220.127.116.11. DQS Postamble Circuitry
There are preamble and postamble specifications for both read and write operations in DDR3 and DDR2 SDRAM. The DQS postamble circuitry ensures that data is not lost if there is noise on the DQS line during the end of a read operation that occurs while DQS is in a postamble state.
The Cyclone® V devices contain dedicated postamble registers that you can control to ground the shifted DQS signal that is used to clock the DQ input registers at the end of a read operation. This function ensures that any glitches on the DQS input signal during the end of a read operation and occurring while DQS is in a postamble state do not affect the DQ IOE registers.
- For preamble state, the DQS is low, just after a high-impedance state.
- For postamble state, the DQS is low, just before it returns to a high-impedance state.
For external memory interfaces that use a bidirectional read strobe (DDR3 and DDR2 SDRAM), the DQS signal is low before going to or coming from a high-impedance state.
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