Specifications

AN93
98 Rev. 0.9
3.4. Digital Interface
The Si2493/57/34/15/04 can be connected to a host
processor through either a serial or parallel interface.
Direct connection to the chip requires low-voltage
CMOS signal levels from the host and any other
circuitry directly interfacing with the Si2493/57/34/15/04.
The following sections describe in detail the serial and
parallel digital interface options.
3.4.1. Serial Interface/UART
The DTE rate is set by the autobaud feature after reset.
On the 24-pin package, if a pulldown resistor ≤ 10 kΩ is
placed between EESD/D2 (Si2493/57/34/15/04, pin 18)
and GND (Si2493/57/34/15/04, pin 6), the UART is
configured to 19.2 kbps 8-bit data no parity and 1 stop
bit on reset. The UART data rate is programmable from
300 bps to 307 kbps with the AT\Tn command. After the
AT\Tn command is issued, the ISOmodem echoes the
result code at the old DTE rate. After the result code is
sent, all subsequent communication is at the new DTE
rate.
The DTE baud clock is within the modem crystal
tolerance (typically ±50 ppm), except for DTE rates that
are uneven multiples of the modem clock. All DTE rates
are within the +1%/–2.5% required by the V.14
specification. Table 69 shows the ideal DTE rate, the
actual DTE rate, and the approximate error.
The UART interface synchronizes on the start bits of
incoming characters and samples the data bit field and
stop bits. The interface is designed to accommodate
character lengths of 8, 9, 10, and 11 bits giving data
fields of 6, 7, 8, or 9 bits. Data width can be set to 6, 7,
or 8 bits with the AT\Bn command. Parity can be set to
odd, even, mark, or space by the AT\Pn command in
conjunction with AT\B2 or AT\B5. Other AT\Bn settings
have no parity.
3.4.2. Autobaud
The Si2493/57/34/15/04 includes an automatic baud
rate detection feature that allows the host to start
transmitting data at any standard DTE rate from
300 bps to 307.2 kbps. This feature is enabled by
default. When autobaud is enabled, it continually
adjusts the baud rate, and the Si2493/57/34/15/04
always echoes result codes at the same baud rate as
the most recently-received character from the host.
Autobaud can be turned off using the AT commands,
\T0 through \T15 and \T17. Autobaud can be turned on
again using the AT command, \T16.
Autobaud is off when dialing, answering, and in data
mode and set to the most recently-active baud rate prior
to entering one of these states. When in autobaud
mode, autoparity is performed when either an “at” or an
“AT” is detected. Autoparity detects the following
formats: 7N1, 7N2, 7O1, 7E1, 8N1, 8E1, 8O1, and 9N1.
Note: For 7N1, the modem is programmed to 7 data bits,
mark parity, and this may be changed with the AT\P
and AT\B commands. In autobaud mode, 7N1 is prop-
erly interpreted and echoed, but the AT\P and AT\B
commands must be sent prior to dialing in order to lock
the parity and format to 7N1. Otherwise, the Si2493/57/
34/15/04 locks to 7 bits, mark parity mode (7N2).
3.4.3. Flow Control
The Si2493/57/34/15/04 supports flow control through
RTS
/CTS and XON/XOFF. RTS (request-to-send) is a
control signal from the terminal (DTE) to the modem
(DCE) indicating data may be sent from the modem to
the terminal. CTS
(clear-to-send) is a control signal from
the modem (DCE) to the terminal (DTE) indicating data
may be sent from the terminal to the modem for
transmission to the remote modem. This arrangement is
typically referred to as hardware flow control. There is a
14-character FIFO and a 1024 character elastic transmit
buffer (see Figure 16). CTS
goes inactive (high) when
the 1024 character buffer reaches 796 characters then
reasserts (low) when the buffer falls below 128
characters. There is no provision to compensate for
FIFO overflow. Data received on TXD when the FIFO is
full is lost.
Table 69. DTE Rates
Ideal DTE Rate
(bps)
Actual DTE
Rate (bps)
Approximate
Error(%)
300 300
600 600
1200 1200
2400 2400
7200 7202 0.01
9600 9600
12000 12003 0.02
14400 14400
19200 19200
38400 38400
57600 57488 0.2
115200 115651 0.4
230400 228613 0.8
245760 245760
307200 307200