Loopback Operation; Collision; Flow Control; Control Frame Transmission - Realtek RTL8169 Manual

4 16
4 15:0
8 31:0
12 31:0

9.2 Loopback Operation

Loopback mode is normally used to verify that the logic operations up to the Ethernet cable/fiber channel function correctly. The
RTL8169 supports both internal and external loopback capabilities. The RTL8169 internal loopback is actually a digital
loopback inside the RTL8169. To test an external loopback, the RTL8169 must operate in normal mode and the external
PHYceiver should be configured in loopback mode.

9.3 Collision

If the RTL8169 is not in full-duplex mode, a collision event occurs when the receive input is not idle while the RTL8169
transmits. If the collision was detected during the preamble transmission, a jam pattern is transmitted after completing the
preamble (including the JK symbol pair when network speed is 100Mbps). The RTL8169 does not support half-duplex mode in
1000Mbps mode. Therefore, there is no collision when the RTL8169 operates in 1000Mbps mode.

9.4 Flow Control

The RTL8169 supports IEEE802.3X flow control to improve performance in full-duplex mode. It detects and sends PAUSE
packets to achieve the flow control task. Results from the N-Way process with the link partner determine if flow control is
supported for the current connection.

9.4.1. Control Frame Transmission

When the RTL8169 is running out of receive descriptors in full duplex mode, it sends a PAUSE packet (with
pause_time=FFFFh) to inform the source station to stop transmission for the specified period of time. Once the receive
descriptors are available again, the RTL8169 sends another PAUSE packet (with pause_time=0000h) to wake up the source
station to restart transmission.

9.4.2. Control Frame Reception

The RTL8169 enters backoff state for the specified period of time when it receives a valid PAUSE packet (with pause_time=n)
in full duplex mode. If the PAUSE packet is received while the RTL8169 is transmitting, the RTL8169 starts to backoff after the
current transmission is completed. The RTL8169 is free to transmit packets when it receives a valid PAUSE packet (with
pause_time=0000h) or the backoff timer(=n*512 bit time) elapses.
The PAUSE operation cannot be used to inhibit transmission of MAC Control frames (e.g. a PAUSE packet). The N-way flow
control capability can be disabled. Please refer to Section 7, EEPROM (93C46 or 93C56) Contents for further information.
2002/03/27
TAVA Tag Available: When set, the received packet is an IEEE802.1Q VLAN
TAG (0x8100) available packet.
VLAN_TAG
VLAN Tag: If the TAG of the packet is 0x8100, The RTL8169 MAC
extracts four bytes from the after source ID, sets TAVA bit to 1, and
moves the TAG value to this field in the Rx descriptor.
VIDH: The high 4 bits of a 12-bit VLAN ID.
VIDL: The low 8 bits of a 12-bit VLAN ID.
PRIO: 3-bit 8-level priority.
CFI: Canonical Format Indicator.
RxBuffL Low 32-bit Address of Receive Buffer
RxBuffH High 32-bit Address of Receive Buffer
58
RTL8169
Rev.1.21