External Memory Interface Handbook Volume 3: Reference Material

ID 683841
Date 7/24/2019
Document Table of Contents

1.5. Write Datapath

The write datapath passes write data from the memory controller to the I/O. The write data valid signal from the memory controller generates the output enable signal to control the output buffer. For memory protocols with a bidirectional data bus, it also generates the dynamic termination control signal, which selects between series (output mode) and parallel (input mode) termination.

The figure below illustrates a simplified write datapath of a typical half-rate interface. The full-rate DQS write clock is sent to a DDIO_OUT cell. The output of DDIO_OUT feeds an output buffer which creates a pair of pseudo differential clocks that connects to the memory. In full-rate mode, only the SDR-DDR portion of the path is used; in half-rate mode, the HDR-SDR circuitry is also required. The use of DDIO_OUT in both the output strobe and output data generation path ensures that their timing characteristics are as similar as possible. The <variation_name>_pin_assignments.tcl script automatically specifies the logic option that associates all data pins to the output strobe pin. The Fitter treats the pins as a DQS/DQ pin group.

Figure 3. Write Datapath

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