22.214.171.124. Half Data Rate Block
The Arria® V devices contain a half data rate (HDR) block in the postamble enable circuitry.
The HDR block is clocked by the half-rate resynchronization clock, which is the output of the I/O clock divider circuit. There is an AND gate after the postamble register outputs to avoid postamble glitches from a previous read burst on a non-consecutive read burst. This scheme allows half-a-clock cycle latency for dqsenable assertion and zero latency for dqsenable deassertion.
Using the HDR block as the first stage capture register in the postamble enable circuitry block is optional. Altera recommends using these registers if the controller is running at half the frequency of the I/Os.
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