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[C6455] RGMII pin requirements

Naoki Kawada
Guru 19890 points

Hi, 

My customer has a question about pin requirements for RGMII.

They have been shipping their products in the market for long time, which C6455 is on its PCB.
And now they happened to review their schematic and found the connections of F3,G4,F2 and F1 pin were wrong.
The datasheet says that these pins can be connected directly to the ground if the RGMII mode of the EMAC is not used. But in fact, these pins had been connected to the ground in spite of using RGMII in their system.

So they are concerned by the possibilities that there might be some side-effects to cause something problems on RGMII or something.
I know customers should meet the requirements in datasheet and I believe it is difficult for you to answer to the query, but it would be so appreciated if you have any comments on this.

Best Regards,
Naoki

  • 0
    TI__Guru* 81335 points
    Hi,
    Moved this thread over C64x Single Core forum for faster and appropriate response. Thank you.
  • 0
    TI__Mastermind 43660 points
    Naoki,

    I believe there is no requirement of these F3, G4, F2 and F1 pins for proper RGMII operation. The reserved pins and voltage monitor pin are required for boundary scan of RGMII. Please refer section 7.3.4 Preserving Boundary-Scan Functionality on RGMII and DDR2 Memory Pins in device data manual for better understanding.

    Regards,
    Senthil
  • 0 in reply to Senthilkumar Srinivasan
    Guru 19890 points

    Hi Senthil,

    Thank you for your reply. I'll suggest the same to the customer.

    Best Regards,
    Naoki

  • 0 in reply to Naoki Kawada
    Guru 19890 points
    Hello Senthil,

    I got some feedback from the customer. They would like to know the reasons you mentioned there is no pin requirement for proper operation. As I mentioned in the previous post, as of now any problems have not been raised from the market, but they are wondering if something problems might come out after aging operation. Do you have any comments on this ?

    Best Regards,
    Naoki
  • 0 in reply to Naoki Kawada
    TI__Mastermind 43660 points
    Naoki,

    I mentioned F3, G4, F2 and F1 pins are not required for RGMII operation as these are meant for boundary scan of RGMII. I will also check and confirm this with the design team and update you.

    Regards,
    Senthil
  • 0 in reply to Senthilkumar Srinivasan
    Guru 19890 points

    Hello Senthil,

    Thank you for your help. If connecting these pins to ground is requirement only for boundary scan and it never affects to RGMII proper operation, I think that's fine.
    Anyway, I'll wait for your feedback on this.
    Sorry to have troubled you so much.

    Best Regards,
    Naoki

  • 0 in reply to Naoki Kawada
    Guru 19890 points
    Hello Senthil,

    How is your progress on this query ?
    I got an additional question related to pin setup for RGMII proper operation.

    They are supplying 0.75V to VREFHSTL (B2 pin) via external DDR Termination Regulator, but data sheet says:

    (DVDD15/2)-V reference for HSTL buffer (EMAC RGMII). VREFHSTL can be
    generated directly from DVDD15 using two 1-kΩ resistors to form a resistor
    divider circuit.

    My customer is concerned that using register divider circuit might be requirement...
    I don't think this is requirement, but they want to have your comments on this.

    Best Regards,
    Naoki
  • 0 in reply to Naoki Kawada
    TI__Mastermind 43660 points
    Naoki,

    Since this is older device, I have limitation on getting support from the design team. So you can proceed with my response in above posts.

    Yes, the resistor divider for VREFHSTL is not a requirement. You can connect this pin any other 0.75V source provided that source should be capable to drive the current requirement.

    Regards,
    Senthil
  • 0 in reply to Senthilkumar Srinivasan
    Guru 19890 points

    Hi Senthil,

    Thanks you for your reply. As for VREFHSTL, I understood.

    Sorry to ask you so much, but my customer have a request to investigate the pin requirements for the proper operation of RGMII.

