[FAQ] Can Auto-negotiation link up with Force mode on 100mbps?

It is common for an Ethernet PHY to support multiple speeds, e.g. 10/100/1000Mbps. For various use cases, it is desired to operate at a single speed. There are two main ways to do this that are discussed below:

• Auto-Negotiation
  • By default, auto-neg will communicate with the link partner and establish link at the highest available speed. Auto-negotiation can be checked by reading bit 12 of Reg 0x0 (BMCR)
    • If both PHYs support 10/100/1000Mbps, they will link up at 1000Mbps
    • After the cable is connected, Registers 0x5 (for 10/100) & 0xA (for 1000) can be read to check Link-partner's capabilities
  • If we wanted to fix the speed at 100Mbps, we need to de-advertise both 10/1000Mbps capabilities
    • This can be done by writing to Registers 0x4 (for 10/100) & 0x9 (for 1000)
• Forced Speed
  • To enable Forced Speed functionality, Auto-negotiation needs to be disabled
    • This can be done via Hardware Straps or by Register Write
      • Reg 0x0 can disable Auto-neg and select the desired speed.
      • Please read the data sheet's Register description for your part

It is important to note that for both options, Auto-neg and Forced, both parties of the link chain need to be in the same mode. I.e. one PHY with Auto-Neg enabled and the other with Forced Speed will NOT link up. Both need to be Auto-neg or both Forced. Please refer to the table 1 below.

Table 1- Auto-neg/Forced Speed Set up

Note:

1. For the most robust configuration, TI recommends keeping Auto-negotiation enabled and de-advertising unwanted speeds.
2. When the PHY is initially strapped to enable auto-negotiation and you want to configure the PHY into force 100mbps mode, Software reset (0x001F to 4000) is required after the software configuration in order to perform a link up in Force mode 100Mbps.
3. Auto-negotiation does not apply to Fiber communication. With fiber, both PHY will need to be manually configured to the same speed.
4. We do not recommend forcing 1000Mbps

Regards,

Hillman Lin,

Alvaro Reyes