![](http://datasheet.mmic.net.cn/Exar-Corporation/XRT94L33IB-L_datasheet_100163/XRT94L33IB-L_204.png)
XRT94L33
xr
Rev.1.2.0.
3-CHANNEL DS3/E3/STS-1 TO STS-3/STM-1 MAPPER IC DATA SHEET
204
Transmit ATM Control – Byte 0 (Address = 0xNF03)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
HEC Byte
Invert
HEC Byte
Check
Enable
Parity Check
Enable
Discard Cell
upon Parity
Error
Odd Parity
Unused
Cell Payload
Scramble
Enable
R/W
R/O
R/W
0
X
0
X
In this mode, all ATM cells (even those with HEC byte errors) will pass through the “HEC Byte Calculation &
Insertion” block, without any flagging of HEC byte errors.
Configuring the HEC Byte Calculation & Insertion block to Regenerate (e.g., recompute and insert) the
HEC byte, prior to transmission via the “Transmit Data Path”
In addition to being capable of detecting and flagging HEC byte errors, the “HEC Byte Calculation & Insertion”
block can be configured to recompute and insert a new HEC byte into the fifth octet position within all ATM
cells that are processed by the “HEC Byte Calculation & Insertion” block. The user can implement this
configuration by writing a “1” into Bit 0 (Re-Calculate HEC Byte Enable), within the “Transmit ATM Control –
Byte 1” Register, as depicted below.
Transmit ATM Control – Byte 1 (Address = 0xNF02)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
Test Cell
Mode
Enable
Test Cell
Generator –
One Shot
GFC_Enable[3:0]
Coset
Addition
Re-
Calculate
HEC Byte
Enable
R/W
0
1
Notes:
Setting this bit-field to “1” configures the “HEC Byte Calculation & Insertion” block to unconditionally compute a new HEC
byte (based upon the value of the first four header bytes of a given ATM cell) and insert this value into the 5
th octet
position within each outbound ATM cell.
Further, this configuration setting also configures the “HEC Byte
Calculation & Insertion” block to NOT discard any ATM cells that it receives from upstream circuitry, even those cells
that it determines to contain HEC byte errors.
This bit-field is ignored if Bit 6 (HEC Byte Check Enable) within the “Transmit ATM Control – Byte 0” register (Address =
0xNF03) is set to “0”.
Other Options available via the “HEC Byte Calculation & Insertion” block
The user has the following additional options via the “HEC Byte Calculation & Insertion” block.
To modulo-2 add the Coset Polynomial to the HEC byte, prior to transmission.
To invert the HEC byte
Each of these options is discussed below.
Adding the Coset Polynomial to the HEC Byte
The “HEC Byte Calculation & Insertion” block permits the user to configure it to modulo-2 add the Coset
Polynomal (e.g., x
6 + x4 + x2 + 1) to the CRC-8 value (e.g., the HEC byte). Afterwards, this “newly computed”
HEC byte would be written back into the fifth octet position within each outbound ATM cell.