[FAQ] How to Confirm Ethernet PHY Strapping

Our Ethernet PHYs have Hardware Bootstraps that allow the PHY to be configured without the need for register access and also set the PHY ID. Either one of these being mismatched can cause issues, e.g. PHY is configured for Fiber but is connected via Copper, or the PHY_ID gets changed which won't allow any register access. 

Whenever debugging our Ethernet PHY, our first recommendation is to confirm that the PHY is strapped as intended. With register access, a register can be read to confirm what strap values the PHY booted up with. If this register read does not align with what was intended with the schematic, this could explain why the PHY is misbehaving in the first place. 

A common reason for a PHY to strap into an incorrect mode is because the line connected to the strap pin is being driven by something else in the system. Often times the strap pins are also the MAC pins, so if the MAC boots up before the PHY, it might start transmitting across the MAC lines, which would cause the PHY to boot up into an unintended mode. 

TI's recommendation would be to confirm all of the lines connected to the strap pins are silent during bootup. The Strap Registers are provided below.

Note: These Registers are Extended Registers and require special steps in order to access. More details can be found here.