5-2
Mindspeed Technologies
28229-DSH-001-B
Transmission Convergence Block
CX28224/5/9 Data Sheet
5.1.1
HEC Generation
In normal operation, the CX2822x calculates the HEC for the four header bytes of
each cell coming from the ATM layer. It then adds the HEC coset (55 hex, by ATM
standards) and inserts the result in octet 5 of the outgoing cell. HEC calculation can be
disabled by setting bit 7 of CGEN (0x08) to a 1. When HEC is disabled, the CX2822x
leaves the contents of the HEC field unchanged and transmits whatever data is placed
in that field by the ATM layer.
The HEC coset is used to maintain a value other than zero in the HEC field. If the first
four bytes in the header are zero, the HEC derived from these bytes is also zero. When
this occurs and there are strings of zeros in the data, the receiver cannot determine cell
boundaries. Therefore, it is recommended that the value 55 hex be added to the HEC
before transmission. To enable the HEC coset on the transmit side, set bit 6 in register
CGEN (0x08) to one. To enable the receive HEC coset, set bit 5 in register CVAL
(0x0C) to one.
5.2
ATM Cell Receiver
The ATM cell receiver performs cell delineation on incoming data cells by searching
for the position of a valid HEC field within the cell. The HEC coset can be either
active or inactive; this is determined in bit 5 in the CVAL (0x0C) register.
Figure 5-1. Details of the TC Block (Bits 7 and 6 in ATMINTFC, address 0x202)
UTOPIA
Level 2
Interface
IMA
Transmit
UTOPIA
Level 2
atmUTxClk
atmURxClk
Microprocessor
Interface
Loopback
Control
One Second Interface
Interrupt Control
ATM Cell Transmitter
TC Transmit Port
IMA
Receive
UTOPIA
Level 2
MicroInt*
MicroAddr[10:0]
MicroData[7:0]
Control Lines
SPTxData
SPTxClk
SPTxSync
atmUTxClAv
atmUTxEnb*
atmUTxSOC
atmUTxPrty
atmUTxAddr[4:0]
atmURxClAv
atmURxEnb*
atmURxPrty
atmURxAddr[4:0]
TCK
8kHzIn
OneSecIO
TRST*
TMS
TDI
TDO
SPRxData
SPRxClkI
SPRxSync
atmURxData[15:0]
atmUTxData[15:0]
TC Receive Port
This segment is replicated for Ports 0 - 7
JTAG Controller
Status and Control
StatOut[0:1]
ATM Cell Receiver
Cell
Alignment
Cell Validation
VPI/VCI Screening
4-cell
FIFO
4-cell
FIFO
Framer
(Line)
Interface
500027_063
atmURxSOC