Charles Powe

i.MX6Q: Using an external reference for PCIe

Discussion created by Charles Powe on Feb 18, 2013
Latest reply on Apr 29, 2016 by karthik AS
Branched to a new discussion

I'm working on a design in which the i.MX6Q communicates with a single PCIe endpoint.  Unlike the SABRE reference designs the reference clock from the endpoint comes from an on-board oscillator, not the i.MX6.  We recently got the i.MX6Q to use this external clock as a reference for PCIe.  I wanted to share our experiences -- and hopefully compare/contrast to other solutions.


We were able to bring the PCIe link up initially using the PCIe "internal" reference.  The PCIe was driven by the ENET PLL.  This PLL provides clocks for Ethernet, PCIe, and SATA.  This wasn't a ideal setup, however, as the endpoint was on an asynchronous reference.


Our solution to using an external reference for PCIe has three parts:


1. Configure the CLK pins to accept a clock as input.

2. Bypass the ENET PLL, using the CLK pins as the alternate source.

3. Configure the PCIe PHY to accept a clock other than 125 MHz.


To configure CLK pins as input you must adjust the PMU_MISC1n register (reference section 50.7.6 in Rev. O of the reference manual).  Clear the appropriate OBEN bit and set the IBEN bit.  The clock source bits are don't-care; they're only used as an output.  In my case, I modified the _clk_pcie_enable() and _clk_pcie_disable() routines in the Freescale BSP @ linux/arch/arm/mach-mx6/clock.c.


The ENET PLL is bypassed by adjusting the CCM_ANALOG_PLL_ENETn register (section 18.7.15 in the Rev. O RM).  Set the BYPASS_CLK_SRC (bits [15:14]) to the source clock and set the BYPASS bit (bit 16).  I updated the _clk_pll_enable() routine in clock.c to make these adjustments.  See below.


static int _clk_pll_enable(struct clk *clk)


    unsigned int reg;

    void __iomem *pllbase;


    pllbase = _get_pll_base(clk);


    reg = __raw_read(pll_base);


    /* Bypass the ENET PLL; use CLK1 as the source */

    if (clk == &pll8_enet_main_clk) {


        reg |= (0x1 << ANADIG_PLL_BYPASS_CLK_SRC_OFFSET);



    /* power-up the PLL */



    /* ...snip... */


    /* Enable the PLL output now */

    reg = __raw_readl(pllbase);

    if (clk != &pll8_enet_main_clk)

        reg &= ~ANADIG_PLL_BYPASS;


    __raw_writel(reg, pllbase);


    return 0;



Finally, the PCIe PHY needs to be adjusted to accept a clock at the provided frequency.  In our case, the reference clock is 100MHz.  By default the PCIe PHY wants a 125MHz reference.  To accept another rate adjust the MPLL according to table 49-1 in the reference manual.  Use the ATEOVRD (49.5.8) and MPLL_OVRD_IN_LO  (49.5.9) registers.  NOTE:  The ATEOVRD shows bit 0 as reserved.  I determined through experimentation that this isn't the case.  Bit 0 is ref_clkdiv2, bit 1 is ref_usb2_en, and bit 2 is ateovrd_en.  See code below.


#define PCIE_PHY_ATEOVRD          (0x10)

#define PCIE_PHY_MPLL_OVRD_IN_LO  (0x11)

static int imx_pcie_override_phy_mpll(u32 mpll_multiplier, u32 ref_clkdiv2)


    u32 ref_usb2_en;

    u32 reg;

    int ret;


    pr_info("overriding PCIe PHY MPLL config: multiplier = %d, clkdiv2 = %d\n",

        mpll_multiplier, ref_clkdiv2);


    /* set the MPLL override value to 'disabled' */

    pcie_phy_cr_read(PCIE_PHY_MPLL_OVRD_IN_LO, &reg);

    reg &= ~(0x1 << 1);

    pcie_phy_cr_write(PCIE_PHY_MPLL_OVRD_IN_LO, reg);


    /* enable MPLL override */

    reg |= (0x1 << 0);

    pcie_phy_cr_write(PCIE_PHY_MPLL_OVRD_IN_LO, reg);


    /* set the new MPLL multiplier */

    reg &= ~(0x7F << 2);

    reg |=  (mpll_multiplier << 2);

    pcie_phy_cr_write(PCIE_PHY_MPLL_OVRD_IN_LO, reg);


    /* enable multiplier override */

    reg |= (0x1 << 9);

    pcie_phy_cr_write(PCIE_PHY_MPLL_OVRD_IN_LO, reg);



     * set the ref_clkdiv2.  when this override is enabled it

     * overrides both ref_clkdiv2 and ref_usb2_en.  make sure

     * the overriden ref_usb2_en reflects the original value.


     pcie_phy_cr_read(PCIE_PHY_ATEOVRD, &reg);

     ref_usb2_en = (reg >> 3) & 0x1;


     /* set the current value of ref_usb2_en as the override */

     reg &= ~(0x1 << 1);

     reg |=  (ref_usb2_en << 1);


     /* set the ref_clkdiv2 override */

     reg &= ~(0x1 << 0);

     reg |=  (ref_clkdiv2 << 0);


     pcie_phy_cr_write(PCIE_PHY_ATEOVRD, reg);


     /* enable the ref_clkdiv2 override */

     reg |= (0x1 << 2);  /* ateovrd_en */

     pcie_phy_cr_write(PCIE_PHY_ATEOVRD, reg);


     /* disable the MPLL override */

     pcie_phy_cr_read(PCIE_PHY_MPLL_OVRD_IN_LO, &reg);

     reg &= ~(0x1 << 0);

     reg |=  (0x1 << 1);

     pcie_phy_cr_write(PCIE_PHY_MPLL_OVRD_IN_LO, reg);


     return 0;



I call this routine from the imx_pcie_pltfm_probe() routine in linux/arch/arm/mach-mx6/pcie.c just before enabling the LTSSM.


That's it.  This seems to work for me.  If anyone has other experiences I'd love to hear about it.