S29AS008J datasheet(18/56 Pages) CYPRESS

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Document Number: 002-00870 Rev.*K

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S29AS008J

7.14

Hardware Data Protection

The command sequence requirement of unlock cycles for programming or erasing provides data protection against inadvertent

writes (refer to Table on page 30 for command definitions). In addition, the following hardware data protection measures prevent

accidental erasure or programming, which might otherwise be caused by spurious system level signals during VCC power-up and

power-down transitions, or from system noise.

7.14.1

Low VCC Write Inhibit

When VCC is less than VLKO, the device does not accept any write cycles. This protects data during VCC power-up and power-down.

The command register and all internal program/erase circuits are disabled, and the device resets. Subsequent writes are ignored

until VCC is greater than VLKO. The system must provide the proper signals to the control pins to prevent unintentional writes when

VCC is greater than VLKO.

7.14.2

Write Pulse Glitch Protection

Noise pulses of less than 5 ns (typical) on OE#, CE# or WE# do not initiate a write cycle.

7.14.3

Logical Inhibit

Write cycles are inhibited by holding any one of OE# = VIL, CE# = VIH or WE# = VIH. To initiate a write cycle, CE# and WE# must be

a logical zero (VIL) while OE# is a logical one (VIH).

7.14.4

Power-Up Write Inhibit

If WE# = CE# = VIL and OE# = VIH during power up, the device does not accept commands on the rising edge of WE#. The internal

state machine is automatically reset to reading array data on power-up.

Secured Silicon Sector Flash Memory Region

The Secured Silicon Sector feature provides a 256-byte Flash memory region that enables permanent part identification through an

Electronic Serial Number (ESN). The Secured Silicon Sector uses a Secured Silicon Sector Indicator Bit (DQ7) to indicate whether

or not the Secured Silicon Sector is locked when shipped from the factory. This bit is permanently set at the factory and cannot be

changed, which prevents cloning of a factory-locked part. This ensures the security of the ESN once the product is shipped to the

field.

Cypress offers the device with the Secured Silicon Sector either factory-locked or customer-lockable. The factory-locked version is

always protected when shipped from the factory, and has the Secured Silicon Sector Indicator Bit permanently set to a 1. The

customer-lockable version is shipped with the Secured Silicon Sector group unprotected, allowing customers to utilize the that sector

in any manner they choose. The customer-lockable version has the Secured Silicon Sector Indicator Bit permanently set to a 0.

Thus, the Secured Silicon Sector Indicator Bit prevents customer-lockable devices from being used to replace devices that are

factory locked.

The system accesses the Secured Silicon Sector through a command sequence (see Enter Secured Silicon Sector/Exit Secured

Silicon Sector Command Sequence on page 26). After the system writes the Enter Secured Silicon Sector command sequence, it

may read the Secured Silicon Sector by using the addresses normally occupied by the boot sectors. This mode of operation

continues until the system issues the Exit Secured Silicon Sector command sequence, or until power is removed from the device. On

power-up, or following a hardware reset, the device reverts to sending commands to the boot sectors.

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