9.3 Device Response to a Write Operation Within an Enabled ROM Zone

The AT21CS01/AT21CS11 will respond differently to a write operation in a memory zone that has been set to ROM in comparison with a write operation in a memory zone that has not been set to ROM. Writing to the EEPROM is accomplished by sending a Start condition, followed by a device address byte with the opcode of 1010b (Ah), the appropriate client address combination, and the Read/Write bit set as a logic ‘0’. Since a memory address has not been input at this point in the sequence, the device returns an ACK. Next, the 8‑bit word address is sent, which will result in an ACK from the device, regardless of whether that address is in a memory zone that has been set to ROM or not. However, upon sending the data input byte, a write operation to an address that is in a memory zone that is set to ROM will result in a NACK response from the AT21CS01/AT21CS11, and the device will be immediately ready to accept a new command. If the address being written is in a memory zone that has not been set to ROM, the device will return an ACK to the data input byte as per normal operation for write operations as described in Write Operations.