LAN89218 datasheet(107/148 Pages) MICROCHIP

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DS60001255B-page 107

LAN89218

5.4.8

FLOW—FLOW CONTROL REGISTER

This register controls the generation and reception of the Control (Pause command) frames by the MAC’s flow control

block. The control frame fields are selected as specified in the 802.3x Specification and the Pause-Time value from

this register is used in the “Pause Time” field of the control frame. In full-duplex mode the FCBSY bit is set until the

control frame is transferred onto the cable. In half-duplex mode FCBSY is set while back pressure is being asserted.

The host has to make sure that the Busy bit is cleared before writing the register. The Pass Control Frame bit

(FCPASS) does not affect the sending of the frames, including Control Frames, to the Application Interface. The Flow

Control Enable (FCEN) bit enables the receive portion of the Flow Control block.

This register is used in conjunction with the AFC_CFG register in the Slave CSRs to configure flow control. Software

flow control is initiated using the AFC_CFG register.

Offset:

Attribute:

R/W

Default Value:

00000000h

Size:

32 bits

Note:

The LAN89218 will not transmit pause frames or assert back pressure if the transmitter is disabled.

Bits

Description

31:16

Pause Time (FCPT). This field indicates the value to be used in the PAUSE TIME field in the control

frame. This field must be initialized before full-duplex automatic flow control is enabled.

15:3

Reserved

Pass Control Frames (FCPASS). When set, the MAC sets the Packet Filter bit in the Receive packet

status to indicate to the Application that a valid Pause frame has been received. The Application must

accept or discard a received frame based on the Packet Filter control bit. The MAC receives, decodes

and performs the Pause function when a valid Pause frame is received in Full-Duplex mode and when

flow control is enabled (FCE bit set). When reset, the MAC resets the Packet Filter bit in the Receive

packet status.

The MAC always passes the data of all frames it receives (including Flow Control frames) to the Appli-

cation. Frames that do not pass Address filtering, as well as frames with errors, are passed to the Appli-

cation. The Application must discard or retain the received frame’s data based on the received frame’s

STATUS field. Filtering modes (Promiscuous mode, for example) take precedence over the FCPASS

bit.

Flow Control Enable (FCEN). When set, enables the MAC Flow Control function. The MAC decodes

all incoming frames for control frames; if it receives a valid control frame (PAUSE command), it disables

the transmitter for a specified time (Decoded pause time x slot time). When reset, the MAC flow control

function is disabled; the MAC does not decode frames for control frames.

Note:

Flow Control is applicable when the MAC is set in Full Duplex Mode. In Half-Duplex mode,

this bit enables the Backpressure function to control the flow of received frames to the MAC.

Flow Control Busy (FCBSY). This bit is set high whenever a pause frame or back pressure is being

transmitted. This bit should read logical 0 before writing to the Flow Control (FLOW) register. During a

transfer of Control Frame, this bit continues to be set, signifying that a frame transmission is in prog-

ress. After the PAUSE control frame’s transmission is complete, the MAC resets to 0.

Note:

When writing this register the FCBSY bit must always be zero.

Note:

Applications must always write a zero to this bit

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