
XRT74L74
4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.1.1
PRELIMINARY
120
Upon power up or reset, the Cell Scrambler function
will be enabled. Therefore, a “0” must be written to
this bit in order to disable cell scrambling.
3.2.2.3
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 their own
values for the GFC nibble-field into each outbound
cell, via a serial input port. This serial input port (the
“Transmit GFC-Nibble-field” Serial Input port) will be
activated by writing a “1” to bit 3 (GFC Insert Enable)
of the “TxCP Control” Register, as depicted below.
Once the “Transmit GFC Nibble-field” Serial input port
is activated, it will accept the 4 bit GFC value via the
TxGFC pin during each cell processing period. 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, TxGFCClk. Since
these four bits must be provided for each cell; TxG-
FCClk 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 sig-
nificant bit) of the GFC field. Figure 17 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 OAM
cell into the “Transmit OAM Cell” buffer, the local μP
must write this data into this address location within
the UNI IC, via the Microprocessor Interface. After-
wards, whenever the OAM cell is to be transmitted,
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
17. 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