you do not know the PSC then writing the chip will not be possible.

Michael Karas