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
299
TXC/RXC[2] is used to indicate the validity of the upper 8-bits of data. It should be the same as TXC/RXC[0]
except for the final data in a packet that has an odd number of bytes.
16-B
IT
E
NCODED
P
ACKET
FIFO
The 16-bit encoded Packet FIFO mode is shown in
. The TXC[2:0] signals indicate the status of the
data they accompany. The encoding is chosen to allow easy conversion to and from POS-PHY Level 3
signalling, which is a wider slower packet bus. The flow control signal runs in the opposite direction to the data.
shows two packets being sent. In the first the sender is unable to supply data for one cycle and
removed the valid signal. In the second packet the receiver asserts the flow control signal for a cycle requesting
a pause from the sender. The transmitter will see the flow control request on the next rising edge of the clock
and will send data for 2-4 cycles before suspending transmission one cycle later. The interface will assert its
flow control output when receive channel flow control is invoked either by one of the DMA channels having
fewer descriptors than the low watermark or by an explicit processor request.
Figure 65: 16-Bit Encoded Packet FIFO
Table 174: Codes for 16-Bit GMII Style Packet FIFO
TXC/RXC[2:0]
Data Not Valid
000
Start of Packet, 2 Bytes Valid
000->101 at start of cycle
Middle of Packet, 2 Bytes Valid
101
End of Packet, 1 Byte Valid
001->000 at end of cycle
End of Packet, 2 Bytes Valid
101->000 at end of cycle
Error
111, bit [1] held set until 000
TXC/RXC[2:0]
TXD/RXD[7:0]
TXD/RXD[15:8]
TCLKO/RCLK
TXFC/RXFC
000
101 111 000 111 100 111 111 010
000
101 111 111 000
111 110
SOP
EOP
55
SOP
55
EOP
FC
Table 175: Codes for 16-Bit Encoded Bypass Mode
TXC/RXC[2:0]
Data Not Valid
000
Reserved
001
End of Packet, 1 Byte Valid
010
End of Packet With Error
011
Reserved
100