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CHAPTER 4 ATM CELL PROCESSOR
Preliminary User’s Manual S14767EJ1V0UM00
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If indication bits for LLC or PPP are 1 in VC table, ATM Cell Processor generates LLC or PPP header and
writes it in SAR FIFO. LLC and PPP encapsulations are exclusive.
<3> 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. However, if LLC or PPP encapsulation is executed, the number of
bytes to fetch are 40 or 46 bytes ( if RAS PPP encapsulation is used, 44 bytes). When the 53rd byte of
the segment is written, SAR FIFO is updated.
<4> Calculating the CRC-32 value and the length
Each time a segment is read from system memory, the CRC-32 value is calculated for that segment and
transmitted bytes are also counted. ATM Cell Processor writes those results in VC table.
<5> Updating the VC table
Updates "Buffer Read Address" and "Remaining Bytes in Current Buffer" fields.
(h) Sending the last cell
When the L flag of the current send buffer indicates that the buffer is the last one and the value in the field
indicating the number of bytes remaining in the VC table is less than 40 bytes, the cell is the last cell of the
packet.
<1> If the current cell is the last cell of the packet, and the remaining payload data is less than 40 bytes, zero
padding and the 8 byte trailer are added. If the remaining payload data is more than 40 bytes and there
are not enough space to add an 8-byte AAL-5 trailer, ATM Cell Processor just adds zero padding to make
48 byte payload and sends a cell containing only a trailer and padding next.
<2> When the last segment of the AAL-5 PDU is read, the final CRC-32 value and the packet length are
inserted into the trailer of the AAL-5 PDU, and the contents of the first word in the VC table are inserted
into the CPCS-UU and CPI fields, thereby completing the AAL-5 trailer.
(i) ATM Cell Processor checks whether there is a subsequent packet ( checks Last Packet Info address and First
Packet Info address in Tx VC table). When a subsequent packet exists, (f) and (g) are repeated.
(j) For each packet, ATM Cell Processor stores send indication as status information in the mailbox and
generates an interrupt. Then ATM Cell Processor releases Packet Info area.
(l) V
R
4120A RISC Processor reads the mailbox and updates the read pointer of the mailbox.
4.8.2.2 Transmit queue
Tx_Ready command has to be issued in order to transmit a packet. However, V
R
4120A RISC Processor doesn’t
have to wait Tx indication before issuing next Tx_Ready command for the same VC. When V
R
4120A RISC Processor
issues Tx_Ready command before completing transmission process for the previous packet, ATM Cell Processor
builds Tx Queue for that VC.