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CH1786

CH1786 Family of Ultra Small 2400bps Modems

Unspecified 

Cermetek Microelectronics, Inc.

normal operation the CH1786 decodes analog signals

from the telephone line that are in the millivolt range.

Steps must be taken by the user to guarantee that

power supply noise on all supply lines, including

ground, does not exceed 50 mV peak to peak. Any

frequency between 20 kHz and 150 kHz must be less

than 500 micro volts peak. If necessary, use

dedicated power and ground planes. Failure to

provide such operating conditions could cause the

CH1786 to malfunction or to function erratically.

The CH1786 requires a single +5V ±5% supply. It is

recommended that by-pass capacitors be placed on

the power supply as close to the modem’s supply

input as practical. It’s recommended a 10 µF

Tantalum capacitor in parallel with a 0.01 µF ceramic

capacitor be used.

Voice/DTMF Tone Injection Port. The CH1786

provides two pins to allow the user to share the

telephone line interface associated with the modem

for voice and DTMF applications. Figure 4 contains a

schematic indicating one possible configuration for

voice/ tone utilization.

WARNING: The CH1786 has been FCC Part 68

approved as a data modem only. Utilization of

the Voice/DTMF Tone Port requires further

registration. FCC will require that the system,

including the CH1786 and the handset or DTMF

transceiver, adhere to Part 68 rules. Voice/Tone

Injection is not active on CH1786NH.

Typically, voice communication would precede data

communications. For this case, the following

commands should be used to configure the CH1786

(Refer to Figure 4):

CH1786 Family of Ultra Small 2400bps Modems

Enter: ATS7=255<CR> Disables data-carrier time

Result: OK

out

Enter: ATS10=255<CR> Disables lost-carrier time

Result: OK

out

The following command sequences illustrate

implementation of common Voice/Tone Port

applications.

1. To answer a voice call.

(a) Enter: ATH1<CR>

Result: OK

(b) Drive V.INJ. HIGH to activate relay.

(c) Begin voice conversation.

2. To switch to data mode.

(a) Drive V.INJ. LOW

(b) At the originate modem:

Enter: ATX1<CR>

Result: OK

3. To disconnect (hang-up) a voice call or a data

call.

(a) Enter: ATH<CR>

Result: OK

4. To place a call to 1234567 using the CH1786’s

DTMF tone generator.

(a) Enter: ATDT1234567;C0<CR>

Result: OK

(b) Drive V.INJ. HIGH

MODEM CONTROL

The CH1786 modem may be controlled by sending

serial ASCII command sequences. The commands

are sent to the modem serially on the TXD pin. After

execution of a received command, the modem

returns a serial status message that can be read on

pin RXD. This message indicates command

completion status. Refer to Table 3 for a complete

list of status messages.

Figure 4. Voice/Tone Injection.

Enter: ATS0=<CR>

Result: OK

Disables auto-answer

2003 Cermetek Microelectronics, Inc.

INITIALIZING THE MODEM

Before commands may be sent to the CH1786, the

CH1786 must be initialized. Initialization is a two step

activity consisting of:

1. Hard Reset. This is accomplished by applying a

hardware reset pulse to pin RST or by switching

the power off and then back on.

2. Initial modem training. The CH1786 must be

trained to the host ’s speed (2400,1200,300bps)

and parity (odd, even, mark, space or none).

These activities are briefly described below.

Page 4

Document No. 607-0004 Revision L1 (06/03)

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