
XRT7245
DS3 UNI FOR ATM
á
PRELIMINARY
REV. 1.03
170
A “1” in this bit-field enables the Cell Scrambler. Con-
versely, a “0” in this bit-field disables the Cell-Scrambler.
Upon power up or reset, the Cell Scrambler function
will be enabled. Therefore, the user must write a “0”
to this bit in order to disable cell scrambling.
6.2.2.3
The first four bits in the first header byte of each cell
are allocated for carrying “Generic Flow Control” (GFC)
GFC Nibble-Field Serial Input Port
information. The user can externally insert his/her own
values for the GFC nibble-field into each outbound
cell, via a serial input port. The user will activate this
serial input port (the “Transmit GFC-Nibble-field” Serial
Input port) by writing a “1” to bit 3 (GFC Insert Enable)
of the “TxCP Control” Register, as depicted below.
Once the user has activated the “Transmit GFC Nibble-
field” Serial input port, it will accept the 4 bit GFC val-
ue via the TxGFC pin during each cell processing pe-
riod. The TxGFC serial input port will be expecting
the bits of the GFC nibble-field in descending order
(MSB first). The GFC bits are clocked into the serial
input port via the rising edge of the clock signal,
TxG-FCClk. Since these four bits must be provided
for each cell; TxGFCClk will provide four clock edges
during each cell processing period. The “Transmit
GFC Nibble-field” Serial input port will also provide a
“framing pulse” in the form of the TxGFCMSB output
pin pulsing “high”. This output pin will pulse “high”
when the Transmit Cell Processor is ready to receive
the MSB (most significant bit) of the GFC field.
Figure 45 presents a timing diagram illustrating the
role of each of these signals during GFC insertion.
6.2.2.4 OAM Cell Processing
The UNI chip provides on-chip RAM space for the
storage of the complete contents (header and pay-
load) of an OAM cell. This RAM space is known as
the “Transmit OAM Cell” buffer (consisting of 54
bytes) and is located at 136h through 16Bh in the
UNI address space. Therefore, in order to “l(fā)oad” the
TxCP Control Register (Address = 60h)
B
IT
7
B
IT
6
B
IT
5
B
IT
4
B
IT
3
B
IT
2
B
IT
1
B
IT
0
Scrambler
Enable
Coset
Enable
HEC Insert
Enable
TDPChk
Pattern
GFC Insert
Enable
TDPErr
Interrupt
Enable
Idle Cell
HEC CalEn
TDPErr
Interrupt
Status
R/W
R/W
R/W
R/W
R/W
R/W
R/W
RUR
1
1
1
1
x
0
1
0
F
IGURE
45. B
EHAVIOR
OF
T
X
GFC, T
X
GFCC
LK
,
AND
T
X
GFCMSB
DURING
GFC
INSERTION
INTO
THE
“O
UTBOUND
” C
ELL
TxGFCClk
TxGFCMSB
t14
TxGFC
t16
BIT 3
BIT 2
BIT 1
BIT 0
t17
t13
t15