Functional Description
For the support of LAPD the ISAC-S contains an HDLC transceiver which is responsible for
flag generation/recognition, bit stuffing mechanism, CRC check and address recognition.
A powerful FIFO structure with two 64-byte pools for transmit and receive directions and an
intelligent FIFO controller permit flexible transfer of protocol data units to and from the µC
system.
2.8.1
Message Transfer Modes
The HDLC controller can be programmed to operate in various modes, which are different in
the treatment of the HDLC frame in the receive direction. Thus, the receive data flow and the
address recognition features can be programmed in a flexible way, which satisfies different
system requirements.
In the auto mode the ISAC-S handles elements of procedure of the LAPD (S and I frames)
according to CCITT I.441 fully autonomously.
For the address recognition the ISAC-S contains four programmable registers for individual
SAPI and TEI values SAP1-2 and TEI1-2, plus two fixed values for "group" SAPI and TEI,
SAPG and TEIG.
There are 5 different operating modes which can be set via the MODE register (addr. 22 ):
H
Auto mode (MDS2, MDS1 = 00)
Characteristics:
– Full address recognition (1 or 2 bytes).
– Normal (mod 8) or extended (mod 128) control field format
– Automatic processing of numbered frames of an HDLC procedure
If a 2-byte address field is selected, the high address byte is compared with the fixed value
FE or FC (group address) as well as with two individually programmable values in SAP1
H
H
and SAP2 registers. According to the ISDN LAPD protocol, bit 1 of the high byte address will
be interpreted as command/response bit (C/R) dependent on the setting of the CRI bit in
SAP1, and will be excluded from the address comparison.
Similarly, the low address byte is compared with the fixed value FF (group TEI) and two
H
compare values programmed in special registers (TEI1, TEI2). A valid address will be
recognized in case the high and low byte of the address field match one of the compare values.
The ISAC-S can be called (addressed) with the following address combinations:
– SAP1/TEI1
– SAP1/FF
H
– SAP2/TEI2
– SAP2/FF
H
– FE (FC )/TEI1
H
H
– FE (FC )/TEI2
H
H
– FE (FC )/FF
H
H
H
Semiconductor Group
108
INFINEON [ Infineon ]
