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Enable Spread Spectrum Clocking on the DDR Interface

Introduction​

This article brings one possible approach to enable spread spectrum clocking using the DDR interface. The reference implementation in this article is done using a Verdin iMX8M Plus System on Module.

Why Enabling Spread Spectrum Clocking?​

In some situations, users might face EMI issues due to spikes around specific frequencies, such as 4 Ghz. So, this is indicated to reduce the electromagnetic peak arround that specific frequency.

Prerequisites​

Setup your development environmet for Yocto Project. Refer to Build a Reference Image with Yocto Project/OpenEmbedded.

Customize U-Boot and ATF to Enable Spread Spectrum Clocking​

  1. On your layers directory, create a layer to contain your patches (e.g. meta-spread-spectrum). You can use bitbake-layers to do this.

  2. In the meta-spread-spectrum layer create a recipes-bsp folder that will contain the modifications on Toradex BSP. 

    $ mkdir recipes-bsp && cd recipes-bsp

  3. Inside the recipes-bsp, create the directories that will contain the patch on the Arm Trusted Firmware (imx-atf)

    $ mkdir -p imx-atf/imx-atf/verdin-imx8mp
    $ tree .
    .
    └── imx-atf
        └── imx-atf
            └── verdin-imx8mp

  4. Add your patch to the imx-atf/imx-atf/verdin-imx8mp folder. You can find a reference implementation of the patch for Verdin iMX8M Plus at 0001-PATCH-Add-DRAM-PLL-for-1866MHz-and-enable-SSCG.patch.

  5. Create the append file imx-atf_2.2.bbappend as follows:

    imx-atf_2.2.bbappend
    # check imx-atf bbappend in meta-toradex-nxp
    # it strongly defines SRC_URI
    FILESEXTRAPATHS:prepend:verdin-imx8mp := "${THISDIR}/${PN}:"

    SRC_URI:append:verdin-imx8mp = " \
        file://0001-PATCH-Add-DRAM-PLL-for-1866MHz-and-enable-SSCG.patch"

  6. Inside the recipes-bsp, create the directories that will contain the patches to be applied on U-Boot (u-boot).

    $ mkdir -p u-boot/u-boot-toradex/verdin-imx8mp
    $ tree .
    .
    ├── imx-atf
    │   └── imx-atf
    │       └── verdin-imx8mp
    └── u-boot
        └── u-boot-toradex
            └── verdin-imx8mp

  7. Add your patch to the u-boot/u-boot-toradex/verdin-imx8mp folder. You can find a reference implementation of the patch for Verdin iMX8M Plus at 0001-PATCH-Enable-SSCG-on-DRAM.patch.

  8. Create the append file u-boot-toradex_2020.04.bbappend as follows:

    u-boot-toradex_2020.04.bbappend
    FILESEXTRAPATHS:prepend:verdin-imx8mp := "${THISDIR}/${PN}:"

    SRC_URI += "\
        file://0001-PATCH-Enable-SSCG-on-DRAM.patch"

  9. Verify if your layer directory looks like this:

    $ tree
    .
    ├── conf
    │   └── layer.conf
    ├── COPYING.MIT
    ├── README
    └── recipes-bsp
        ├── imx-atf
        │   ├── imx-atf
        │   │   └── verdin-imx8mp
        │   │       └── 0001-PATCH-Add-DRAM-PLL-for-1866MHz-and-enable-SSCG.patch
        │   └── imx-atf_2.2.bbappend
        └── u-boot
            ├── u-boot-toradex
            │   └── verdin-imx8mp
            │       └── 0001-PATCH-Enable-SSCG-on-DRAM.patch
            └── u-boot-toradex_2020.04.bbappend

  10. (Optional) Check if your layer is being correctly loaded by using the bitbake-layers [show-layers] [show-recipes] command.

  11. Build your customized image using the Bitbake command. For this, follow the steps described on the Build a Reference Image with Yocto Project/OpenEmbedded article.



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