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DS90UB913A-Q1: DS90UB913A No Data Output

Devin Mullen
Prodigy 10 points

Part Number: DS90UB913A-Q1
Other Parts Discussed in Thread: DS90UB913A-CXEVM, DS90UB914A-CXEVM

Hello,

I am using a DS90UB913A/DS90UB914A pair (currently on evaluation boards via DS90UB913A-CXEVM and DS90UB914A-CXEVM), but I have the DOUT+/- pins from DS90UB913A pins tied to TD+/- on an SFP module (1Gbps capable) and then the SFP connected via single mode fiber to another SFP with its RD+/- pins connected to the RIN+/- pins on the DS90UB914A. I am using the DS90UB913A/DS90UB914A in 12-bit LF mode, with a PCLK of 27MHz going into the DS90UB913A. I am only using 8 bits of data, with both HSYNC and VSYNC. I believe this should yield an overall high-speed bit rate of 28*27MHz = 756Mbps. The SFP modules are plugged into SFP breakout boards with SMA connections for TD+/- and RD+/-. The physical cable connections between the DS90UB913A-CXEVM/DS90UB914A-CXEVM and the SFP breakout boards are standard 50 Ohm Coax cables that are about 1 foot in length.

When hooked up to the SFP modules, I see a lock on the DS90UB914A (via the lock LED on the eval board, and measuring this with an oscilloscope shows it to be steady in the logic high state. I am also seeing a 27MHz PCLK (which is the same as the PCLK into DS90UB913A) out of the DS90UB914A which indicates to me that the deserializer is locked and recovering the proper clock from the serializer. However, the issue is that I see no HSYNC, VSYNC or any data bits. HSYNC, VSYNC and D0-D7 are all stuck low. Reading the registers of the DS90UB913A, bit 0 (LINK Detect) of the "General Status" Register (Address 0x0C) is logic low - indicating a fault of some kind. If I take the SFP modules out of the setup and wire the DS90UB913A outputs directly to the DS90UB914A inputs, everything works properly and I am able to recover PCLK, HSYNC, VSYNC, and D0-D7. 

 I understand that the SFP connections will prevent the back-channel from operating between the DS90UB914A and the DS90UB913A. I had read in other forum posts that these parts are able to operate without the back channel. Is the back channel still required during initialization, to synchronize these two parts, in order to establish the link? If not, shouldn't these two parts work over an SFP connection scheme as described above?

Any insight you can provide into resolving this issue would be greatly appreciated. I have attached the register values that I am reading from the two devices.

Thank you,

Devin Mullen

DS90UB913_914 Registers with Fiber.xlsx 

  • 0
    TI__Guru** 116790 points

    Hello Devin,

    Please allow 1-2 business days until I review the provided details and get back to you.

  • 0 in reply to Hamzeh Jaradat
    TI__Guru** 116790 points

    Hello Devin,

    I reviewed the provided data. Everything look okay to me. I see no issue in your link. Maybe the conversion from electrical to optic is not functioning as expected?!

    Devin Mullen said:
    Reading the registers of the DS90UB913A, bit 0 (LINK Detect) of the "General Status" Register (Address 0x0C) is logic low - indicating a fault of some kind

    The Link Detect bit is getting updated over the back channel, and since you do not have a functioning BC that means this bit will stay low.

    Please make sure your both devices are configured into the same Mode.

    Make sure also GPIO2 on the SER is pulled low with a ~40Kohm resistor.

  • 0
    Prodigy 10 points

    Hello Hamzeh,

    After posting this, I was able to get the link working. I was originally using PCLK as the input clock for the serializer in the cable configuration. When hooked up to the SFP, I switched to external oscillator as the input clock for the serializer, and changed the external oscillator to match the PCLK frequency. After that I was able to get the link working through the SFP.

    The DS90UB913A datasheet does not recommend using PCLK as the input clock for the serializer. I measured the jitter of the PCLK signal from our imager and it was about 3.6ns, which is about 10x the maximum allowable jitter per the DS90UB913A datasheet (0.3*UI, UI=1/(PCLK*28)=1.32ns). Given this measurement, I'm surprised that I was able to get the link working over the cable. My coax cables were pretty short (~1ft) with minimal insertion loss so maybe that was enough to get the link to work in the cable configuration.

    Thank you for your help.

    Devin

  • 0 in reply to Devin Mullen
    TI__Guru** 116790 points

    Hello Devin,

    Thanks for the update. Glad to hear that the issue is resolved.