BCM1250/BCM1125/BCM1125H
User Manual
10/21/02
B r o a d c o m C o r p o r a t i o n
Page
298
Section 9: Ethernet MACs
Document
1250_1125-UM100CB-R
16-B
IT
P
ACKET
FIFO O
PERATION
The 16-bit Packet FIFO mode provides an interface that can run bidirectionally at OC-48 data rates. It can
externally be converted into a POS-PHY level 3 interface using a simple gearbox PLD or FPGA. Alternatively,
it may be connected directly to an ASIC.
On the BCM1250, there are two Packet FIFO interfaces. On the BCM1125/H there is one. Interface F0 (F on
the BCM1125/H) replaces the Ethernet MACs E0 and E1, and reuses pins from both. It uses the DMA
controller associated with interface E0. On the BCM1250 Packet interface F1 reuses pins from all three
Ethernet MACs (E0, E1 and E2) and uses the DMA engine associated with E2. It is possible to run the F0
packet interface along with the E2 Ethernet interface.
The receive filtering options are described in section
Section: “Packet FIFO Interfaces” on page 292
. If an
unframed data stream is being received software must ensure that the
mac_chup
and
mac_chlo
tables select
the same DMA channel at all indices.
Packets transmitted in any of the Packet FIFO modes can have a CRC-32 appended by setting the append
CRC bit in the transmit packet descriptor. Packets received have their final four bytes checked for a valid CRC-
32 and if the bypass_fcs_chk bit is set in the
mac_cfg
register the status bit in the receive descriptor will be
set accordingly (if the bypass_fcs_chk bit is clear the status word will never flag a CRC error). If the CRC-32
is used on the receive side there must be at least three clock cycles between the end of packet and the start
of the next packet. If this inter-frame gap is not provided then the CRC checker will give UNPREDICTABLE
results.
The 16 bit Packet FIFO mode uses three control signals that accompany the data and, in encoded mode, a
flow control signal in the reverse direction. The 16 bit wide data path is treated as two bytes where the data on
bits [7:0] is earlier in the packet (and therefore will be DMAed to/from a lower memory address) than the data
on [15:8]. The last 16 bit half-word of a packet that is an odd number of bytes long will have valid data on bits
[7:0] and an UNPREDICTABLE value on bits [15:8].
16-B
IT
GMII S
TYLE
P
ACKET
FIFO
The 16-bit GMII style of Packet FIFO interface is shown in
. The packet data is framed by the TXC[0]
or RXC[0] signal. The first byte that has it active is marked as the start of a packet and the last byte that has it
active is the end of the packet, all bytes between are valid and part of the packet. The TXC[1] or RXC[1] signal
can be used to signal an error whenever the frame signal is active, it should be asserted when the error is
detected and held high until the frame signal falls.
Figure 64: 16-Bit GMII Style Packet FIFO
TXC/RXC[0]
TXC/RXC[1]
TXC/RXC[2]
TXD/RXD[7:0]
TXD/RXD[15:8]
TCLKO/RCLK
SOP
EOP
55
SOP
55
EOP