The Cyclone 10Gx LVDS Pad requirement with DC-Coupled links

[KJian6]

In out PCB layout, we are using the DC-Coupled connection for Cyclone 10GX transceiver LVDS refclk pad. ( Pin Name: REFCLK_GXB1C_CHTP and REFCLK_GXB1C_CHTN).

The input clock frequency is 125MHz. The Vcm is 1.0V, The AC swing range is 600mV. We measured these numbers from the PCB when FPGA is not configured.

But, we can't see the this refclk waveform inside the Cyclone 10GX fpga.

It always stuck to high in our signaltap waveform. We can see our other LVDS (not for refclk transceivers) clocks in signaltap.

Question 1: Can we use DC-Coupled link besides the AC-Coupled link for Cyclone 10Gx transciver RefCLK_GXB1C_CHTP LVDS pad ?  

Question 2: What is the spec for Cyclone 10Gx transceiver LVDS RefClk_GXBL1C pad with DC-Coupled link ? What is the required Vcm ? In the datasheet, it only says AC couple VICM is 0.95V, is the requirement for DC couple different?

[CheePin_C_Intel]

Hi Jiang, As I understand it, you have some inquiries related to the C10GX refclks. Allow me some time to look into your issue. I shall come back to you with findings. Thank you for your patience. Best regards, Chee Pin

[KJian6]

Hi:

1, For questions 1---" Can we use DC-Coupled link besides the AC-Coupled link for Cyclone 10Gx transceiver RefCLK_GXB1C_CHTP LVDS pad ? "

We have found the following statement from intel Cyclone 10gx IO guide line statement:

https://www.intel.com/content/www/us/en/programmable/documentation/osf1485840198234.html#czh1485915417341

REFCLK_GXB[L1] Input  High speed differential reference clock positive receiver channels, specific to each transceiver bank of the left (L) side of the device.

                                         These pins must be AC-coupled if the selected REFCLK I/O standard is not HCSL.

So please confirm that we can't use DC-Coupled link for  Cyclone 10Gx transceiver RefCLK_GXB1C_CHTP LVDS pad.

2, One more question about the transceiver refclk Vicm valuefor AC-Coupled link:

In Cyclone 10GX datasheet  (2c10gx-51002.pdf:  page 23), The Vicm is 0.95V when Vccr_GXB = 0.95V.

At the same time, We have measured the transceiver refclk Vicm from Intel Cyclone 10Gx development kit.

(https://www.intel.com/content/www/us/en/products/programmable/fpga/cyclone-10/gx/design-tools.html.)

T he Vicm is only 0.8V in our measurement which is smaller than your spec.

Can you help check it again ?

Thanks,

Ke

[CheePin_C_Intel]

Hi, Sorry for the delay and thanks for your patience. Please see my response to your inquiries as following: 1, For questions 1---" Can we use DC-Coupled link besides the AC-Coupled link for Cyclone 10Gx transceiver RefCLK_GXB1C_CHTP LVDS pad ? " [CP] For your information, since you are using the refclk with LVDS IO standard, you must use AC coupled for the transceiver refclk. The DC coupled is allowed for HCSL IO standard which is for PCIe application. 2. 2, One more question about the transceiver refclk Vicm value for AC-Coupled link [CP] Yes, you are right, when the VCCR_GXB = 0.95V, you would need to supply VICM value of 0.95V to the refclk. Regarding the measurement that you did at for the devkit, would you mind to share with me how you measure the voltage? Generally the VICM need to be measure at the FPGA ball to more precise. For your information, if you are enabling the on-chip termination (which is default setting in Quartus except for HCSL) for the refclk, there will be internal biasing circuitry to construct the right VICM internal. You will not need to build external on board biasing circuitry. Please let me know if there is any concern. Thank you. Best regards, Chee Pin

[CheePin_C_Intel]

Hi, Just would like to follow up with you on this. Please let me know if there is any concern. Thank you. Best regards, Chee Pin

[KJian6]

Hi Chee:

Many thanks for your great help ! These info are very helpful!

Also the issue has been resolved in our side.

Best Regards,

Ke

[CheePin_C_Intel]

Hi Ke, Thanks for your update. Glad to hear that you have manage to resolve the issue. I will set this case to closed. Thank you.