CAT33C704 Datasheet, PDF (9/14 Pages) Catalyst Semiconductor – 4K-Bit Secure Access Serial E2PROM

Preliminary
CAT33C704
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
EWEN
WRITE
[access code]
[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
EWEN
MACC
[old access code]
[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 CAT33C704 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
EWEN
OVMPR
WRITE
[access code]
[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
EWEN
WMPR
WRITE
[access code]
[address]
[address ][data ]
Figure 11. ERAL Timing
CS
CLK
DI
OP7
DO
OP CODE
OP0 OP7
HIGH-Z
OP CODE
DON’T CARE
tEW
tSV
OP0
BUSY
READY
5074 FHD F14
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