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DS80PCI800: Gen-3 questions

Miranda Gu
Genius 16632 points
Part Number: DS80PCI800

Hi team,

My customer is estimating DS80PCI800. Could you help to answer their questions as below?

  1. If DS80PCI800 is configured as Gen-3 mode (RATE pin tie 20 kΩ to GND), does DS80PCI800 automatically enter non-limiting mode? Can it enter limiting mode?
  2. The datasheet mentions that in gen-3 mode, the output VOD is dependent on VID & frequency content. Is output VOD also dependent on VOD Control register setting?
  3. My customer use different VOD Control register settings in gen-3 mode, while the test results are different. VOD control=111b has better eye opening than other settings. What may cause it?

  • 0
    TI__Mastermind 43795 points

    Hi Miranda,

    If DS80PCI800 is configured as Gen-3 mode (RATE pin tie 20 kΩ to GND), does DS80PCI800 automatically enter non-limiting mode? Can it enter limiting mode?

    Answer:  RATE = 20K to GND; The DS80PCI800 will always be in GEN3 mode having non-limiting outputs. It will not be in limiting mode.

    The datasheet mentions that in gen-3 mode, the output VOD is dependent on VID & frequency content. Is output VOD also dependent on VOD Control register setting?

    Answer:  Yes, in non-limiting mode the output VOD setting defines a maximum dynamic output amplitude.  If the input signal amplitude is equal to or larger than the VOD control setting the output will look like it is limiting even though it is non-limiting mode.  

    My customer use different VOD Control register settings in gen-3 mode, while the test results are different. VOD control=111b has better eye opening than other settings. What may cause it?

    Answer: The maximum VOD control setting will tend to produce the largest vertical eye opening and results in the most linear response.  For many PCIe applications using the higher VOD control settings with the DS80PCI800 produces the best result.

    Regards,

    Lee

  • 0 in reply to Lee Sledjeski
    TI__Genius 16632 points

    Hello Lee,

    Thanks for your kind support!

    One more question:

    If customer use de-emphasis Gen-3 mode (leave RATE-pin float), will your answers above be changed?

  • 0 in reply to Miranda Gu
    TI__Mastermind 43795 points

    Hi Miranda,

    With the RATE = Float the DS80PCI800 will use an internal circuit to determine the actual datarate.  If the rate is PCIe Gen1 or Gen2 then the DS80PCI800 will be in limiting mode.  When the PCIe link switches speeds to Gen3, then the DS80PCI800 will shift into a non-limiting mode automatically.

    For PCIe Gen1 and Gen2 operation the device output VOD will be defined by the VOD setting - even if the input is smaller or larger.

    For PCIe Gen3 the output is dependent on the input VOD and VOD setting as indicated previously.

    Regards,

    Lee

  • 0 in reply to Lee Sledjeski
    TI__Genius 16632 points

    Hello Lee,

    For 810 device, I understand that VOD = (VOD / VID Ratio) *  VID, where the VOD / VID Ratio is fixed and set by users.

    For 800 device, I still don't know the dependence between VOD and VID & VOD Setting in non-limiting mode. How dynamic VOD works? 

    Appreciate your kind explanation.

  • 0 in reply to Miranda Gu
    TI__Mastermind 43795 points

    Hi Miranda,

    For the DS80PCI800 in non-limiting mode the VOD setting bounds the maximum output amplitude.  The output amplitude can go as low as 0V differential if there is no signal on the input.  Once there is a valid input signal, the output amplitude is typically equal to 1.25x the input amplitude.  Once the output amplitude has reached the VOD setting value (for example 1200mV) the output amplitude will no longer increase with respect to any further input amplitude increases.  

    Regards,

    Lee 

  • 0 in reply to Lee Sledjeski
    TI__Genius 16632 points

    Clearly understood! Many thanks Lee.