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DS90UB940-Q1: No signal output when paired with UB947 or UB927

Part Number: DS90UB940-Q1

We are designing a board with UB940 to pair with UB947 or UB927 to output CSI2 signal to downstream IC. While the UB940 eval board can give output when paired with same source device, our board does not produce any valid waveform. Please help review the schematic linked below to see if there are any mistakes. Thank you.

https://www.dropbox.com/scl/fi/bxrf50elb9iesls3eepkf/WNKD_CAN_LVDS_940_V3.0.pdf?rlkey=lctaurt35x1hn5llpng7dekxe&dl=0

  • Hi Wei,

    The power rail filtering does not have the recommended filtering. Please see the typical applications section of the datasheet, as well as the comparison below.

    However, the issue may be instead related to the layout. especially regarding issues like length matching (both the FPD-Link traces and the CSI traces), stubs, or other factors that may impact signal integrity.

    Has the SER and DES achieved lock?

    Regards,

    Ben

  • Thank you for the  insights. Please help review the layout from top to bottom layers to see if any problems contribute to the no signal issue.

  • Hi Wei,

    Are the connectors for the FPD-Link signal and the device on the same layer? It is hard to tell from these images. If they are, this would cause a stub if the connector is thru-hole. A way around this would be to route the FPD-Link traces to the bottom layer with a via, and connecting to the connector there.

    Please make sure the FPD-Link traces are length matched according to the spec in the datasheet.

    Please make sure the CSI traces are length matched according to the MIPI guidelines.

    Also, can you confirm whether there is lock between the SER and DES when there is black screen? Can you provide a reg dump of the 940?

    Regards,

    Ben