SMSC LAN1198 7.3 Flow Control Function

LAN9118 Family Programmer Reference Guide

Revision 1.0 (12-14-09) 40 SMSC AN 12.12

APPLICATION NOTE

RX_DATA_FIFO must be read an appropriate number of DWORDS to purge the miniscule packet.

This should only happen if the MAC_CR is configured to PASSBAD packets ( == ‘1’).

7.3 Flow Control Function

The flow control function monitors space available in the RX Data FIFO. When the threshold set by

the in AFC_CFG:AFC_HI field is reached, the Flow Controller will instruct the MAC to send a pause

frame (in full-duplex mode), or to jam incoming frames (in half-duplex mode). Logic residing in the

MAC is then responsible for inserting the PAUSE frame or generating backpressure at the appropriate

time.

When operating in half-duplex mode with the transmitter and flow control enabled, the controller will

automatically assert backpressure based on the setting of AFC_CFG register. When the RX Data FIFO

reaches the level set by AFC_HI the Backpressure Duration Timer will start. The controller will assert

backpressure for any received frames as defined by the FCANY, FCADD, FCMULT and FCBRD

control bits. This continues until the Backpressure Duration Timer reaches the time specified by the

BACK_DUR field. After the BACK_DUR time period has elapsed the receiver will accept one frame.

If, after receiving one RX frame, the RX Data FIFO is still above the threshold set by AFC_LO, the

controller will again start the Backpressure duration timer and will assert backpressure for subsequent

frames repeating the process described here until the RX Data FIFO level drops below the AFC_LO

setting. If the RX Data FIFO drops below AFC_LO before the Backpressure Duration Timer has

expired, the timer will immediately reset and backpressure will not be asserted until the RX Data FIFO

level exceeds AFC_HI.

If the AFC_LO value is set to all ones (0xff) and the AFC_HI value is set to all zeros (0x00), the flow

controller will assert backpressure for received frames as if AFC_HI threshold is always exceeded. The

driver can use this mechanism to direct flow control by enabling and disabling the FCANY, FCADD,

FCMULT and FCBRD bits.The BACK_DUR field can be used to avoid excessive collisions, and the

FCANY bit can be used to eliminate late collisions, unless the LAN topography is known before hand.

When operating in full-duplex mode with the transmitter and flow control enabled, the controller will

automatically transmit pause frames based on the settings of AFC_CFG and FLOW registers. When

the RX Data FIFO reaches the level set by AFC_HI the controller will transmit a pause frame. The

pause time field that is transmitted is set in the Pause Time (FCPT) field of the FLOW register. When

the RX Data FIFO drops below AFC_LO the controller will automatically transmit a pause frame with

a pause time of zero. The controller will only send another pause frame when the RX Data FIFO level

falls below AFC_LO and then exceeds AFC_HI again.