External Memory Interfaces Intel® Stratix® 10 FPGA IP User Guide

ID 683741
Date 3/11/2022
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4.2.6. AFI Shadow Register Management Signals

Shadow registers are a feature that enables high-speed multi-rank support. Shadow registers allow the sequencer to calibrate each rank separately, and save the calibrated settings—such as deskew delay-chain configurations—of each rank in its own set of shadow registers.

During a rank-to-rank switch, the correct set of calibrated settings is restored just in time to optimize the data valid window. The PHY relies on additional AFI signals to control which set of shadow registers to activate.

Table 168.  Shadow Register Management Signals

Signal Name

Direction

Width

Description

afi_wrank

Input

AFI_WRANK_WIDTH

Signal from controller specifying which rank the write data is going to. The signal timing is identical to that of afi_dqs_burst. That is, afi_wrank must be asserted at the same time and must last the same duration as the afi_dqs_burst signal.

afi_rrank

Output

AFI_RRANK_WIDTH

Signal from controller specifying which rank is being read. The signal must be asserted at the same time as the afi_rdata_en signal when issuing a read command, but unlike afi_rdata_en, afi_rrank is stateful. That is, once asserted, the signal value must remain unchanged until the controller issues a new read command to a different rank.

Both the afi_wrank and afi_rrank signals encode the rank being accessed using the one-hot scheme (e.g. in a quad-rank interface, 0001, 0010, 0100, 1000 refer to the 1st, 2nd, 3rd, 4th rank respectively). The ordering within the bus is the same as other AFI signals. Specifically the bus is ordered by time slots, for example:

Half-rate afi_w/rrank = {T1, T0}
Quarter-rate afi_w/rrank = {T3, T2, T1, T0}

Where Tx is a number of rank-bit words that one-hot encodes the rank being accessed at the y th full-rate cycle.

Additional Requirements for Shadow Register Support

To ensure that the hardware has enough time to switch from one shadow register to another, the controller must satisfy the following minimum rank-to-rank-switch delays (tRTRS):

  • Two read commands going to different ranks must be separated by a minimum of 3 full-rate cycles (in addition to the burst length delay needed to avoid collision of data bursts).
  • Two write commands going to different rank must be separated by a minimum of 4 full-rate cycles (in addition to the burst length delay needed to avoid collision of data bursts).

The FPGA device supports a maximum of 4 sets of shadow registers, each for an independent set of timings. More than 4 ranks are supported if those ranks have four or fewer sets of independent timing. For example, the rank multiplication mode of an LRDIMM allows more than one physical rank to share a set of timing data as a single logical rank. Therefore the device can support up to 4 logical ranks, though that means more than 4 physical ranks.

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