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W3E16M72S-250BI Datasheet(PDF) 9 Page - White Electronic Designs Corporation |
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W3E16M72S-250BI Datasheet(HTML) 9 Page - White Electronic Designs Corporation |
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9 / 17 page  9 White Electronic Designs Corporation • (602) 437-1520 • www.wedc.com White Electronic Designs W3E16M72S-XBX February 2005 Rev. 7 allowed as long as it does not interrupt the data transfer in the current bank and does not violate any other timing parameters. Input A10 determines whether one or all banks are to be precharged, and in the case where only one bank is to be precharged, inputs BA0, BA1 select the bank. Otherwise BA0, BA1 are treated as “Don’t Care.” Once a bank has been precharged, it is in the idle state and must be activated prior to any READ or WRITE commands being issued to that bank. A PRECHARGE command will be treated as a NOP if there is no open row in that bank (idle state), or if the previously open row is already in the process of precharging. AUTO PRECHARGE AUTO PRECHARGE is a feature which performs the same individual-bank PRECHARGE function described above, but without requiring an explicit command. This is accomplished by using A10 to enable AUTO PRECHARGE in conjunction with a specific READ or WRITE command. A precharge of the bank/row that is addressed with the READ or WRITE command is automatically performed upon completion of the READ or WRITE burst. AUTO PRECHARGE is nonpersistent in that it is either enabled or disabled for each individual READ or WRITE command. The device supports concurrent auto precharge if the command to the other bank does not interrupt the data transfer to the current bank. AUTO PRECHARGE ensures that the precharge is initiated at the earliest valid stage within a burst. This “earliest valid stage” is determined as if an explicit precharge command was issued at the earliest possible time, without violating tRAS (MIN).The user must not issue another command to the same bank until the precharge time (tRP) is completed. This is determined as if an explicit PRECHARGE command was issued at the earliest possible time, without violating tRAS (MIN). BURST TERMINATE The BURST TERMINATE command is used to truncate READ bursts (with auto precharge disabled). The most recently registered READ command prior to the BURST TERMINATE command will be truncated. The open page which the READ burst was terminated from remains open. AUTO REFRESH AUTO REFRESH is used during normal operation of the DDR SDRAM and is analogous to CAS#-BEFORE-RAS# (CBR) REFRESH in conventional DRAMs. This command is nonpersistent, so it must be issued each time a refresh is required. The addressing is generated by the internal refresh controller. This makes the address bits “Don’t Care” during an AUTO REFRESH command. Each DDR SDRAM requires AUTO REFRESH cycles at an average interval of 7.8125 µs (maximum). To allow for improved efficiency in scheduling and switching between tasks, some flexibility in the absolute refresh interval is provided. A maximum of eight AUTO REFRESH commands can be posted to any given DDR SDRAM, meaning that the maximum absolute interval between any AUTO REFRESH command and the next AUTO REFRESH command is 9 x 7.8125 µs (70.3µs). This maximum absolute interval is to allow future support for DLL updates internal to the DDR SDRAM to be restricted to AUTO REFRESH cycles, without allowing excessive drift in tAC between updates. Although not a JEDEC requirement, to provide for future functionality features, CKE must be active (High) during the AUTO REFRESH period. The AUTO REFRESH period begins when the AUTO REFRESH command is registered and ends tRFC later. SELF REFRESH* The SELF REFRESH command can be used to retain data in the DDR SDRAM, even if the rest of the system is powered down. When in the self refresh mode, the DDR SDRAM retains data without external clocking. The SELF REFRESH command is initiated like an AUTO REFRESH command except CKE is disabled (LOW). The DLL is automatically disabled upon entering SELF REFRESH and is automatically enabled upon exiting SELF REFRESH (200 clock cycles must then occur before a READ command can be issued). Input signals except CKE are “Don’t Care” during SELF REFRESH. The procedure for exiting self refresh requires a sequence of commands. First, CLK must be stable prior to CKE go- ing back HIGH. Once CKE is HIGH, the DDR SDRAM must have NOP commands issued for tXSNR, because time is required for the completion of any internal refresh in progress. A simple algorithm for meeting both refresh and DLL re- quirements is to apply NOPs for 200 clock cycles before applying any other command. * Self refresh available in commercial and industrial temperatures only. |
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