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AM29BDS128HD9 데이터시트(PDF) 20 Page - SPANSION |
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AM29BDS128HD9 데이터시트(HTML) 20 Page - SPANSION |
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20 / 89 page  18 Am29BDS128H/Am29BDS640H 27024B3 May 10, 2006 D A TA SH EE T Locking Bit. This will permanently set the part to op- erate only using Persistent Sector Protection. If the customer decides to use the password method, they must set the Password Mode Locking Bit. This will permanently set the part to operate only using pass- word sector protection. It is important to remember that setting either the Per- sistent Sector Protection Mode Locking Bit or the Password Mode Locking Bit permanently selects the protection mode. It is not possible to switch between the two methods once a locking bit has been set. It is important that one mode is explicitly selected when the device is first programmed, rather than relying on the default mode alone. This is so that it is not possible for a system program or virus to later set the Password Mode Locking Bit, which would cause an unexpected shift from the default Persistent Sector Protection Mode into the Password Protection Mode. The device is shipped with all sectors unprotected. AMD offers the option of programming and protecting sectors at the factory prior to shipping the device through AMD’s ExpressFlash™ Service. Contact an AMD representative for details. It is possible to determine whether a sector is pro- tected or unprotected. See “Autoselect Command Se- quence” section on page 36 for details. Persistent Sector Protection The Persistent Sector Protection method replaces the old 12 V controlled protection method while at the same time enhancing flexibility by providing three dif- ferent sector protection states: ■ Persistently Locked—A sector is protected and cannot be changed. ■ Dynamically Locked—The sector is protected and can be changed by a simple command ■ Unlocked—The sector is unprotected and can be changed by a simple command In order to achieve these states, three types of “bits” are going to be used: Persistent Protection Bit (PPB) A single Persistent (non-volatile) Protection Bit is as- signed to a maximum four sectors (“Am29BDS128H Boot Sector/Sector Block Addresses for Protec- tion/Unprotection” section on page 16). All 4 Kbyte boot-block sectors have individual sector Persistent Protection Bits (PPBs) for greater flexibility. Each PPB is individually modifiable through the PPB Program Command. Note: If a PPB requires erasure, all of the sector PPBs must first be preprogrammed prior to PPB erasing. All PPBs erase in parallel, unlike programming where in- dividual PPBs are programmable. It is the responsibil- ity of the user to perfor m the preprogramming operation. Otherwise, an already erased sector PPBs has the potential of being over-erased. There is no ha rdwa re mechan ism to preve n t se ct or PPBs over-erasure. Persistent Protection Bit Lock (PPB Lock) A global volatile bit. When set to “1”, the PPBs cannot be changed. When cleared (“0”), the PPBs are changeable. There is only one PPB Lock bit per de- vice. The PPB Lock is cleared after power-up or hard- ware reset. There is no command sequence to unlock the PPB Lock. Dynamic Protection Bit (DYB) A volatile protection bit is assigned for each sector. After power-up or hardware reset, the contents of all DYBs is “0”. Each DYB is individually modifiable through the DYB Write Command. When the par ts are first shipped, the PPBs are cleared. The DYBs and PPB Lock are defaulted to power up in the cleared state – meaning the PPBs are changeable. When the device is first powered on the DYBs power up cleared (sectors not protected). The Protection State for each sector is determined by the logical OR of the PPB and the DYB related to that sector. For the sectors that have the PPBs cleared, the DYBs control whether or not the sector is protected or unprotected. By issuing the DYB Write command sequences, the DYBs will be set or cleared, thus placing each sector in the protected or unprotected state. These are the so-called Dynamic Locked or Unlocked states. They are called dynamic states because it is very easy to switch back and forth between the protected and un- protected conditions. This allows software to easily protect sectors against inadvertent changes yet does not prevent the easy removal of protection when changes are needed. The DYBs maybe set or cleared as often as needed. The PPBs allow for a more static, and difficult to change, level of protection. The PPBs retain their state across power cycles because they are Non-Volatile. Individual PPBs are set with a command but must all be cleared as a group through a complex sequence of program and erasing commands. The PPBs are also limited to 100 erase cycles. The PBB Lock bit adds an additional level of protec- tion. Once all PPBs are programmed to the desired settings, the PPB Lock may be set to “1”. Setting the PPB Lock disables all program and erase commands to the Non-Volatile PPBs. In effect, the PPB Lock Bit locks the PPBs into their current state. The only way to clear the PPB Lock is to go through a power cycle. |
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