Ethernet PHY Link Configuration
Link Configuration Guidelines
The success in establishing a reliable Ethernet link depends on different factors including configuration parameters of the link partners like mode, speed and duplexity, and also on choosing the right cabling for a given speed.
The following sections provide general guidelines to configure link partners when establishing Ethernet links.
Manual Mode
Half-Duplex Mode
When either link partner is in manual mode with duplexity set to half-duplex, the other link partner should be configured in any of the following modes:
Manual mode with the exact same speed and duplexity. Otherwise:
Speed mismatch can cause link not to be established at all.
Duplexity mismatch can result in an established link, but lower performance due to frame collisions.
Auto-negotiation mode supporting same speed and duplexity. The auto-negotiating partner always switches to half-duplex if FLPs are not being received from the other link partner.
Full-Duplex Mode
When either link partner is in manual mode with duplexity set to full-duplex, the other link partner should be configured in any of the following modes:
Manual mode with the exact same speed and duplexity. Otherwise:
Speed mismatch can cause link not to be established at all.
Duplexity mismatch can result in an established link, but lower performance due to frame collisions.
Auto-negotiation mode is not recommended because link may be established if both devices are at the same speed but they will end with duplexity mismatch as the auto-negotiating device always switches to half-duplex if no FLPs are received from the other link partner. This results in lower performance due to frame collisions.
Auto-Negotiation Mode
When either link is in auto-negotiation mode, then other link partner can be configured also in auto-negotiation mode or in manual mode following the recommendations outlined in the Manual Mode section.
Auto-negotiation mode should always succeed as long as the two link partners have common capabilities. The negotiated speed and duplexity is the highest supported mode supported by both link partners.
It’s worth mentioning that, as per standard, auto-negotiation is the only mode to be used for 1 Gbps. However, it’s also possible to use manual mode if both link partners are configured to the same speed and duplexity.
Strapping
PHY strapping allows certain settings to be configured at start-up time, for instance, auto-negotiation enable/disable, advertise capabilities, auto-MDIX, etc.
The Ethernet PHY abstraction layer supports PHYs which are strapped to enable auto-negotiation and whose advertise abilities are also set via strap pins. This mode is enabled when EnetPhy_Cfg.isStrapped configuration parameter is set to true.
In this mode, the driver follows a shorter path in its state machine. PHY reset and auto-negotiation configuration related states are bypassed, directly jumping to wait for link up.