3.18. AFI 4.0 Specification
The AFI is a single-data-rate interface, meaning that data is transferred on the rising edge of each clock cycle. Most memory interfaces, however, operate at double-data-rate, transferring data on both the rising and falling edges of the clock signal. If the AFI interface is to directly control a double-data-rate signal, two single-data-rate bits must be transmitted on each clock cycle; the PHY then sends out one bit on the rising edge of the clock and one bit on the falling edge.
The AFI convention is to send the low part of the data first and the high part second, as shown in the following figure.
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