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Ixiasoft
33.7.3. Estimating the Valid Signal Window
Calculating the window is a two-step process. First, determine by how much time the SDRAM clock can lag the controller clock, and then by how much time it can lead. After finding the maximum lag and lead values, calculate the midpoint between them.
These calculations provide an estimation only. The following delays can also affect proper PLL tuning, but are not accounted for by these calculations.
- Signal skew due to delays on the printed circuit board — These calculations assume zero skew.
- Delay from the PLL clock output nodes to destinations — These calculations assume that the delay from the PLL SDRAM-clock output-node to the pin is the same as the delay from the PLL controller-clock output-node to the clock inputs in the SDRAM controller. If these clock delays are significantly different, you must account for this phase shift in your window calculations.
Lag is a negative time shift, relative to the controller clock, and lead is a positive time shift. The SDRAM clock can lag the controller clock by the lesser of the maximum lag for a read cycle or that for a write cycle. In other words, Maximum Lag = minimum(Read Lag, Write Lag). Similarly, the SDRAM clock can lead by the lesser of the maximum lead for a read cycle or for a write cycle. In other words, Maximum Lead = minimum(Read Lead, Write Lead).