CAT35C704 データシートの表示（PDF） - Catalyst Semiconductor => Onsemi

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CAT35C704

4K-Bit Secure Access Serial E2PROM

Catalyst Semiconductor => Onsemi 

CAT35C704

Preliminary

PASSWORD PROTECTION

The CAT35C704 is a 4K-bit E2PROM that features a

password protection scheme to prevent unauthorized

access to the information stored in the device. It contains

an access code register which stores one to eight bytes

of access code along with the length of that access code.

Additionally, a memory pointer register stores the ad-

dress that partitions the memory into protected and

unprotected areas. As shipped from the factory, the

device is unprogrammed and unprotected. The length of

the access code is equal to zero and the memory pointer

register points to location zero. Every byte of the device

is fully accessible without an access code. Setting a

password and moving the memory pointer register to

cover all or part of the memory secures the device. Once

secured, the memory is divided into a read/write area

and a read-only area with the entry of a valid access

code. If no access code is entered, the memory is

Figure 1. A.C. Timing

VCC

tVCCS

CS

tCKH tCKL

tCSS

CLK

tDIS

DI

DO

tDIH

tPD

HIGH-Z

divided into a read-only area and a non-access area.

Figure 2 illlustrates this partitioning of the memory array.

WRITE PROTECTION

Another feature of the CAT35C704 is WRITE-protection

without the use of an access code. If the memory pointer

register is set to cover all or part of the memory, without

setting the access code register, the device may be

divided into an area which allows full access, and an

area which allows READ-only access. To write into the

READ-only area, the user can override the memory

pointer register for every WRITE instruction or he can

simply move the address in the memory pointer register

to uncover this area, and then write into the memory.

This mechanism prevents inadvertent overwriting of

important data in the memory without the use of an

access code. Figure 3 illustrates this partitioning of the

memory array.

tCSH

tHZ

tPD

HIGH-Z

5074 FHD F03

Figure 2. Secure Mode

ACCESS REGISTER:

ACCESS CODE LENGTH:

MEMORY POINTER:

ACCESS CODE (1–8 BYTES)

1 TO 8

a…a

POINTER

REGISTER

ADDRESS

IN MEMORY

READ-ONLY

ACCESS

PASSWORD-ONLY

ACCESS

255 (x16)

511 (x8)

a…a

0

5074 FHD F04

Doc. No. 25045-00 2/98

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