Si2401
Preliminary Rev. 0.9
13
Serial Interface
The Si2401 has a universal asynchronous receiver/
transmitter (UART) serial interface compatible with
standard microcontroller serial interfaces. After powerup
or reset, the speed of the serial (Data Terminal
Equipment—DTE) interface is set by default to
2400 bps with the 8-bit, no parity, and one-stop bit (8N1)
format described below.
The serial interface DTE rate can be modified by writing
SE0[2:0] (SD) with the value corresponding to the
desired DTE rate. (See Table 8.) This is accomplished
with the command, ATSE0=xx, where xx is the
hexadecimal value of the SE0 register.
Immediately after the ATSE0=xx string is sent, the host
UART must be reprogrammed to the new DTE rate in
order to communicate with the Si2401.
The carriage return character following the ATSE0=xx
string must be sent at the new DTE rate to observe the
“O” response code. See Table 12 on page 21 for the
response code summary.
Configurations and Data Rates
The Si2401 can be configured to any of the Bell and
CCITT operation modes listed in Table 9. When
configured for V.22bis, the modem connects at
1200 bps if the far end modem is configured for V.22.
This device also supports SIA and other protocols for
the security industry. Table 7 provides the modulation
method, carrier frequencies, data rate, baud rate, and
notes on standard compliance for each modem
configuration of the Si2401. Table 9 shows example
register settings (S07) for some of the modem
configurations.
As shown in Figure 3, 8-bit and 9-bit data modes refer to
the DTE format over the UART. Line data formats are
configured through registers S07 (MF1) and S15 (MLC).
If the number of bits specified by the format differs from
the number of bits specified by the DCE data
communications equipment or line (DTE) format, the
MSBs are either dropped or bit-stuffed, as appropriate.
For example, if the DTE format is 9 data bits (9N1), and
the line data format is 8 data bits (8N1), the MSB from
the DTE is dropped as the 9-bit word is passed from the
DTE side to the DCE (line) side. In this case, the
dropped ninth bit can then be used as an escape
mechanism. However, if the DTE format is 8N1, and the
line data format is 9N1, an MSB equal to 0 is added to
the 8-bit word as it is passed from the DTE side to the
DCE side.
The Si2401 UART does not continuously check for stop
bits on the incoming digital data. Therefore, if the TXD
pin is not high, the RXD pin may echo meaningless
characters to the host UART. This requires the host
UART to flush its receiver FIFO upon initialization.
Figure 3. Link and Line Data Formats
Table 8. DTE Rates
DTE Rate (bps)
SE0[2:0] (SD)
300
000
1200
001
2400
010
9600
011
19200
100
38400
101
115200
110
307200
111
Table 9. Modem Configuration Examples
(S07[7] (HDEN) = 0, S07[6] (BD) = 0)
Modem Protocol
Register S07 Values
V.22bis
0x06
V.22
0x02
V.21
0x03
Bell 212A
0x00
Bell 103
0x01
V.23 (1200 tx, 75 rx)
0x14
V.23 (75 tx, 1200 rx)
0x24
V.23 (600 tx, 75 rx)
0x10
V.23 (75 tx, 600 rx)
0x20
DTE Interface
Data Rate: SE0[2:0] (SD)
Data Format: SE0[3] (ND)
DCE (Line) Interface
Data Rate: S07 (MF1)
Data Format: S15 (MLC)
Si3010
Si2401
RJ11
TXD
RXD
