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AR# 50752

MIG 7 Series RLDRAM II - Combinatorial path can fail timing when using larger components at high speed


Version Found: v1.4
Version Resolved and other Known Issues: See (Xilinx Answer 45195).

The following timing failure can occur when targeting larger components (e.g., "mt49h16m36xx-18") with 72-bit data widths running at speeds greater than 450 MHz.

Slack (setup path): -0.728ns (requirement - (data path - clock path skew + uncertainty))

Source: u_mig_7series_v1_4/c5_u_rld_memc_ui_top_std/u_rld_phy_top/u_qdr_rld_phy_read_top/nd_io_inst[2].u_qdr_rld_phy_read_data_align/rd_ptr_0 (FF)

Destination:u_mig_7series_v1_4/c5_u_rld_memc_ui_top_std/u_rld_phy_top/u_qdr_rld_phy_read_top/u_qdr_rld_phy_read_stage2_cal/mem_latency_31 (FF)

Requirement: 4.000ns

Data Path Delay:4.507ns (Levels of Logic = 5)


This is a result of a combinatorial path within the read data alignment logic that has a high number of fan-outs and route delays.

To work around the timing failure, the qdr_rld_phy_read_data_align.v module must have the following lines replaced:

Lines 103-108

reg [BYTE_LANE_WIDTH-1:0] memory_rd0 [15:0];
reg [BYTE_LANE_WIDTH-1:0] memory_fd0 [15:0];
reg [BYTE_LANE_WIDTH-1:0] memory_rd1 [15:0];
reg [BYTE_LANE_WIDTH-1:0] memory_fd1 [15:0];
reg [3:0] wr_ptr;
reg [3:0] rd_ptr;

replaced with:

reg [BYTE_LANE_WIDTH/2-1:0] memory_rd0_0 [15:0];
reg [(BYTE_LANE_WIDTH+1)/2-1:0] memory_rd0_1 [15:0];
reg [BYTE_LANE_WIDTH/2-1:0] memory_fd0_0 [15:0];
reg [(BYTE_LANE_WIDTH+1)/2-1:0] memory_fd0_1 [15:0];
reg [BYTE_LANE_WIDTH/2-1:0] memory_rd1_0 [15:0];
reg [(BYTE_LANE_WIDTH+1)/2-1:0] memory_rd1_1 [15:0];
reg [BYTE_LANE_WIDTH/2-1:0] memory_fd1_0 [15:0];
reg [(BYTE_LANE_WIDTH+1)/2-1:0] memory_fd1_1 [15:0];
reg [3:0] wr_ptr /* synthesis syn_maxfan = (BYTE_LANE_WIDTH+1)/2 */;
reg [3:0] rd_ptr /* synthesis syn_maxfan = (BYTE_LANE_WIDTH+1)/2 */;

and Lines 141-150

memory_rd0[wr_ptr] <= iserdes_rd0;
memory_fd0[wr_ptr] <= iserdes_fd0;
memory_rd1[wr_ptr] <= iserdes_rd1;
memory_fd1[wr_ptr] <= iserdes_fd1;

assign rise_data0 = memory_rd0[rd_ptr];
assign fall_data0 = memory_fd0[rd_ptr];
assign rise_data1 = memory_rd1[rd_ptr];
assign fall_data1 = memory_fd1[rd_ptr];

replaced with

memory_rd0_0[wr_ptr] <= iserdes_rd0[BYTE_LANE_WIDTH/2-1:0];
memory_rd0_1[wr_ptr] <= iserdes_rd0[BYTE_LANE_WIDTH-1:BYTE_LANE_WIDTH/2];
memory_fd0_0[wr_ptr] <= iserdes_fd0[BYTE_LANE_WIDTH/2-1:0];
memory_fd0_1[wr_ptr] <= iserdes_fd0[BYTE_LANE_WIDTH-1:BYTE_LANE_WIDTH/2];
memory_rd1_0[wr_ptr] <= iserdes_rd1[BYTE_LANE_WIDTH/2-1:0];
memory_rd1_1[wr_ptr] <= iserdes_rd1[BYTE_LANE_WIDTH-1:BYTE_LANE_WIDTH/2];
memory_fd1_0[wr_ptr] <= iserdes_fd1[BYTE_LANE_WIDTH/2-1:0];
memory_fd1_1[wr_ptr] <= iserdes_fd1[BYTE_LANE_WIDTH-1:BYTE_LANE_WIDTH/2];

assign rise_data0 = {memory_rd0_1[rd_ptr],memory_rd0_0[rd_ptr]};
assign fall_data0 = {memory_fd0_1[rd_ptr],memory_fd0_0[rd_ptr]};
assign rise_data1 = {memory_rd1_1[rd_ptr],memory_rd1_0[rd_ptr]};
assign fall_data1 = {memory_fd1_1[rd_ptr],memory_fd1_0[rd_ptr]};

Revision History
07/25/2012 - Initial release

AR# 50752
Date Created 07/11/2012
Last Updated 01/16/2013
Status Active
Type Known Issues
  • Artix-7
  • Kintex-7
  • Virtex-7
  • MIG 7 Series