Specifications

Si2404
Rev. 0.91 15
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 59. If
A0 = 1, the data bus represents a read/write to the
“Parallel Interface 1 (0x01)” register on page 60).
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 32 and
enters data mode.
Data Mode
The Si2404 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 Si2404 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, and Bell103 modulations. See “AN93:
Modem Designer’s Guide” for details.
V.80 Synchronous Access Mode
The Si2404 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 Si2404 contains an on-chip phase-locked loop
(PLL) and clock generator. Using either a single crystal
or master clock input, the Si2404 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 Si2404 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 Si2404 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 Si2404 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 Si2404 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 Si2404 ISOmodem can employ error correction
(reliable) protocols to ensure error-free delivery of
asynchronous data sent between the host and the
remote end. The Si2404 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
Si2404 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 wire
mode.