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DDR3L Design Choice Questions

Former Member

I am doing my own hardware design based on the MicroZed.  I am looking at the DDR3 implementation, and found a couple of design choices made on the MicroZed that I did not understand.  I am using the Rev F schematic.

I would appreciate it very much if someone could tell me why the following choices were made:
1.  Why is 1.5V used for the DDR voltage?  DDR3L memory is used.  This requires 1.35V for VDD.    1.5V is used on DDR3 and the part used on the MicroZed is 1.5V compatible.  Also look at my second question which is related to this.
2.  The Vivado Board Definition file for the Microzed selects "DDR3 (Low Voltage)" as the memory type.  This selects 1.35V interface, and specifically sets the internal reference voltage.  The Board Definition file also selects "Internal Vref".  Does this combination not set the wrong reference voltage?
3.  Why does the board definition file select "Internal Vref"?  The hardware is design to use the external reference (pins H6 and P6 on U9J connected to VTTREF) and according to the Xilinx documentation, it should be used because it provides better performance.

Thanks.

  • 0

    As stated in the MicroZed UG:

    The MicroZed DDR3 interface uses 1.5V SSTL-compatible inputs. When MicroZed was first designed, Zynq did not support 1.35V signaling. The Micron MT41K DDR3L components are 1.35V components that are backward compatible with 1.5V interfaces. Since the MicroZed release, Xilinx has qualified the Zynq-7000 DDR3 interface at 1.35V as well. Preliminary testing indicates that MicroZed operates with the Vcco_ddr interface at 1.35V. To experiment with this, change R39 to 18.7KΩ. To request this change on a volume order, please contact customize@avnet.com.

    I will investigate the issues with the board definition files. These should be set to 1.5V operation with external Vref. Zynq ES silicon did not support external Vref, but MicroZed has never shipped with ES silicon, so I believe you are correct that this is an error, although we have never seen any DDR3 failures related to this.

     

    Thanks for bringing this to our attention.

    Bryan

  • 0

    An updated board definition archive is now posted for MicroZed. You will find that it references DDR3 as the memory type and does not use the internal Vref.

     

    Thanks for the feedback.

    Bryan