SI2414 SILABS [Silicon Laboratories], SI2414 Datasheet - Page 20

• Manufacturer Part Number: SI2414
• Manufacturer: SILABS [Silicon Laboratories]

Si2456/Si2433/Si2414
parameter can be negotiated rather than making
specific assignments.
The host can cause the Si2456/33/14 to enter Data
mode and initiate dialing by issuing an AT command to
dial.
Fast Connect
The Si2456/33/14 supports a Fast Connect mode of
operation to reduce the time of a connect sequence in
originate mode. The Fast Connect modes are enabled
via U7A. Each of the stages (answer tone detect time,
unscrambled ones detect time, etc.) in the connect
sequence may be shortened. The amount that each of
these is shortened when in Fast Connect mode
depends on the modulation. (See Table 9.)
Transparent HDLC/
Synchronous DCE Mode
The Si2456/33/14 also supports a "transparent HDLC"
mode
asynchronous DTE and a synchronous DCE. The
Si2456/33/14 performs HDLC frame packing and
unpacking, frame opening and closing, flag generation
and detection, CRC computation and checking, and 0
insertion and deletion. To use this mode, the DTE rate
must be greater than the DCE rate, and either CTS or
/Q and /S must be used. (See Table 10 on page 21.)
On the transmit side, if no data is received on TXD, the
Si2456/33/14 continually transmits HDLC flags at the
DCE. As soon as there are 10 characters sent into the
20
Bell103, V.21
V.22, Bell212
V.22, Bell212
Protocol
Bell212,
V.22bis
V.22bis
Table 9. Fast Connect/Transparent HDLC
V.22,
All
of
operation,
Normal, Asynchronous
Normal, Transparent
Transparent HDLC
Transparent HDLC
Asynchronous
Asynchronous
Fast connect,
Fast connect,
Fast connect,
HDLC
DCE
which
operates
U7A = 0002
U7A = 0001
U7A = 0001
U7A = 0003
U7A = 0002
&H6, 7, 8
Register
Settings
&H9, 10
&H7, 8
&H7, 8
with
&Hn
&H6
\N0
\N0
\N0
\N0
\N0
Rev. 0.9
an
transmit buffer, the Si2456/33/14 begins an HDLC frame
at the DCE. The reason for this 10-character “headstart”
is to reduce the likelihood of an underrun once the HDLC
frame has begun at the DCE. As long as the host
continues to send data, the Si2456/33/14 continues to
zero insert, update the CRC value, and send data within
an HDLC frame. To properly end the frame, the host must
send a /Zn (see Table 10) indicating to the Si2456/33/14
the end of the frame. Once the Si2456/33/14 encounters
the /Zn, it computes and sends the final CRC and begins
transmitting HDLC flags. If an HDLC frame is smaller
than the 10-character “head start”, the HDLC frame is
started at the DCE upon receipt of the /Zn character. The
/Tn metacharacter is sent to the host to provide an
indication that an HDLC frame was sent successfully.
The “n” in the /Zn and /Tn is a host-defined tag that can
be used to track multiple HDLC frames. To facilitate
transmit flow control, the modem sends the /S and /Q
metacharacters to the host. If the transmit buffer (512
bytes) is three-quarters full, the /S metacharacter is sent
to the host. The host must then stop transmitting. When
the transmit buffer empties down to half full, the /Q
metacharacter is sent to the host to indicate that it is okay
to begin transmitting again. If a transmit underrun occurs,
the current frame is aborted, and a /Un is sent to the host.
All data from the underrun to the receipt of the /Zn
metacharacter is discarded by the modem. A design goal
of the host software should be to eliminate any
occurrence of the /Un metacharacter.
Because the “/” is an escape character, the host must
send a “//” when a “/” appears in the transmit data stream.
The Si2456/33/14 removes one “/” for each instance of “//
” that appears on TXD.
On the receive side, as long as HDLC flags are received
by the Si2456/33/14 at the DCE, it does not pass data out
RXD. Once the first non-flag word is detected, the
Si2456/33/14 performs zero deletion, calculates the CRC
value, and passes the data out RXD. The Si2456/33/14
continues in this manner until detecting the HDLC flags,
which indicate the end of the frame. At this point, the
HDLC frame is complete, and the Si2456/33/14
calculates the final CRC and compares it to the CRC
value received in the frame. If the CRC matches, the
Si2456/33/14 passes /G to the host. If the CRC does not
match, the Si2456/33/14 passes /B to the host.
Because the / is an escape character, the Si2456/33/14
sends a // when a / appears in the receive data stream.
The host must remove one / for each instance of // that
appears on RXD. Table 10 on page 21 lists additional
escape characters that are used to control the flow of
data between the Si2456/33/14 and the host in the
"transparent HDLC" mode.