M29W400DT datasheet(41/48 Pages) STMICROELECTRONICS

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M29W400DT, M29W400DB

Block protection

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Appendix B

Block protection

Block protection can be used to prevent any operation from modifying the data stored in the

Flash. Each block can be protected individually. Once protected, Program and Erase

operations on the block fail to change the data.

There are three techniques that can be used to control block protection, these are the

programmer technique, the in-system technique and temporary unprotection. temporary

unprotection is controlled by the Reset/Block Temporary Unprotection pin, RP; this is

described in the Section 2: Signal descriptions.

Unlike the command interface of the Program/Erase controller, the techniques for protecting

and unprotecting blocks change between different Flash memory suppliers. For example,

the techniques for AMD parts will not work on STMicroelectronics parts. Care should be

taken when changing drivers for one part to work on another.

B.1

Programmer technique

The programmer technique uses high (VID) voltage levels on some of the bus pins. These

cannot be achieved using a standard microprocessor bus, therefore the technique is

recommended only for use in programming equipment.

To protect a block follow the flowchart in Figure 19: Programmer equipment block protect

flowchart. To unprotect the whole chip it is necessary to protect all of the blocks first, then all

blocks can be unprotected at the same time. To unprotect the chip follow Figure 20:

Programmer equipment chip unprotect flowchart. Table 23: Programmer technique bus

operations, BYTE = VIH or VIL, gives a summary of each operation.

The timing on these flowcharts is critical. Care should be taken to ensure that, where a

pause is specified, it is followed as closely as possible. Do not abort the procedure before

reaching the end. Chip Unprotect can take several seconds and a user message should be

provided to show that the operation is progressing.

B.2

In-system technique

The in-system technique requires a high voltage level on the Reset/Blocks Temporary

Unprotect pin, RP. This can be achieved without violating the maximum ratings of the

components on the microprocessor bus, therefore this technique is suitable for use after the

Flash has been fitted to the system.

To protect a block follow the flowchart in Figure 21: In-system equipment block protect

flowchart. To unprotect the whole chip it is necessary to protect all of the blocks first, then all

the blocks can be unprotected at the same time. To unprotect the chip follow Figure 22: In-

system equipment chip unprotect flowchart.

The timing on these flowcharts is critical. Care should be taken to ensure that, where a

pause is specified, it is followed as closely as possible. Do not allow the microprocessor to

service interrupts that will upset the timing and do not abort the procedure before reaching

the end. Chip Unprotect can take several seconds and a user message should be provided

to show that the operation is progressing.

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