Specifications

Si2457/Si2434/Si2415
Rev. 0.91 17
Parallel Interface
The parallel interface is an 8-bit data bus with a single
bit address. Figure 3 on page 9 shows the required
timing for the parallel interface.
If A0 = 0, the data bus represents a read/write to the
“Parallel Interface 0 (0x00)” register on page 63. If
A0 = 1, the data bus represents a read/write to the
“Parallel Interface 1 (0x01)” register on page 64).
Command Mode
Upon reset, the ISOmodem® is in command mode and
accepts “AT” commands. An outgoing modem call can
be made using the “ATDT#” (tone dial) or “ATDP#”
(pulse dial) command after the device is configured. If
the handshake is successful, the modem responds with
the response codes detailed in Table 13 on page 35 and
enters data mode.
Data Mode
The Si2457/34/15 ISOmodem is in data mode while it
has a telephone line connection to another modem or is
in the process of establishing a connection.
Data protocols are available to provide error correction
to improve reliability (V.42 and MNP2-4) and data
compression to increase throughput (V.42bis and
MNP5).
Each connection between two modems in data mode
begins with a handshaking sequence. During this
sequence, the modems determine the line speed, data
protocol, and related parameters for the data link.
Configuration through AT commands determines the
range of choices available to the modem during the
negotiation process.
Fast Connect
The Si2457/34/15 supports a fast connect mode of
operation to reduce the time of a connect sequence in
originate mode. The Fast Connect modes can be
enabled for V.21, V.22, Bell103, and V.29 modulations.
See “AN93: Modem Designer’s Guide” for details.
V.80 Synchronous Access Mode
The Si2457/34/15 supports a V.80 synchronous access
mode of operation, which operates with an
asynchronous DTE and a synchronous DCE. See
“AN93: Modem Designer’s Guide” for complete details.
Clocking/Low Power Modes
The Si2457/34/15 contains an on-chip phase-locked
loop (PLL) and clock generator. Using either a single
crystal or master clock input, the Si2457/34/15 can
generate all the internal clocks required to support the
featured modem protocols. Either a 27 MHz or
4.9152 MHz clock (3.3 V max input—see Table 5 on
page 7) on XTALI or a 4.9152 MHz crystal across XTALI
and XTALO form the master clock (±100 ppm max) for
the ISOmodem. This clock source is sent to an internal
PLL that generates all necessary internal system clocks
including the DSP clock. By default, the Si2457/34/15
assumes a 4.9152 MHz clock input. If a 27 MHz clock
on XTALI is used with the serial (UART) interface, a
pulldown resistor <
10 kΩ must be placed between DCD
(pin 23) and GND. If a 27 MHz clock on XTALI is used
with the parallel interface, a pulldown resistor <
10 kΩ
must be placed between CTS
/CS (pin 11) and GND and
AOUT/INT
and GND (see Table 8).
Using the S24 S-register, the Si2457/34/15 can be set
to automatically enter sleep mode after a pre-
programmed time of inactivity with either the DTE or the
remote modem. The sleep mode is entered after (S24)
seconds have passed since the TX FIFO has been
empty. The ISOmodem remains in the sleep state until
either a 1 to 0 transition on TXD (serial mode) or a 1 to 0
transition on CS
(parallel mode) occurs.
Additionally, the Si2457/34/15 may be placed in a
complete powerdown mode or wake-on-ring mode.
Complete powerdown is accomplished via
U65[13] (PDN). Once the PDN bit is written, the Si2457/
34/15 completely powers down and can only be
powered back on via the RESET
pin.
A clock input may be produced on the CLKOUT pin.
See “AN93: Modem Designer’s Guide” for details.
Data Compression
The modem can achieve DTE (host-to-ISOmodem)
speeds greater than the maximum DCE (modem-to-
modem) speed through the use of a data compression
protocol. The compression protocols available are the
ITU-T V.42bis and MNP5 protocols. Data compression
attempts to increase throughput by compressing the
data before actually sending it. Thus, the modem is able
to transmit more data in a given period of time.
Error Correction
The Si2457/34/15 ISOmodem can employ error
correction (reliable) protocols to ensure error-free
delivery of asynchronous data sent between the host
and the remote end. The Si2457/34/15 supports V.42
and MNP2-4 error correction protocols. V.42 (LAPM) is
most commonly used and is enabled by default.
Wire Mode
Wire mode is used to communicate with standard non-
error correcting modems. When optioned with \N3, the
Si2457/34/15 falls back to wire mode if it fails in an
attempt to negotiate a V.42 link with the remote modem.
Error correction and data compression are not active in