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Intel
®
IXP45X and Intel
®
IXP46X Product Line of Network Processors
August 2006
Developer’s Manual
Order Number: 306262-004US
233
Ethernet MACs—Intel
®
IXP45X and Intel
®
IXP46X Product Line of Network Processors
6.1.4
Transmitting Ethernet Frames with MII Interfaces
Using processor API calls, the Intel XScale processor can request that packets be
transmitted by the xMII Interface. The Intel XScale processor prepares an Ethernet
packet to be transmitted. When the preparation is complete, the Intel XScale processor
uses the Intel-supplied API calls to inform the Ethernet NPE that a packet is ready to be
transmitted. The Ethernet NPE fetches the packet from the memory attached to the
IXP45X/IXP46X network processors and forwards the data over the NPE coprocessor
interface to the 256-byte Transmit FIFO contained in the Ethernet coprocessor.
Once the data has reached a predefined trigger point – known as the Buffer Size for
Transmit Register (TXBUFFSIZE) in the Transmit FIFO – or the End-of-Frame signal is
received, a data packet will begin to be transmitted over the MII interface. For each
frame transmitted, TXBUFFSIZE holds the minimum number of bytes that must be
contained in the transmit FIFO before the frame transmission may start. If the total
size of the frame is less than the Buffer Size for Transmit Register, the frame will be
transmitted when the end-of-frame signal is received.
When entries leave the bottom of the Transmit FIFO, the entries are 32 bits. The
settings of the Buffer Size for Transmit Register (TXBUFFSIZE), the Threshold for
Partially Full (THRESHPF), and the Threshold for Partially Empty (THRESHPE) are tightly
coupled with the code written for the NPE core. Manipulation of the values will result in
unpredictable behavior.
• The Threshold for Partially Full parameter sets a status flag going to the NPE core
when the number of entries in the Transmit or Receive FIFOs is larger than the
value programmed in this register.
• The Threshold for Partially Empty parameter sets a status flag going to the NPE
core when the number of entries in the Transmit or Receive FIFOs is smaller than
the value programmed in this register.
After the data begins leaving the FIFO, the data is sent through a converter function
that is used to convert the bits from 32-bits to byte-wide entries to supply to the
Transmit Engine. The Transmit Engine will take the bytes supplied from the converter,
Figure 31.
MDIO Read
Notes:
1.
ST (Start Bits) is a signal that is logic 0 followed by logic 1 — after a PREAMBLE stage.
2.
OC (Op Code) is a two-bit signal that informs the destination PHYs, if the current requested transaction is a read or a
write. Logic 0 followed by logic 1 indicates a write transaction is requested. Logic 1 followed by logic 0 indicates a read
transaction.
3.
TA (Turn Around) is a two-bit, turn-around time used to allow the control of the MDIO to change directions. For write
operations, the TA bits will be logic 1 followed by logic 0. For read transactions, the TA bits will be high-impedance (Z)
followed by the selected PHY driving the MDIO signal with logic 0.
4.
For write operations, the Management Interface Master will drive the MDIO signal for the duration of the access.
5.
For read operation, the Management Interface Master will drive the MDIO signal until the turn around cycle. The PHY will
drive the MDIO signal for the second bit of the turn around and the remaining 16 data bits.
MDC
MDIO
PHY ADDR
(4:0)
REG ADDR
(4:0)
MDIOCMD2
(7:0)
MDIOCMD1
(7:0)
PREAMBLE
32 consecutive 1s
ST
OC
TA
4 3 2 1 0 4 3 2 1
15 14 13 12 1110 9 8 7 6 5 4 3 2 1 0
0
B2170-01