CX82100 Home Network Processor Data Sheet
7.7
TMAC Architecture
Before the host requests transmission of a frame, it constructs the data (LLC data) field of
the frame in memory. The TMAC appends a preamble and a SFD to the beginning of the
frame. Using information from the descriptor, TMAC also appends a PAD at the end of
the data field of sufficient length to ensure that the transmitted frame length satisfies a
minimum frame. TMAC then attempts to avoid contention with other traffic on the
medium by monitoring the carrier sense signal provided by the Ethernet PHY and
deferring to passing traffic. When the medium is clear, frame transmission is initiated
(after a brief interframe delay to provide recovery time for other devices on the medium).
The TMAC then provides data nibbles to the EPHY on the MII.
The EPHY monitors the medium and generates the collision detect signal, which, in the
contention-free case, remains off for the duration of the frame. When transmission has
completed without contention, the TMAC informs the host by writing status into the
memory and awaits the next request.
7.7.1
Transmit Frame Structure
Before the TMAC can start transmitting a frame containing the LLC data, a transmit
message structure as shown in Figure 7-5 must be constructed by the host in ARM's
memory. TMAC reads data from the memory (via DMA channel 1 or 3) to transmit via
MII and writes data into the memory to update the status. The ARM host is the master for
TMAC transmit operations and serves data to the TMAC via the APB. It is also the host's
task to assemble Ethernet frames to be sent out by the TMAC. The transmit descriptor
(TDES), the transmit status (TSTAT), and the sequence of transmitter DMA operation
are described below. Note that the qword count which is to be loaded into the
DMAC_{x}_CNT1 register should always include the first qword reserved for the
transmit status.
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