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Ixiasoft
6.2.3.6. Clock Signals
The memory uses these clock signals to generate the DQS signal during a read through the DLL inside the memory. The SDRAM data sheet specifies the following timings:
- tDQSCK is the skew between the CK_T or CK_C signals and the SDRAM-generated DQS signal.
- tDSH is the DQS falling edge from CK_T rising edge hold time.
- tDSS is the DQS falling edge from CK_T rising edge setup time.
- tDQSS is the positive DQS latching edge to CK_T rising edge.
SDRAM devices have a write requirement (tDQSS) that states the positive edge of the DQS signal on writes must be within ± 25% (± 90°) of the positive edge of the SDRAM clock input. Therefore, you should generate the CK_Tand CK_C signals using the DDR registers in the IOE to match with the DQS signal and reduce any variations across process, voltage, and temperature. The positive edge of the SDRAM clock, CK_T, is aligned with the DQS write to satisfy tDQSS.