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M93S66 Datasheet(PDF) 14 Page - STMicroelectronics

Part No. M93S66
Description  4Kbit, 2Kbit and 1Kbit 16-bit wide MICROWIRE Serial Access EEPROM with Block Protection
Download  34 Pages
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Maker  STMICROELECTRONICS [STMicroelectronics]
Homepage  http://www.st.com
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M93S66 Datasheet(HTML) 14 Page - STMicroelectronics

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M93S66, M93S56, M93S46
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WRITE PROTECTION AND THE PROTECTION REGISTER
The Protection Register on the M93Sx6 is used to
adjust the amount of memory that is to be write
protected. The write protected area extends from
the address given in the Protection Register, up to
the top address in the M93Sx6 device.
Two flag bits are used to indicate the Protection
Register status:
Protection Flag: this is used to enable/disable
protection of the write-protected area of the
M93Sx6 memory
OTP bit: when set, this disables access to the
Protection Register, and thus prevents any
further modifications to the value in the
Protection Register.
The lower-bound memory address is written to the
Protection Register using the Protection Register
Write (PRWRITE) instruction. It can be read using
the Protection Register Read (PRREAD) instruc-
tion.
The Protection Register Enable (PREN) instruc-
tion must be executed before any PRCLEAR,
PRWRITE or PRDS instruction, and with appropri-
ate levels applied to the Protection Enable (PRE)
and Write Enable (W) signals.
Write-access
to
the
Protection
Register
is
achieved by executing the following sequence:
Execute the Write Enable (WEN) instruction
Execute the Protection Register Enable
(PREN) instruction
Execute one PRWRITE, PRCLEAR or PRDS
instructions, to set a new boundary address in
the Protection Register, to clear the protection
address (to all 1s), or permanently to freeze
the value held in the Protection Register.
Protection Register Read
The Protection Register Read (PRREAD) instruc-
tion outputs, on Serial Data Output (Q), the con-
tent of the Protection Register, followed by the
Protection Flag bit. The Protection Enable (PRE)
signal must be driven High before and during the
instruction.
As with the Read Data from Memory (READ) in-
struction, a dummy 0 bit is output first. Since it is
not possible to distinguish between the Protection
Register being cleared (all 1s) or having been writ-
ten with all 1s, the user must check the Protection
Flag status (and not the Protection Register con-
tent) to ascertain the setting of the memory protec-
tion.
Protection Register Enable
The Protection Register Enable (PREN) instruc-
tion is used to authorize the use of instructions that
modify
the
Protection
Register
(PRWRITE,
PRCLEAR, PRDS). The Protection Register En-
able (PREN) instruction does not modify the Pro-
tection Flag bit value.
Note: A Write Enable (WEN) instruction must be
executed before the Protection Register Enable
(PREN) instruction. Both the Protection Enable
(PRE) and Write Enable (W) signals must be driv-
en High during the instruction execution.
Protection Register Clear
The Protection Register Clear (PRCLEAR) in-
struction clears the address stored in the Protec-
tion Register to all 1s, so that none of the memory
is write-protected by the Protection Register. How-
ever, it should be noted that all the memory re-
mains protected, in the normal way, using the
Write Enable (WEN) and Write Disable (WDS) in-
structions.
The Protection Register Clear (PRCLEAR) in-
struction clears the Protection Flag to 1. Both the
Protection Enable (PRE) and Write Enable (W)
signals must be driven High during the instruction
execution.
Note: A Protection Register Enable (PREN) in-
struction must immediately precede the Protection
Register Clear (PRCLEAR) instruction.
Protection Register Write
The Protection Register Write (PRWRITE) instruc-
tion is used to write an address into the Protection
Register. This is the address of the first word to be
protected. After the Protection Register Write
(PRWRITE) instruction has been executed, all
memory locations equal to and above the speci-
fied address are protected from writing.
The Protection Flag bit is set to 0, and can be read
with Protection Register Read (PRREAD) instruc-
tion. Both the Protection Enable (PRE) and Write
Enable (W) signals must be driven High during the
instruction execution.
Note: A Protection Register Enable (PREN) in-
struction must immediately precede the Protection
Register Write (PRWRITE) instruction, but it is not
necessary to execute first a Protection Register
Clear (PRCLEAR).
Protection Register Disable
The Protection Register Disable (PRDS) instruc-
tion sets the One Time Programmable (OTP) bit.
This instruction is a ONE TIME ONLY instruction
which latches the Protection Register content, this
content is therefore unalterable in the future. Both
the Protection Enable (PRE) and Write Enable (W)
signals must be driven High during the instruction
execution. The OTP bit cannot be directly read, it
can be checked by reading the content of the Pro-
tection Register, using the Protection Register
Read (PRREAD) instruction, then by writing this
same value back into the Protection Register, us-


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