
(SMDS/802.6) is selectable by HEC Coverage [bit 1] of CONFIG_3 [0x02]. The 
remaining 48 octets are payload and are taken from the FIFO interface. The 
CN8223 calculates and overwrites the CRC field to complete the 53-octet cell.
A cell-ready indication controls the cell generation process from the external 
ATM interface circuit to the cell generation block. When the ATM interface 
indicates that it has the first cell of a message ready, the cell generation block 
begins formatting a non-idle cell for transmission using the octet data and cell 
delineation control inputs at the interface. The cell generation circuitry 
automatically generates idle cells until the external FIFO indicates that another 
cell is ready for transfer. The header and payload for idle cells are programmable 
via control registers. When the next cell is ready, the host presents the data and 
cell delineation control inputs. 
2.0 Functional Description
CN8223
2.6 ATM Cell Processing
ATM Transmitter/Receiver with UTOPIA Interface
2-26
Conexant
100046D
2.6  ATM Cell Processing
The ATM cell processing block is located between the line framers and FIFO port 
blocks of the CN8223 (see 
Figure 1-3
). This functional block interfaces between 
the octet data and cell data portions of the chip. The CN8223 supports cell 
delineation via either PLCP or HEC alignment for DS1, E1, DS3, E3, E4, STS-1, 
and STS-3c/STM-1 rates. At DS3 and E3 rates, all required stuffing functions are 
supported. 
2.6.1  Cell Generation for Transmit
Cell generation refers to the formatting of 53-octet ATM cells from 48- or 
52-octet payload data from the FIFO interface for hand-off to the line framer 
transmitter. The CN8223 provides modes that generate complete cells as well as 
modes that pass entire 53- or 57-octet cells directly from the FIFO interface. Cell 
modes and other per-port controls are in the four CELL_GEN_x registers 
[0x04
–
0x07].
The generation process operates autonomously with a handshake protocol 
through the FIFO interface. Cells are forwarded automatically to the line framer 
for transmission.
When full ATM cell generation is performed, a 5-octet header is generated by 
the CN8223. The VCI and other fields in the first 4 octets come from 
microprocessor control registers. The HEC in octet 5 is calculated and inserted by 
the CN8223. HEC coverage over 4-header octets (ATM) or 3-header octets