Virtex® Family FPGAs

[bryant_barbara@bah.com]

I'm looking to configure a DCM for the following operation....is it simply done by connecting the Clk2x to the Clkfb?

Clkin = 100 MHz

Clk0 = 200MHz

Clk90,Clk180, Clk270 = the associated phases of Clk0 (200MHz) respectively

Clkdv = 100Mhz

Thank you.

[johnnyrocketfingers · Accepted solution]

The CLK0 will always reflect your CLKIN so by using CLK2X you will not see CLK0 doubled. If you want to get 200MHz at 90 180 and 270 then you will ned to cascade DCMs use CLK2X from DCM 1 as an input to DCM2 tehn use CLK0 from DCM2.

Answer record 18181 discusses cascading DCMs and also teh reset circuitary required:  http://www.xilinx.com/support/answers/18181.htm

[tijanat]

Hello,

I need to do the same: generate 4 phases at approx. 300MHz. I was hoping to cascade 2 DCMs:

DCM1:

freq. settign LOW,

CLKIN = 150MHz, use CLK2x and CLK2x180 to drive other logic. (phases 0 and 180 @ 300MHZ)

DCM2:

CLK90 as input to DCM2.Use CLK2x and CLK2x180 to drive th erest of the logic. (phases 90 and 270 @300MHz)

The challenges are:

 - Core_IP Wizard for DCM_SP primitive in ISE does not allow frequencies higher than 120MHz for frequency setting LOW. I thought that Spartan 3E family has a DLL LOW frequency mode  range of up to 180MHz (based on xapp462.pdf).

 - It appears that BOTH DCMs need to have the same DLL Freq Mode Setting, i.e. I can't select HIGH for the second DCM in the cascade.

Could someone please explain to me:

- whether Spartan 3E indeed has LOW FREQUENCY MODE limitation of less than 120MHz?

- Could I set DCM1 to LOW and DCM2 to HIGH frequency mode in a DCM cascade?

Thank you.

Tijana