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IS42S16800A1 Datasheet(PDF) 5 Page - Integrated Silicon Solution, Inc |
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IS42S16800A1 Datasheet(HTML) 5 Page - Integrated Silicon Solution, Inc |
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5 / 63 page  Integrated Silicon Solution, Inc. — www.issi.com — 1-800-379-4774 5 Rev. 00B 0 5/01/06 ISSI® IS42S16800A1 Power On and Initialization The default power on state of the mode register is supplier specific and may be undefined. The following power on and initializa- tion sequence guarantees the device is preconditioned to each users specific needs. Like a conventional DRAM, the Synchronous DRAM must be powered up and initialized in a predefined manner. During power on, all VDD and VDDQ pins must be built up simultaneously to the specified voltage when the input signals are held in the “NOP” state. The power on voltage must not exceed VDD+0.3V on any of the input pins or VDD supplies. The CK signal must be started at the same time. After power on, an initial pause of 200µs is required followed by a precharge of all banks using the precharge command. To prevent data contention on the DQ bus during power on, it is required that the DQM and CKE pins be held high during the initial pause period. Once all banks have been precharged, the Mode Register Set Command must be issued to ini- tialize the Mode Register. A minimum of two Auto Refresh cycles (CBR) are also required. These may be done before or after programming the Mode Register. Failure to follow these steps may lead to unpredictable start-up modes. Programming the Mode Register For application flexibility, CAS latency, burst length, burst sequence, and operation type are user defined variables and must be programmed into the SDRAM Mode Register with a single Mode Register Set Command. Any content of the Mode Register can be altered by re-executing the Mode Register Set Command. If the user chooses to modify only a subset of the Mode Register variables, all four variables must be redefined when the Mode Register Set Command is issued. After initial power up, the Mode Register Set Command must be issued before read or write cycles may begin. All banks must be in a precharged state and CKE must be high at least one cycle before the Mode Register Set Command can be issued. The Mode Register Set Command is activated by the low signals of RAS, CAS, CS, and WE at the positive edge of the clock. The address input data during this cycle defines the parameters to be set as shown in the Mode Register Operation table. A new command may be issued following the mode register set command once a delay equal to tRSC has elapsed. CAS Latency The CAS latency is a parameter that is used to define the delay from when a Read Command is registered on a rising clock edge to when the data from that Read Command becomes available at the outputs. The CAS latency is expressed in terms of clock cycles and can have a value of 2 or 3 cycles. The value of the CAS latency is determined by the speed grade of the device and the clock frequency that is used in the application. A table showing the relationship between the CAS latency, speed grade, and clock frequency appears in the Electrical Characteristics section of this document. Once the appropriate CAS latency has been selected it must be programmed into the mode register after power up, for an explanation of this procedure see Programming the Mode Register in the previous section. |
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