SMSC LAN9312 - Manual (Page 189)

High Performance Two Port 10/100 Managed Ethernet Switch with 32-Bit Non-PCI CPU Interface
Datasheet
SMSC LAN9312 189 Revision 1.4 (08-19-08)
DATASHEET
14.2.2.9 Host MAC Automatic Flow Control Configuration Register (AFC_CFG)
This read/write register configures the mechanism that controls the automatic and software-initiated
transmission of pause frames and back pressure from the Host MAC to the switch fabric. This register
is used in conjunction with the
Host MAC Flow Control Register (HMAC_FLOW) in the Host MAC CSR
space. Pause frames and backpressure are sent to the switch fabric to stop it from sending packets
to the Host MAC. Network data into the switch fabric is affected only if the switch fabric buffering fills.
Note: The Host MAC will not transmit pause frames or assert back pressure if the transmitter is
disabled. This register controls only the Host MAC flow control and not the Switch Engine
MAC’s flow control.
Refer to section Section 9.2, "Flow Control" for additional information.
Offset: 0ACh Size: 32 bits
BITS DESCRIPTION TYPE DEFAULT
31:24 RESERVED RO -
23:16 Automatic Flow Control High Level (AFC_HI)
This field specifies, in multiples of 64 bytes, the level at which flow control
will trigger. When this limit is reached, the chip will apply back pressure or
will transmit a pause frame as programmed in bits [3:0] of this register.
During full-duplex operation only a single pause frame is transmitted when
this level is reached. The pause time transmitted in this frame is
programmed in the FCPT field of the
Host MAC Flow Control Register
(HMAC_FLOW) in the Host MAC CSR space.
During half-duplex operation each incoming frame that matches the criteria
in bits [3:0] of this register will be jammed for the period set in the
BACK_DUR field.
Note: This level is also used for hard-wired flow control when
HW_FC_EN is set in the
Port 0(Host MAC) Manual Flow Control
Register (MANUAL_FC_MII) .
R/W 00h
15:8 Automatic Flow Control Low Level (AFC_LO)
This field specifies, in multiples of 64 bytes, the level at which a pause frame
is transmitted with a pause time setting of zero. When the amount of data
in the RX Data FIFO falls below this level the pause frame is transmitted. A
pause time value of zero instructs the other transmitting device to
immediately resume transmission. The zero time pause frame will only be
transmitted if the RX Data FIFO had reached the AFC_HI level and a pause
frame was sent. A zero pause time frame is sent whenever automatic flow
control in enabled in bits [3:0] of this register.
Note: When automatic flow control is enabled the AFC_LO setting must
always be less than the AFC_HI setting.
Note: This level is also used for hard-wired flow control when
HW_FC_EN is set in the
Port 0(Host MAC) Manual Flow Control
Register (MANUAL_FC_MII) .
R/W 00h
7:4 Backpressure Duration (BACK_DUR)
When the Host MAC automatically asserts back pressure, it will be asserted
for this period of time. In full-duplex mode, this field has no function and is
not used. Please refer to
Table 14.2 , describing Backpressure Duration bit
mapping for more information.
R/W 0h