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CHAPTER 4 ATM CELL PROCESSOR
Preliminary User’s Manual S14767EJ1V0UM00
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4.8.2 Transmission function
ATM Cell Processor has two transmission operation mode. One is ATM mode and another is IPoA mode. In ATM
mode, V
R
4120A RISC Processor sets the VC information in VC table prior to the every packet transmission, and issue
Tx_Ready command with VC number in order to transmit packet belongs to the VC. In IPoA mode, V
R
4120A RISC
Processor also has to sets the VC information in VC table at first, and also sets the mapping between IP flow and VC.
Once V
R
4120A RISC Processor sets those information, V
R
4120A RISC Processor only sets the transmitting data in
SDRAM and issue Tx_Ready command with packet start address. ATM Cell Processor finds out TCP/IP header from
the packet and chooses corresponding VC automatically.
The transmitting data structure is described in Section. 4.5.1.
4.8.2.1 Transmission procedure
(1) ATM mode
(a) Setting transmitting data
Before transmitting a packet, V
R
4120A RISC Processor stores transmitting data in SDRAM and sets the packet
descriptor.
(b) Opening the send channel
If V
R
4120A RISC Processor needs a new channel for transmission, V
R
4120A RISC Processor issues
Open_Channel command. When V
R
4120A RISC Processor issues the command, ATM Cell Processor assigns
the new block from the VC Table pool in Work RAM and reports its start address to V
R
4120A RISC Processor
using a command indication. V
R
4120A RISC Processor sets the assigned block as a send VC address.
(c) Setting the send VC table
The 16-word block assigned from the VC Table pool in Work RAM is set as the send VC table for each VC.
(d) Issuing the Tx_Ready command -> making preparations for transmission of the first cell
When V
R
4120A RISC Processor issues Tx_Ready command, ATM Cell Processor sets the Packet Info
Structure in Work RAM, fetch the packet descriptor and store it in the area. ATM Cell Processor checks
Transmit Queue if any packet is waiting for transmission. If Transmit Queue is not empty, ATM Cell Processor
adds the Packet Info structure at the end of the queue. If no packet is waiting in the queue, ATM Cell
Processor also schedules next transmission time with “current time plus 1”.
(e) Sending a single cell (called when hit by the scheduler)
<1> Generating a header
A header is generated from Word1 in the VC table and written into SAR FIFO. "00H" is inserted into the
GFC field of the header.
<2> Sending a segment data from SDRAM to SARFIFO
ATM Cell Processor reads a transmitting segment (48-byte payload data of the cell) from SDRAM and set
it in SAR FIFO using Scatter/Gather DMA. The starting address of the segment is indicated by the "Buffer
Read Address" field in the VC table. When the 53rd byte of the segment is written, SAR FIFO is updated.
<3> Calculating the CRC-32 value and the length
Each time a segment is read from SDRAM, the CRC-32 value is calculated for that segment and
transmitted bytes are also counted. ATM Cell Processor writes those results in VC table.
<4> Updating the VC table
Updates "Buffer Read Address" and "Remaining Bytes in Current Buffer" fields.
(f) Sending the last cell