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M12L128168A Datasheet(PDF) 12 Page - Elite Semiconductor Memory Technology Inc. |
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M12L128168A Datasheet(HTML) 12 Page - Elite Semiconductor Memory Technology Inc. |
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12 / 22 page  ESMT M12L128168A Operation temperature condition -40 °C ~85°C Elite Semiconductor Memory Technology Inc. Publication Date: Oct. 2007 Revision: 1.2 12/43 DEVICE OPERATIONS (Continued) AUTO REFRESH The storage cells of SDRAM need to be refreshed every 64ms to maintain data. An auto refresh cycle accomplishes refresh of a single row of storage cells. The internal counter increments automatically on every auto refresh cycle to refresh all the rows. An auto refresh command is issued by asserting low on CS , RAS and CAS with high on CKE and WE . The auto refresh command can only be asserted with all banks being in idle state and the device is not in power down mode (CKE is high in the previous cycle). The time required to complete the auto refresh operation is specified by tRFC(min). The minimum number of clock cycles required can be calculated by driving tRFC with clock cycle time and them rounding up to the next higher integer. The auto refresh command must be followed by NOP’s until the auto refresh operation is completed. The auto refresh is the preferred refresh mode when the SDRAM is being used for normal data transactions. The auto refresh cycle can be performed once in 15.6us. SELF REFRESH The self refresh is another refresh mode available in the SDRAM. The self refresh is the preferred refresh mode for data retention and low power operation of SDRAM. In self refresh mode, the SDRAM disables the internal clock and all the input buffers except CKE. The refresh addressing and timing is internally generated to reduce power consumption. The self refresh mode is entered from all banks idle state by asserting low on CS , RAS , CAS and CKE with high on WE . Once the self refresh mode is entered, only CKE state being low matters, all the other inputs including clock are ignored to remain in the refresh. The self refresh is exited by restarting the external clock and then asserting high on CKE. This must be followed by NOP’s for a minimum time of tRFC before the SDRAM reaches idle state to begin normal operation. It is recommended to use burst 4096 auto refresh cycles immediately before and after self refresh. |
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