User Manual
BCM1250/BCM1125/BCM1125H
10/21/02
B r o a d c o m C o r p o r a t i o n
Document
1250_1125-UM100CB-R
Section 9: Ethernet MACs Page
295
8-B
IT
E
NCODED
P
ACKET
FIFO
Encoded mode, shown in
, uses the control lines to signal the four states that can be associated with
the data. This mode should be used for packet data where there can be invalid bytes between the start and
end of packet marks.
shows the encodings used.
Figure 61: 8-Bit Packet FIFO Encoded Style
In this mode flow control is provided in each direction. Transmit flow control is input on the COL pin (normally
the collision sense from the PHY), when this is asserted the interface will stop sending data (and drive the data
not valid control code) two rising clock edges later. The interface will assert the MDIO pin (normally the
management interface to the PHY) as a receive flow control output if (a) the CPU sets the force flow control
configuration bit; (b) the number of descriptors on a receive channel falls below the low watermark and
automatic flow control is enabled for that channel; or (c) there are less than 4 64 bit words left free in the receive
FIFO. If the device the other end of the link honours the flow control request within a few clock times data will
never be lost on the link.
In this mode a transmit FIFO underflow during transmission of a packet causes bubbles to be inserted in the
data stream (the data not valid code will appear during a packet) but transmission will not be aborted. (In all
other modes a transmit FIFO underrun causes the packet to be aborted).
TXEN/RXDV
TXER/RXER
TXD/RXD[7:0]
TCLKO/RCLK
55
SOP
EOP
Table 171: Codes for 8-Bit Encoded Bypass Mode
TXEN / RXDV
TXER / RXER
Valid Data, Start of Packet
1
0
Valid Data
1
1
Valid Data, End of Packet
0
1
Data Not Valid
0
0