1.4. Address and Command Datapath
The PHY is indifferent to address or command—that is, it performs no decoding or other operations—and the circuitry is the same for both. In full-rate and half-rate interfaces, address and command is full rate, while in quarter-rate interfaces, address and command is half rate.
Address and command signals are generated in the Altera PHY interface (AFI) clock domain and sent to the memory device in the address and command clock domain. The double-data rate input/output (DDIO) stage converts the half-rate signals into full-rate signals, when the AFI clock runs at half-rate. For quarter-rate interfaces, additional DDIO stages exist to convert the address and command signals in the quarter-rate AFI clock domain to half-rate.
The address and command clock is offset with respect to the memory clock to balance the nominal setup and hold margins at the memory device (center-alignment). In the example below, this offset is 270 degrees. The Fitter can further optimize margins based on the actual delays and clock skews. In half-rate and quarter-rate designs, the full-rate cycle shifter blocks can perform a shift measured in full-rate cycles to implement the correct write latency; without this logic, the controller would only be able to implement even write latencies as it operates at half the speed. The full-rate cycle shifter is clocked by either the AFI clock or the address and command clock, depending on the PHY configuration, to maximize timing margins on the path from the AFI clock to the address and command clock.
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