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Visitor ligen
Visitor
274 Views
Registered: ‎02-14-2019

MMCME2_DRP fractional setting

MMCME2_DRP CLKOUT0 reconfigure,but when CLKOUT0 set fractional,it wrong.such as:

clk_in = 100M,I want clkout0 output 74.25.The parameters I set as following:

CLKFBOUT_MULT = 7'd37;

CLKFBOUT_PHASE = 32'd0;

CLKFBOUT_FRAC = 10'd125;

CLKFBOUT_FRAC_EN = 1'b1;

DIVCLK_DIVIDE = 8'd4;

CLKOUT0_DIVIDE = 8'd12;

CLKOUT0_PHASE = 32'd0;

CLKOUT0_DUTY = 32'd50000;

CLKOUT0_FRAC = 32'd500;

CLKOUT0_FRAC_EN = 1'b1;

but when CLKFBOUT_FRAC_EN = 1'b0 and CLKOUT0_FRAC_EN = 1'b0,all reconfigure clk out0 is right.

Does the MMCME2_DRP can set fractional? Is some register I configure wrong.

 

 

 

 

 

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2 Replies
Moderator
Moderator
223 Views
Registered: ‎02-09-2017

Re: MMCME2_DRP fractional setting

Hi @ligen,

 

Could you please explain how you calculated these DRP values and bit lenght?

On page 3 of the XAPP888 there 's the following info: "The FRAC parameter represents the fractional divide portion of the divider in 0.125 increments. For example FRAC(4) = 0.500.".

In addition, in the same document, on Table 2: ClkReg2 Bitmap for CLKOUT[0] , we see that the FRAC portion of the register is just comprised of  3 bits [14:12], so you cannot really write the number 125 to it.

I believe you have to actually write a number between 0 and 7, and that will be translated into 0, 0.125, 0.250, ..., 0.875.

Could you please try that and let us know if it works?

Thanks. 

Andre Guerrero

Product Applications Engineer

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Visitor ligen
Visitor
218 Views
Registered: ‎02-14-2019

Re: MMCME2_DRP fractional setting

Hi@Andre Guerrero

I use the reference design,the parameter I set is mmcme2_drp.v's parameter.It will caculate for ClkReg2 Bitmap of CLKOUT[0] with mmcm_frac_count_calc function。such as,I set CLK0_FRAC = 125,it caculate ClkReg2[14:12] = 1 in real。

 

Thanks 

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