CH1794 データシートの表示（PDF） - Cermetek Microelectronics

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CH1794

HIGH SPEED Modem and FAX FAMILY

Cermetek Microelectronics 

Cermetek Microelectronics, Inc.

CONNECTING TO THE HOST UART

Since a modem communicates data serially and most

host products handle data in a parallel format, a UART

is needed to make parallel-to-serial and serial-to-

parallel translations.

SUPPORTED FEATURES

AT Command Set. The HS Modem supports the AT

Command set with extensions and can operate with

industry standard communication software. The AT

Commands are compatible with TR302.2/88-08006.

High Speed Modem and FAX Family

Modem is powered down. The internal blocking diode

provides supply isolation.

The pins of the HS Modem will be in an undefined

state when powered is switched off. This must be

taken into consideration to assure that no unwanted

signals are presented to the HS Modem during power

down.

Serial Host Interface. The HS Modem family

operates in asynchronous mode and supports a 5V,

RS232- like interface called V.24 (EIA-232-E). The HS

Modem supports a full RS-232C/V.24 serial interface.

Signal levels are TTL rather than RS-232C level

compatible, which allows direct connection of the HS

Modem to the host’s UART without level translating

circuitry. See Table 3 for a complete set of pin

descriptions.

The modem is controlled by sending serial commands

over TXD and by monitoring serial status messages

returned on RXD. All other serial interface lines may

be utilized for the convenience of a particular

application but are not required by the modem.

Unused outputs from the HS Modem should be left

unconnected. Unused inputs should be tied to the

proper logic level. See Table 3.

Speaker Interface. The SPK output reflects the

receiver analog input and provides a signal that can be

used to monitor call progress. The SPK signal can

drive a 300-ohm load directly. Typically, the SPK

signal is input into an audio power amplifier and the

amplifier, in turn, drives the speaker coil. The speaker

is activated with the ATM command. The speaker

volume is adjusted using the ATLn command; where n

is 0, 1, 2, or 3. Increasing numbers correspond to

higher volume. Figure 3 indicates one method of

driving an external 8-ohm speaker.

Zero Power Mode. If an application calls for zero

power during standby periods, the power may be

switched off using external circuitry and then reapplied

when the HS Modem’s RI pin becomes active. The

HS Modem can thus be powered down to zero.

Referring to Figure 2, the controller activates the relay

switch, supplying power (+5V) to the HS Modem. In

this configuration, the ring signal is used to “wake up”

the HS Modem when the HS Modem is in the power

down state.

Figure 2. Zero Power Operation.

Power Supply. The HS Modem module is a complex

set of sensitive subsystems. During the course of

normal operation, the HS Modem 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 25mV peak to peak. If

necessary, use dedicated power and ground planes.

Failure to provide such operating conditions could

cause the HS Modem to malfunction or to function

erratically.

The HS Modem family requires +5V ±5% supply. It is

recommended that bypass capacitors be placed on the

power supply as close to the HS Modem’s supply input

pins as practical. It is recommended that a 10 µF or

greater tantalum capacitor in parallel with a 0.1 µF

ceramic capacitor be used. The user must also

provide shielding from nearby sources of EMI, such as

relays, motors and other power supplies.

Automatic Speed/Format Sensing. The HS Modem

supports both Command and Data modes of

operation.

The internal structure of the RI pin consists of an open

collector opto transistor with a 30k ohm pull-up through

a blocking diode to 5V. The user should apply his own

pull-up resistor to an active 5V source when the HS

The HS Modem can automatically determine the

speed and format of the data sent from the DTE over

the serial interface; and can sense the following broad

range of speeds:

2003 Cermetek Microelectronics, Inc.

Page 3

Document No. 607-0003 Revision K3 (09/03)

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