External Memory Interface Handbook Volume 2: Design Guidelines

ID 683385
Date 5/08/2017
Document Table of Contents

1.7. DLL

The Intel® FPGA memory interface IP uses one DLL. The DLL is located at the corner of the device and can send the control signals to shift the DQS pins on its adjacent sides for Stratix-series devices, or DQS pins in any I/O banks in Arria II GX devices.

For example, the top-left DLL can shift DQS pins on the top side and left side of the device. The DLL generates the same phase shift resolution for both sides, but can generate different phase offset to the two different sides, if needed. Each DQS pin can be configured to use or ignore the phase offset generated by the DLL.

The DLL cannot generate two different phase offsets to the same side of the device. However, you can use two different DLLs to for this functionality.

DLL reference clocks must come from either dedicated clock input pins located on either side of the DLL or from specific PLL output clocks. Any clock running at the memory frequency is valid for the DLLs.

To minimize the number of clocks routed directly on the PCB, typically this reference clock is sourced from the memory controllers PLL. In general, DLLs can use the PLLs directly adjacent to them (corner PLLs when available) or the closest PLL located in the two sides adjacent to its location.

Note: By default, the DLL reference clock in Intel® FPGA external memory IP is from a PLL output.

When designing for 780-pin packages with EP3SE80, EP3SE110, EP3SL150, EP4SE230, EP4SE360, EP4SGX180, and EP4SGX230 devices, the PLL to DLL reference clock connection is limited. DLL2 is isolated from a direct PLL connection and can only receive a reference clock externally from pins CLK[11:4]p in EP3SE80, EP3SE110, EP3SL150, EP4SE230, and EP4SE360 devices. In EP4SGX180 and EP4SGX230 devices, DLL2 and DLL3 are not directly connected to PLL. DLL2 and DLL3 receive a reference clock externally from pins CLK[7:4]p and CLK[15:12]p respectively.

For more DLL information, refer to the respective device handbooks.

The DLL reference clock should be the same frequency as the memory interface, but the phase is not important.

The required DQS capture phase is optimally chosen based on operating frequency and external memory interface type (DDR, DDR2, DDR3 SDRAM, and QDR II SRAM, or RLDRAM II). As each DLL supports two possible phase offsets, two different memory interface types operating at the same frequency can easily share a single DLL. More may be possible, depending on the phase shift required.

Intel® FPGA memory IP always specifies a default optimal phase setting, to override this setting, refer to Implementing and Parameterizing Memory IP .

When sharing DLLs, your memory interfaces must be of the same frequency. If the required phase shift is different amongst the multiple memory interfaces, you can use a different delay chain in the DQS logic block or use the DLL phase offset feature.

To simplify the interface to IP connections, multiple memory interfaces operating at the same frequency usually share the same system and static clocks as each other where possible. This sharing minimizes the number of dedicated clock nets required and reduces the number of different clock domains found within the same design.

As each DLL can directly drive four banks, but each PLL only has complete C (output) counter coverage of two banks (using dual regional networks), situations can occur where a second PLL operating at the same frequency is required. As cascaded PLLs increase jitter and reduce timing margin, you are advised to first ascertain if an alternative second DLL and PLL combination is not available and more optimal.

Select a DLL that is available for the side of the device where the memory interface resides. If you select a PLL or a PLL input clock reference pin that can also serve as the DLL input reference clock, you do not need an extra input pin for the DLL input reference clock.

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