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Moteur de recherche de fiches techniques de composants électroniques |
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Moteur de recherche de fiches techniques de composants électroniques |
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AM29BDS128H Fiches technique(PDF) 21 Page - Advanced Micro Devices |
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AM29BDS128H Fiches technique(HTML) 21 Page - Advanced Micro Devices |
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21 / 89 page  May 10, 2006 27024B3 Am29BDS128H/Am29BDS640H 19 D A TA SH EE T System boot code can determine if any changes to the PPB are needed e.g. to allow new system code to be downloaded. If no changes are needed then the boot code can set the PPB Lock to disable any further changes to the PPBs during system operation. The WP# write protect pin adds a final level of hard- ware protection to the four highest and four lowest 4 Kbyte sectors. When this pin is low it is not possible to change the contents of these four sectors. These sec- tors generally hold system boot code. So, the WP# pin can prevent any changes to the boot code that could override the choices made while setting up sector pro- tection during system initialization. It is possible to have sectors that have been persis- tently locked, and sectors that are left in the dynamic state. The sectors in the dynamic state are all unpro- tected. If there is a need to protect some of them, a simple DYB Write command sequence is all that is necessary. The DYB write command for the dynamic sectors switch the DYBs to signify protected and un- protected, respectively. If there is a need to change the status of the persistently locked sectors, a few more steps are required. First, the PPB Lock bit must be dis- abled by either putting the device through a power-cy- cle, or hardware reset. The PPBs can then be changed to reflect the desired settings. Setting the PPB lock bit once again will lock the PPBs, and the de- vice operates normally again. Note: to achieve the best protection, it’s recommended to execute the PPB lock bit set command early in the boot code, and protect the boot code by holding WP# = VIL. Table 6. Sector Protection Schemes Table 6 contains all possible combinations of the DYB, PPB, and PPB lock relating to the status of the sector. In summary, if the PPB is set, and the PPB lock is set, the sector is protected and the protection can not be removed until the next power cycle clears the PPB lock. If the PPB is cleared, the sector can be dynami- cally locked or unlocked. The DYB then controls whether or not the sector is protected or unprotected. If the user attempts to program or erase a protected sector, the device ignores the command and returns to read mode. A program command to a protected sector enables status polling for approximately tPSP before the device returns to read mode without having modi- fied the contents of the protected sector. An erase command to a protected sector enables status polling for approximately tSEA after which the device returns to read mode without having erased the protected sec- tor. The programming of the DYB, PPB, and PPB lock for a giv e n s e cto r ca n be v e r i fied b y wr itin g a DYB/PPB/PPB lock verify command to the device. Persistent Sector Protection Mode Locking Bit Like the password mode locking bit, a Persistent Sec- tor Protection mode locking bit exists to guarantee that the device remain in software sector protection. Once set, the Persistent Sector Protection locking bit pre- vents programming of the password protection mode locking bit. This guarantees that a hacker could not place the device in password protection mode. Password Protection Mode The Password Sector Protection Mode method allows an even higher level of security than the Persistent Sector Protection Mode. There are two main differ- ences between the Persistent Sector Protection and the Password Sector Protection Mode: ■ When the device is first powered on, or comes out of a reset cycle, the PPB Lock bit is set to the locked state, rather than cleared to the unlocked state. ■ The only means to clear the PPB Lock bit is by writ- ing a unique 64-bit Password to the device. The Password Sector Protection method is otherwise identical to the Persistent Sector Protection method. A 64-bit password is the only additional tool utilized in this method. The password is stored in a one-time programmable (OTP) region of the flash memory. Once the Password Mode Locking Bit is set, the password is permanently set with no means to read, program, or erase it. The password is used to clear the PPB Lock bit. The Pass- word Unlock command must be written to the flash, along with a password. The flash device internally compares the given password with the pre-pro- DYB PPB PPB Lock Sector State 000 Unprotected—PPB and DYB are changeable 001 Unprotected—PPB not changeable, DYB is changeable 010 Protected—PPB and DYB are changeable 100 110 011 Protected—PPB not changeable, DYB is changeable 101 111 |
Numéro de pièce similaire - AM29BDS128H |
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Description similaire - AM29BDS128H |
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