    Let me share the more detailed background with you.
    In fact, they had some products returned from the market and after their investigations, they concluded that the root cause was that some RGMII related pins (RGRX0/1/2/3, RGRXC and RGRXCTL) were not functional -- Some pins were shorten to DVDD15(H7), and some had very small impedance (such like 2 Ohm) compared with the functional devices (60 kOhm).
    They are wondering if the current connections (connecting F3,G4,F2 and F1 pins to ground) might be related to this problem and that's why they are asking the current connections are allowed by RGMII design perspective.
    Also, please don't forget that the returned products were working for two years at least. This is just my thoughts, but I'm concerned that the current connections could accumulate permanent damages in device.

    Could you please try again to get help from your design team ?

    Best Regards,
    Naoki

  • 0 in reply to Naoki Kawada
    TI__Mastermind 43660 points
    Naoki,

    I understand your problem. I am trying to get the design expert suggestion on this query. I will update you soon.

    Regards,
    Senthil
  • 0 in reply to Senthilkumar Srinivasan
    Guru 19890 points
    Hello Senthil,

    Do you have any update on this ? My customer is waiting for your reply.
    Thank you for your cooperation.

    Best Regards,
    Naoki
  • 0 in reply to Naoki Kawada
    TI__Mastermind 43660 points
    Naoki,

    I tired to get some suggestion from the device experts. But i did not get a response yet.

    To my understanding, the pins F3,G4,F2 and F1 are meant for EMAC boundary scan and i am not sure if these pins are used for internal EMAC logic circuitry. If you feel these pins connection would affect the EMAC operation in any way, please follow the datasheet guidelines for these pins connection.

    Regards,
    Senthil
  • 0 in reply to Naoki Kawada
    TI__Mastermind 42545 points

    Hi Naoki,

    As stated in the data sheet, these pins must be connected correctly for the proper operation of RGMII if that interface is used. The option of shorting these pins to ground is provided as a way of reducing power usage of the device, but only if the entire RGMII subsystem is not in use and only if the supply pins for that interface (DVDD15) are also grounded. F1 and F2 are used to set the proper HSTL reference voltage for the RGMII interface based on the value of the resistor installed between the pin and ground. If the resistor is not present the proper reference voltage will not be available to the interface. F3 is a die-side monitor point for the DVDD15 supply. Grounding this pin would short the DVDD15 power supply directly to ground. 

    Regards,

    Bill

  • 0 in reply to Bill Taboada
    Guru 19890 points
    Hi Bill and Senthil,

    Thank you so much for your investigations.
    So you say these pins should be handled as the data sheet stated for proper RGMII operation.
    Let me ask you one more short question.
    Violating these requirements could accumulate stress in device and cause the problem I mentioned before -- some of these pins are shorten to DVDD15(H7), and some has very small impedance. Correct ?

    Best Regards,
    Naoki
  • 0 in reply to Naoki Kawada
    TI__Mastermind 42545 points

    Hi Naoki,

    Connecting the pins in the manner you described is outside the scope of our testing. I can't speculate on whether it would cause the problems you have described. I am most concerned about the connection of F3. Since this is a monitor connection to the DVDD15 on the die itself, grounding this pin would cause a direct short between the DVDD15 power supply and ground. This would draw large amounts of current across the substrate connection. The resister that is supposed to be connected to F1 and F2 is part of a voltage divider which creates a reference voltage to the drive impedance selection circuit. 0 ohms is outside the range of resistance expected so it is unclear what the effect to the drive impedance of the RGMII outputs would be. 

    I know you are looking for some guidance on the effect to units that are already in the field. Unfortunately, we don't have any good information on the effect to the device for the connections you've described.

    Regards,

    Bill 

  • 0 in reply to Bill Taboada
    Guru 19890 points
    Hi Bill,

    Sorry for my delayed response. Understood. I'll talk with the customer.

    Best Regards,
    Naoki