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AR# 36575

MIG v3.0-3.5, Spartan-6 MCB - Refresh period is too large

Description

The JEDEC standards for DDR SDRAM memories (i.e., DDR, LPDDR, DDR2, DDR3) state a refresh rate of 7.8 microseconds must be adhered to.

In testing, it was found that the refresh rate for the MCB was 7.85 microseconds.
 
The period needs to be modified to ensure that the JEDEC standard refresh rate of 7.8 microseconds is met. 

This Answer Record details the required changes.

Solution

Work-around:

To resolve this issue, the refresh period must be reduced.

Step 1 - Subtract 25 from the Refresh Counter.
 
In testing, subtracting 25 from the period calculation has resulted in a true 7.8 microseconds refresh period.

To reduce the refresh period, modify the MEM_REFI_VAL calculation provided in the mcb_raw_wrapper.v/.vhd module. 

The example below is for Verilog, but similar changes should be made for VHDL.

From:
 

localparam MEM_REFI_VAL = ((C_MEM_TREFI + C_MEMCLK_PERIOD -1) /C_MEMCLK_PERIOD);


To:
 

localparam MEM_REFI_VAL = ((((C_MEM_TREFI + C_MEMCLK_PERIOD -1) /C_MEMCLK_PERIOD)) 25);




Step 2 - Avoid a tRFC Violation.

Simply subtracting 25 from the refresh counter causes tRFC violations.

To avoid these violations, time needs to be added to the top-level parameter C_MEM_TZQINIT_MAXCNT. 

This parameter is set in the memcx_wrapper.v/.vhd module. 

The example below is for Verilog but similar changes should be made for VHDL.

From:
 

localparam C_MEM_TZQINIT_MAXCNT      = 10'd512;


To:

localparam C_MEM_TZQINIT_MAXCNT      = 10'd512 + 10'd16;   // 16 clock cycles are added to avoid trfc violations


The above noted changes will be made in MIG 3.6 which will be released with ISE software 12.3.

Linked Answer Records

Master Answer Records

Answer Number Answer Title Version Found Version Resolved
36211 MIG v3.5 - Release Notes and Known Issues for ISE Design Suite 12.2 N/A N/A

Associated Answer Records

Answer Number Answer Title Version Found Version Resolved
35180 Spartan-6 - 12.4 Software Known Issues related to the Spartan-6 FPGA N/A N/A
AR# 36575
Date Created 07/02/2010
Last Updated 08/21/2014
Status Active
Type General Article
Devices
  • Spartan-6 LX
  • Spartan-6 LXT
IP
  • MIG