Preliminary

CAT35C704

9

Doc. No. 25045-00 2/98

As shown previously in Figure 3, memory locations

below the address set in the memory pointer will be

program/erase protected. Thus, unintentional clearing

or writing of data in this area will be prevented, while

memory locations at or above the protected area still

allow full access. This protection does not apply to the

ERAL and WRAL commands which are not blocked by

the memory pointer.

SECURE MODE

As shown previously in Figure 2, in the secure mode,

memory locations at or above the address set in the

memory pointer allow READ-only access. Memory loca-

tions below that address will require an access code

before they can be accessed. The secure mode is

activated with an MACC (Modify Access Code) instruc-

tion followed by a user access code which can be one to

eight bytes in length.

EWEN

MACC

[old code][new code][new code]

The EWEN instruction enables the device to perform

program/erase operations. The new access code must

be entered twice for verification. If the device already has

an access code, the old access code must be entered

before the new access code can be accepted. The

length of the password is incorporated into the MACC

portion of the instruction.

Once the secure mode is activated, access to memory

locations is under software control. Access (read, write,

and clear instructions) to the memory locations below

the address in the memory pointer is allowed only if the

ENAC (Enable Access) instruction followed by the cor-

rect access code has been previously executed.

ENAC

[access code]

EWEN

WRITE

[address][data]

The ENAC instruction, along with the access code,

enables access to the protected area of the device. The

EWEN instruction enables execution of the program/

erase operations. This portion of the memory is other-

wise inaccessible for any operation. Read-only access

is allowed without the access code for memory locations

at or above the address in the memory pointer.

The access code can be changed by the following

instruction:

ENAC

[old access code]

EWEN

MACC

[old code][new code][new code]

A two-tier protection scheme is implemented to protect

data against inadvertent clearing or writing. To write to

the memory, an EWEN (Program/Erase Enable) must

first be issued. The CAT35C704 will now allow program/

erase operations to be performed only on memory

locations at or above the address set in the memory

pointer. The remaining portion of the memory is still

protected. To override this protection, an OVMPR (Over-

ride Memory Pointer Register—see Memory Pointer

Register) must be issued for every program/erase in-

struction which accesses the protected area:

ENAC

[access code]

EWEN

OVMPR

WRITE

[address][ data]

As an alternative to the OVMPR instruction, the WMPR

(Write Memory Pointer Register) instruction may be

used to move the memory pointer address to uncover

the area where writing is to be performed:

ENAC

[access code]

EWEN

WMPR

[address]

WRITE

[address ][data ]

Figure 11. ERAL Timing

5074 FHD F14

CS

CLK

DI

DO

OP CODE

OP CODE

OP7

OP0

OP0

OP7

tSV

tEW

HIGH-Z

DON’T CARE

BUSY

READY