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M12L128168A Datasheet(PDF) 11 Page - Elite Semiconductor Memory Technology Inc. |
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M12L128168A Datasheet(HTML) 11 Page - Elite Semiconductor Memory Technology Inc. |
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11 / 43 page  ESMT M12L128168A Elite Semiconductor Memory Technology Inc. Publication Date: Oct. 2006 Revision: 2.0 11/43 DEVICE OPERATIONS (Continued) dividing tRCD(min) with cycle time of the clock and then rounding of the result to the next higher integer. The SDRAM has four internal banks in the same chip and shares part of the internal circuitry to reduce chip area, therefore it restricts the activation of four banks simultaneously. Also the noise generated during sensing of each bank of SDRAM is high requiring some time for power supplies to recover before another bank can be sensed reliably. tRRD(min) specifies the minimum time required between activating different bank. The number of clock cycles required between different bank activation must be calculated similar to tRCD specification. The minimum time required for the bank to be active to initiate sensing and restoring the complete row of dynamic cells is determined by tRAS(min). Every SDRAM bank activate command must satisfy tRAS(min) specification before a precharge command to that active bank can be asserted. The maximum time any bank can be in the active state is determined by tRAS (max) and tRAS(max) can be calculated similar to tRCD specification. BURST READ The burst read command is used to access burst of data on consecutive clock cycles from an active row in an active bank. The burst read command is issued by asserting low on CS and RAS with WE being high on the positive edge of the clock. The bank must be active for at least tRCD(min) before the burst read command is issued. The first output appears in CAS latency number of clock cycles after the issue of burst read command. The burst length, burst sequence and latency from the burst read command is determined by the mode register which is already programmed. The burst read can be initiated on any column address of the active row. The address wraps around if the initial address does not start from a boundary such that number of outputs from each I/O are equal to the burst length programmed in the mode register. The output goes into high-impedance at the end of burst, unless a new burst read was initiated to keep the data output gapless. The burst read can be terminated by issuing another burst read or burst write in the same bank or the other active bank or a precharge command to the same bank. The burst stop command is valid at every page burst length. BURST WRITE The burst write command is similar to burst read command and is used to write data into the SDRAM on consecutive clock cycles in adjacent addresses depending on burst length and burst sequence. By asserting low on CS , CAS and WE with valid column address, a write burst is initiated. The data inputs are provided for the initial address in the same clock cycle as the burst write command. The input buffer is deselected at the end of the burst length, even though the internal writing can be completed yet. The writing can be complete by issuing a burst read and DQM for blocking data inputs or burst write in the same or another active bank. The burst stop command is valid at every burst length. The write burst can also be terminated by using DQM for blocking data and precharge the bank tRDL after the last data input to be written into the active row. See DQM OPERATION also. DQM OPERATION The DQM is used mask input and output operations. It works similar to OE during operation and inhibits writing during write operation. The read latency is two cycles from DQM and zero cycle for write, which means DQM masking occurs two cycles later in read cycle and occurs in the same cycle during write cycle. DQM operation is synchronous with the clock. The DQM signal is important during burst interrupts of write with read or precharge in the SDRAM. Due to asynchronous nature of the internal write, the DQM operation is critical to avoid unwanted or incomplete writes when the complete burst write is required. Please refer to DQM timing diagram also. PRECHARGE The precharge is performed on an active bank by asserting low on clock cycles required between bank activate and clock cycles required between bank activate and CS , RAS , WE and A10/AP with valid A13~A12 of the bank to be procharged. The precharge command can be asserted anytime after tRAS(min) is satisfy from the bank active command in the desired bank. tRP is defined as the minimum number of clock cycles required to complete row precharge is calculated by dividing tRP with clock cycle time and rounding up to the next higher integer. Care should be taken to make sure that burst write is completed or DQM is used to inhibit writing before precharge command is asserted. The maximum time any bank can be active is specified by tRAS(max). Therefore, each bank has to be precharge with tRAS(max) from the bank activate command. At the end of precharge, the bank enters the idle state and is ready to be activated again. Entry to power-down, Auto refresh, Self refresh and Mode register set etc. is possible only when all banks are in idle state. AUTO PRECHARGE The precharge operation can also be performed by using auto precharge. The SDRAM internally generates the timing to satisfy tRAS(min) and “tRP” for the programmed burst length and CAS latency. The auto precharge command is issued at the same time as burst write by asserting high on A10/AP, the bank is precharge command is asserted. Once auto precharge command is given, no new commands are possible to that particular bank until the bank achieves idle state. FOUR BANKS PRECHARGE Four banks can be precharged at the same time by using Precharge all command. Asserting low on CS , RAS , and WE with high on A10/AP after all banks have satisfied tRAS(min) requirement, performs precharge on all banks. At the end of tRP after performing precharge all, all banks are in idle state. |
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