CH1787 데이터 시트보기 (PDF) - Cermetek Microelectronics

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CH1787

Small Footprint Hardware Controllable 2400bps Modem

Cermetek Microelectronics 

Cermetek Microelectronics, Inc.

The CH1787 matches the host’s parity when it returns

status messages to the host. During a data connection,

however, the CH1787 passes parity through without

interpretation or alteration.

CH1787 Small Footprint Hardware Controllable 2400bps Modem

POWER SUPPLY

The CH1787 module is a complex sub-system that may

be treated as any other component. Special attention

should be paid to the power supply connections. The

CH1787 decodes analog signals from the telephone

line that are in the millivolt range. All though the

CH1787 is designed to withstand significantly induced

power supply noise, there is a limit. Steps must be

taken to guarantee that power supply noise on all

supply lines, including ground, does not exceed 50 mV

peak to peak. Any frequency between 20kHz and 150

kHz must be less than 50 mV peak. If necessary, use

dedicated power and ground planes. Failure to provide

such operating conditions could cause the CH1787 to

malfunction.

The CH1787 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 is recommended that a 10µF Electrolytic

capacitor in parallel with a 0.01µF ceramic capacitor be

used.

IMPORTANT NOTE

The CH1787 has been FCC Part 68 approved as a

data modem. Utilization of the Voice/Tone Port

requires further registration. FCC will require that

the system, including the CH1787 and the handset

or DTMF transceiver, adhere to Part 68 rules.

MODEM CONTROL

The CH1787 may be controlled by sending serial

ASCII command sequences on TXD (Pin 16). After

execution of the command, the CH1787 returns a

serial status message on RXD (Pin 31).

Initializing the Modem. Before commands may be

sent to the CH1787, the modem must be initialized.

This consists of two events: 1) after power-up, a

hardware reset pulse must be applied to the modem,

and 2) the modem must be trained to the host’s speed

(2400, 1200, 300bps) and parity (odd, even, mark,

space or none).

Power-up Reset. After applying power to the modem,

an internally generated reset pulse is created. The

user can also reset the modem externally by applying a

high-going reset pulse to RST for at least 10ms after

the +5V power supply has stabilized. Delay sending

commands to CH1787 for 100-200ms after power up.

Figure 3.

Voice/Tone Injection.

Training the Modem. The modem must be trained to

match the host’s speed and parity so that it is able to

recognize serial asynchronous commands sent to it by

the host UART. The host must retrain the modem each

time a reset signal is applied on RST or after a RESET

serial command. The modem is trained by sending it

the following three-character sequence.

Enter: AT<CR>

Where: A and T must be upper case or lower

case <CR> represents carriage return

The modem will respond with one of the following

status messages, depending on whether it is optioned

for abbreviated (terse) or English (verbose) status

messages.

Response: 0<CR> terse

<CR><LF>OK<CR><LF> verbose

Where: <CR> represents carriage return (ASCII 13)

<LF> represents line feed (ASCII 13)

The CH1787 may be retrained at any time when in idle.

Another attention sequence A/ is much like the AT

sequence except it repeats the previously entered

command specified with an AT prefix. When given, it

must also be in upper case ASCII. No carriage return

is needed.

THE COMMAND FORMAT

Typical commands consist of three elements: the

attention sequence, the commands themselves, and a

terminating carriage return.

2002 Cermetek Microelectronics, Inc.

Page 3

Document No. 607-0002 Revision B (03/02)

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