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SI2401

V.22BIS ISOMODEM® WITH LOW-COST DAA

Silicon Laboratories 

Si2401/Si3008

Table 10. 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

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.

TXD

RXD

Si2401

Si3008

RJ11

DTE Interface

Data Rate: SE0[2:0] (SD)

Data Format: SE0[3] (ND)

DCE (Line) Interface

Data Rate: S07 (MF1)

Data Format: S15 (MLC)

Figure 3. Link and Line Data Formats

4.2.1. Command/Data Mode

Upon reset, the modem is in command mode and

accepts AT-style 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 “c”, “d”, or “v” string and enters data mode. (The

byte following the “c”, “d”, or “v” is the first data byte.) At

this point, AT-style commands are not accepted. There

are three methods that may be used to return the

Si2401 to command mode:

Use the ESC pin—To program the GPIO3 pin to

function as an ESCAPE input, set GPIO3

SE2[5:4] = 11. In this setting, a positive edge

detected on this pin returns the modem to command

mode. The “ATO” string can be used to reenter data

mode.

Use 9-bit data mode—If 9-bit data format with

escape is programmed, a 1 detected on bit 9 returns

the modem to command mode. (See Figure 2 on

page 8.) This is enabled by setting SE0[3] (ND) = 1

and S15[0] (NBE) = 1. The ATO string can be used

to reenter data mode. Ninth bit escape does not

work in the security modes.

Use “+++”—The escape sequence is a sequence of

three escape characters that are set in S-register

S0F (“+” characters by default). If the ISOmodem®

chipset detects the “+++” sequence and detects no

activity on the UART before or after the “+++”

sequence for a time period set by S-register S10, it

returns to command mode. To disable this escape

sequence, set S-register S10=FF. To remove the

time-dependent behavior, set S-register S10=00.

Whether using an escape method or not, when the

carrier is lost, the modem automatically returns to

command mode and reports “N”.

4.2.2. 8-Bit Data Mode (8N1)

8-bit data mode is the default mode after powerup or

reset and is set by SE0[3] (ND) = 0b. It is asynchronous,

full duplex, and uses a total of 10 bits including a start

bit (logic 0), eight data bits, and a stop bit (logic 1). Data

received from the remote modem is transferred from the

Si2401 to the host on the RXD pin. Data transfer to the

host begins when the Si2401 asserts a logic 0 start bit

on RXD. Data is shifted out of the Si2401 LSB first at

the DTE rate determined by the SE0[2:0] (SD) setting

and terminates with a stop bit. Data from the host for

transmission to the remote modem is shifted to the

Si2401 on TXD beginning with a start bit, LSB first at the

DTE rate determined by the SE0[2:0] setting and

terminates with a stop bit.

Rev. 1.1

13

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