8101 datasheet(61/172 Pages) LSI

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MAC Control Frames

2-43

The controller normally treats MAC control frames according to the

IEEE 802.3, clause 31 algorithm. When the receive MAC detects a MAC

control frame with a pause opcode and the destination address equals

the reserved multicast address or address stored in the MAC Address

1–3 registers, then the transmitter is paused for a time equal to the

number of pause times speciﬁed in the parameter ﬁeld. Each unit of

pause time equals 512 bits (512 ns for Gigabit Ethernet). If a pause

frame is received while another packet is being transmitted, the

transmission is completed for the current packet being transmitted, and

then the transmitter is paused. If there are other packets in the transmit

FIFO their transmission is delayed until the pause timer has expired.

MAC control frames are not normally passed into the receive FIFO; they

are terminated in the receive MAC.

The controller has also incorporated some additional features to facilitate

MAC control frame operation. These features are described in the

following sections.

2.17.1 Automatic Pause Frame Generation

Pause frames can be automatically generated when either the FCNTRL

pin is asserted or the receive FIFO data exceeds the MAC control

AutoSend threshold. These automatically generated pause frames,

referred to as autogenerated pause frames, are internally generated and

transmitted over the 10-bit PHY interface. The reception of a receive

pause frame does not affect the transmission of autogenerated pause

frames. Receive pause frames only inhibit the transmission of regular

packets from the transmit FIFO.

If a packet transmission is in progress when an autogenerated pause

frame is to be transmitted, the controller waits until the transmission of

that packet has completed and then transmits the autogenerated pause

frame before any other subsequent packets in the TX FIFO are

transmitted. When the ﬁrst autogenerated pause frame begins

transmission, an internal timer starts whose value is equal to the

pause_time value in the pause frame (and obtained from “Register 20–

Flow Control 2‚" Section 4.3.17). If the FCNTRL pin is still asserted or

the MAC control frame AutoSend threshold is still exceeded when the

internal pause timer expires, another autogenerated pause frame is

transmitted. This process continues as long as FCNTRL remains

asserted or the MAC control frame AutoSend threshold is exceeded.

8101 Datasheet(PDF) 61 Page - LSI Computer Systems