DP83867E: Bootstrapping Resistors for SGMII vs RGMII and Pin Requirements

Ross

Please see my response below

Question 1: I am a bit confused on the strapping requriements.  Per these notes on sheet 40 of the data sheet:

"Note: RX_D1 is not a strap input, but this pin must be populated with the same strap resistors chosen for RX_D0. RX_D0 and RX_D1 form an SGMII differential pair. The dummy straps on RX_D1 are required to provide a balanced load for this SGMII differential pair."

"Note: RX_D3 is not a strap input, but this pin must be populated with the same strap resistors chosen for RX_D2. RX_D2 and RX_D3 form an SGMII differential pair. The dummy straps on RX_D3 are required to provide a balanced load for this SGMII differential pair."

I am assuming this means that we should be putting placeholder 0 ohm resistors on the RX_D1 pin and the RX_D3 pin as well even though they are not strap inputs?  Just in case we end up having to put strap resistors on the RX_D0 and RX_D2? 

Correct, please see the example below

Also, Does this ONLY apply to SGMII or does it apply to RGMII as well?

This is for SGMII only as SGMII is differential pair, it does not apply toward RGMII.

Question 2: Should we avoid putting any strap resistors on LED_1 and LED_2 if  we have them connected to on board LEDs for test?

No, you can still strap resistors on LED1 and LED2. With the LED output pins are also used as straps, the external components required for strapping and LED usage must be considered to avoid contention. Specifically, when the LED outputs are used to drive LEDs directly, the active state of each output driver is dependent on the logic level sampled by the corresponding AN input upon power up or reset. If a given strap input is resistively pulled low then the corresponding output is configured as an active high driver. In the context of the 4-level straps, this occurs for modes 1, 2, and 3. Conversely, if a given strap input is resistively pulled high, then the corresponding output is configured as an active low driver. In the context of the 4-level straps, this occurs only for mode 4. Below is an example diagram.

Question 3: Should we avoid putting any strap resistors on RX_D2/RX_D3 if we are utilizing them as the outputs for SGMII?

You can still put strapping resistor on RX_D2/RX_D3. For SGMII Mode 4 strap, TI recommends using Rhi = 4kΩ and Rlo = 10kΩ on RX_D0 and RX_D1 , RX_D2 and RX_D3.

Question 4: Per this note in the data sheet "Strap modes 1 and 2 are not applicable for RX_CTRL. The RX_CTRL strap must be configured for strap mode 3 or strap mode 4. If the RX_CTRL pin cannot be strapped to mode 3 or mode 4, bit[7] of Configuration Register 4 (address 0x0031) must be cleared to 0. Autoneg Disable can always be set to 0 when using gigabit Ethernet."  

Does this mean that if we do not have any strapping resistors setting the RX_CTRL pin to 3 or 4 we will run into issues?  Being that Mode 1 is when the resistors are open.

Correct, RX_CTRL must be strapped to either Mode 3 and 4. 

A good place to start the DP83867 design is to use its schematic checklist. They can input the system design parameter into the spreadsheet and it will generate the appropriate strapping configuration.

6811.slvrbn1a.zip

Thanks

David

Ross

RX_CTRL strapping disables/enables auto-negotiation, which is applicable to both SGMII and RGMII.

Thanks

David