AT21CS01 datasheet(32/47 Pages) MICROCHIP

32 / 47 page

ROM Zones

8.1

ROM Zone Size and ROM Zone Registers

Certain applications require that portions of the EEPROM memory array be permanently protected

against malicious attempts at altering program code, data modules, security information, or

encryption/decryption algorithms, keys, and routines. To address these applications, the memory array is

segmented into four different memory zones of 256 bits each. A ROM Zone mechanism has been

incorporated that allows any combination of individual memory zones to be permanently locked so that

they become read

‑only (ROM). Once a memory zone has been converted to ROM, it can never be

erased or programmed again, and it can never be unlocked from the ROM state. Table 8-2 shows the

address range of each of the four memory zones.

8.1.1

ROM Zone Registers

Each 256-bit memory zone has a corresponding single-bit ROM Zone register that is used to control the

ROM status of that zone. These registers are nonvolatile and will retain their state even after a device

power cycle or Reset operation. The following table outlines the two states of the ROM Zone registers.

Each ROM Zone register has specific ROM Zone register address that is reserved for read or write

access.

Table 8-1. ROM Zone Register Values

Value ROM Zone Status

ROM Zone is not enabled and that memory zone can be programmed and erased (the default

state).

ROM Zone is enabled and that memory zone can never be programmed or erased again.

Issuing the ROM Zone command to a particular ROM Zone register address will set the corresponding

ROM Zone register to the logic ‘1’ state. Each ROM Zone register can only be set once; therefore, once

set to the logic ‘1’ state, a ROM Zone cannot be reset back to the logic ‘0’ state.

Table 8-2. ROM Zone Address Ranges

Memory Zone

Starting Memory Address

Ending Memory Address

ROM Zone

1Fh

01h

20h

3Fh

02h

40h

5Fh

04h

60h

7Fh

08h

8.2

Programming and Reading the ROM Zone Registers

8.2.1

Reading the Status of a ROM Zone Register

To check the current status of a ROM Zone register, the master must emulate a random read sequence

with the exception that the opcode 0111b (7h) will be used. The dummy write portion of the random read

sequence is needed to specify which ROM Zone register address is to be read.

AT21CS01/AT21CS11

Datasheet

DS20005857A-page 32

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