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CH1788ET

Surface Mount PLCC 2400bps Modem

Cermetek Microelectronics 

Cermetek Microelectronics, Inc.

CALL PROGRESS MONITORING

Call Progress Monitoring allows a call to be monitored

during the dialing sequence. This feature becomes

active when the CH1788 goes Off-Hook and

automatically disables when DCD is detected. It is

also inactive if a connection attempt should fail and

the CH1788 goes back On-Hook. To enable this

feature, the DCD line must be activated with the

ATC1 command.

Setup. This feature requires additional external

circuitry. See Schematic 2. Although the circuit in

Schematic 2 uses a variable resistor to adjust the

speaker volume, a fixed value resistor may be

substituted.

Alternate Setup. Control of the speaker can

alternately be performed under command control. To

implement this alternate control method, modify the

circuit in Schematic 2 by connecting resistor R5 to A1

instead of DCD. This allows register S101 to be used

to turn the speaker on or off. Set S101 to 1 to turn

the circuit on, set S101 to 0 to turn the circuit off.

FAST CONNECT HANDSHAKING

The CH1788 supports a fast handshaking (often

called “quick connect”) feature. This feature is

controlled via the L3 command.

Typically, the initial handshaking sequence takes

between 10 and 20 seconds to complete. When fast

handshaking is selected, the handshaking sequence

will take between 4-8 seconds. Fast Connect

Handshaking will work with any compatible modem.

LONG SPACE DISCONNECT

The long space disconnect signal is used to force the

CH1788 to release the PSTN line and go back On-

Hook.

Setup. To enable this feature, bit 7 of register S110

most be set to 1. See the details of register S110.

VOICE INJECTION

It is possible to share the CH1788’s internal DAA with

external applications. This allows one PSTN line to

be shared between an external voice application and

the CH1788 modem. This feature is controlled via

register S101.

Setup. This feature requires additional external

circuitry. See Schematic 3. This application

implements Voice Injection via the S1 and S2 pins

and requires a two to four wire converter and a relay

circuit. Set S101 to 1 to activate Voice Inject and set

S101 to 0 to deactivate Voice Inject.

CH1788 Surface Mount PLCC 2400bps Modem

Operation. The two to four wire converter circuit

provides for simultaneous voice in and voice out (with

respect to ground). This circuit must be capacitor

coupled (i.e., C7) to the CH1788 and isolated from

ground (i.e., C6). Voice/data switching is controlled

by setting register S101 appropriately.

During Voice Injection, the voice signal must not

exceed 2.187 Vpp (i.e., zero dB) per FCC maximum

permissible limits. When sending data, the voice

circuit must be fully disconnected from the line (i.e.,

Voice Inject deactivated) or data corruption will occur.

IMPORTANT NOTE

When utilizing the Voice Injection circuit, it is

necessary to submit the end product containing the

CH1788 Voice Injection feature to an FCC approved

testing facility for re-approval of the end product per

FCC Part 68 requirements.

MODEM CONTROL

The CH1788 may be controlled with ASCII AT

commands sequences. The commands are sent to

the CH1788 serially via the TXD pin. After execution

of a received command string, the CH1788 returns a

serial status message on RXD.

INITIALIZING THE MODEM

The CH1788 must be initialized before the CH1788

will recognize commands. Initialization is a two step

activity consisting of:

1. Hard Reset. This is accomplished by applying a

hardware reset pulse to pin RST for 50 ms. The

internal reset may be used by cycling the power.

The modem should not be accessed for 500

msec after a reset has occurred.

IMPORTANT NOTE

Delay the sending commands to the CH1788 for a

period of 500 msec after reset has been completed

to allow the CH1788 time to properly reconfigure.

2. Initial CH1788 Training. This will establish the

word bit length and type for communication with

the host. The word size should not be more than

10 bits including start and stop bits and is typically:

1 start bit, 1 stop bit, 8 data bits and with no parity.

The CH1788 is trained by the host with the

AT<CR>. The serial data rate (i.e., the DTE

speed) at which the host sends the AT<CR>

command initializes the modem DTE Speed.

3. The CH1788 S registers must be re-configured to

their required settings after a soft reset, hardware

reset, or power loss.

2003 Cermetek Microelectronics, Inc.

Page 7

Document No. 607-0018 Revision A2 (07/03)

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