Exact meaning of MEM_PLL_RAT for LS102x products?

[jean-francoisri]

In the "QorIQ LS1021A Reference Manual, Rev. 2 04/2017", MEM_PLL_RAT is described as:

Memory controller complex PLL multiplier/ratio
This field configures the DDR PLL:SYSCLK ratio.

However, the "AN4878: QorIQ LS1021A Design Checklist, Rev. 4, 07/2017" is instead referring to "DDR data rate to DDRCLK" ratio:

In asynchronous DDR mode, the DDR data rate to DDRCLK ratios supported are listed in Table 66. This ratio is determined by the binary value of the RCW configuration field MEM_PLL_RAT (bits 10-15).

Then Table 66 lists the binary values of MEM_PLL_RAT to encode ratios between 6:1 and 24:1, which just happens to be the value on the left side of the ':'. Table 71 confirms this by listing the expected frequency options for DDRCLK and data rate.

So one document describes MEM_PLL_RAT as the ratio between DDR PLL and SYSCLK (i.e. frequency multiplier), while the other doesn't mention MEM_PLL_RAT per se, but describes a ratio between DDR data rate to DDRCLK. This is very confusing because DDR means "Double Data Rate", so the effective rate would double the frequency at which it runs.

Which one is right? And if I wanted to use a LS1022A, which only supports a DDR date rate of 1000MT/s, what value of MEM_PLL_RAT would I need in my RCW?

[r8070z]

Have a great day,

The LS102x has 2 reference clock inputs for DDR PLL – DIF_SYSCLK and DDRCLK. See Figure “Single oscillator source clocking” in the LS1021A manual or AN4878. RCW[DDR_REFCLK_SEL] selects which of these clocks will be applied to the DDR PLL input. RCW[MEM_PLL_RAT]controls DDR PLL and defines its multiplication factor. Clock from DDR PLL output is applied to the DDR memory controller. Provided on the MCK pins SDRAM clock frequency is half of the DDR PLL output clock frequency (it is also half of the data rate). So if the reference clock frequency is 100 MHz and required data rate is 1000 MT/s then we need for 1 GHz on DDR PLL output i.e. we should set RCW[MEM_PLL_RAT]=00_1010 for 10:1 ratio.

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