9.2.3 Freeze ROM Zone Registers
The current ROM Zone state can be frozen so that no further modifications to the ROM Zone registers can be made. Once frozen, this event cannot be reversed.
To freeze the state of the ROM Zone registers, the host must send a Start
condition, followed by the device address byte with the opcode of 0001b
(1h) specified, along with the appropriate client address combination and the
Read/Write bit set to a logic ‘0’. The device
will return either an ACK (logic ‘0’) response if the ROM Zone registers
have not been previously frozen or a NACK (logic ‘1’) response if the
registers have already been frozen.
If the AT21CS01/AT21CS11 returns an ACK, the host
must send a fixed arbitrary address byte value of 55h, to which the device will return an
ACK (logic ‘0’). Following the 55h address byte, a data byte of AAh must
be sent by the host. The device will ACK after the AAh data byte. If an address byte other
than 55h or a data byte other than AAh is sent, the device will NACK (logic
‘1’) and the freeze operation will not be performed.
To complete the Freeze ROM Zone register sequence, a Stop condition is required. If a Stop condition is sent at any other point in this sequence, the operation is aborted. Since a Stop condition is defined as a null bit frame with SI/O pulled high, the host does not need to drive the SI/O line to accomplish this. After the Stop condition is complete, the internally self-timed write cycle will begin.The SI/O pin must be pulled high via the external pull-up resistor during the entire tWR cycle.