Capture DDIO to Read FIFO Path
The capture DDIO block captures input data (DQ) on the rising and falling edges of the capture clock or strobe (DQS).
For Stratix V devices, the capture DDIO block feeds the hard read FIFO or bypasses the hard read FIFO and goes directly to the core. If the capture DDIO block connects directly to the core, the data is transmitted at full rate. For protocols with bidirectional DQS, only an exact number of DQS edge is available for both capturing data in the capture DDIO block and then either in the read FIFO or in the core. The data transfer from the capture DDIO block and the next stage is referred to as zero-cycle transfer. This means that the transfer must happen on the same clock edge.
The hard read FIFO always changes the data rate from full-rate to half-rate, so if you choose to use full-rate, then you cannot use the Read FIFO.
For Arria V and Cyclone V devices, the capture DDIO block to Read FIFO path is similar, with the following exceptions:
- The read FIFO must always be used and cannot be bypassed.
- The read FIFO supports both half-rate and full-rate.
For Arria V, Cyclone V, and Stratix V devices, the hard read FIFO implements the functionality of a generic asynchronous FIFO. You can locate the hard read FIFO in a true dual-ported RAM. Data is written to the write side of the DQS clock domain and read from the read side of the core clock domain. For Arria V and Cyclone V devices, the core clock domain can run at half the frequency and implements a full-rate to half-rate transformation. You can use the write enable and read enable signals to control when to write and read data from the read FIFO. The same signal controls the increment of the write and read pointers. For protocols using a bidirectional strobe, the write enable signal is tied to VCC and DQS gating/ungating implements the write enable functionality.
For Arria V and Cyclone V devices, the hard data valid FIFO internally generates the write enable (gating/ungating) signal and DQS enable signal, while the hard latency FIFO internally generates the read enable signal.
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