Cermetek Microelectronics, Inc.
CH1788 Surface Mount PLCC 2400bps Modem
2006 Cermetek Microelectronics, Inc.
Page 4
Document No. 607-0018 Revision A6 (01/06)
DTMF TONE GENERATION
The CH1788 is capable of generating DTMF tones
without external hardware. DTMF tone generation is
useful for signaling and is supplemental to the DTMF
dialing capability.
Setup.
Generation will only occur when the CH1788
is Off-Hook, in the command mode and prior to
carrier generation.
DTMF Encoding.
By default, the CH1788 generates
DTMF tones at a 70 msec on/off cycle. The cycle can
be modified with register S11.
Issuing an
ATDT
command string with the desired
DTMF digits and terminating the command string with
a semicolon (
;
) will cause the CH1788 to immediately
enter command mode after dialing. Alternatively, the
command mode may be entered from the data mode
by issuing the escape sequence (
+++
) to the
CH1788.
Enter:
ATDT1234567890*#;<CR>
Result: Sends the DTMF tones and returns to
command mode. DO not exceed 63
characters on the command line.
DTMF Tones may also be sent using external
circuitry via the voice injection port. See Schematic
2.
CALLER ID
The CH1788 will detect Caller ID (i.e., CID)
information. CID data, when supplied by the local
telephone service provider, is present between the
first and second ring signals. Valid CID data is
preceded by the “CID” designation. This feature is
enabled via register S95.
Setup.
CID requires additional external circuitry. An
RC network must be added to the CH1788 V+ and V-
pins. See Schematic 4 for details. Register S95
must be set to 8 (the default value) to receive the CID
information. Other caller ID options are available.
See the details of register S95.
Operation.
Using the circuit in Schematic 4, monitor
the RI pin for the period between the first and second
ring signal. The decoded CID data will appear on
RXD.
LINE IN USE
The Line In Use (i.e., LIU) feature allows the CH1788
to signal the host when a shared PSTN line is in use.
When in use, the LINE IN USE status message will
appear on RXD and the CH1788 will NOT go Off-
Hook. This feature is enabled via register S110.
Setup.
The LIU feature requires additional external
circuitry. See Schematic 5 and Schematic 6.
Additionally, register S110 must be set to 243.
PARALLEL PICK-UP
The Parallel Pick-Up (i.e., PIU) feature allows the
termination of a CH1788 modem call in progress, and
then allows a standard phone instrument sharing the
same PSTN line to seize the line and obtain a dial
tone. When terminating the modem call, DCD will go
high, if enabled. The PPU-V status message appears
on RXD.
Setup.
The Parallel Pick-Up feature requires external
circuitry. See Schematic 5.
Operation.
The Parallel Pick-Up circuit detects the
incremental change in Off-Hook voltage on the PSTN
line. If a parallel connected phone instrument is Off-
Hook, the CH1788 will not attempt to go Off-Hook. If
the CH1788 is already Off-Hook and a parallel
connected phone instrument goes Off-Hook, the
CH1788 will terminate the calling progress and allow
the sharing device to seize the line.
Failure To Detect.
The detection circuit in the
CH1788 may not detect the Off-Hook condition for
select phone instruments because some phone
instruments may not change the Off-Hook voltage
sufficiently to allow detection.
There is also the possibility that two devices going
Off-Hook simultaneously may defeat the PPU
detection circuitry.
Recommendation.
To avoid a Failure To Detect
condition, use of a prioritized scheme such as that
employed in the 911 Priority Pick-Up application
circuit described below is strongly suggested.
IMPORTANT NOTE
The Parallel Pick-Up feature should only be used in
non-critical applications and never used as a
substitute for the 911 Priority Pick-Up feature.
911 PRIORITY PICK-UP
The 911 Priority Pick-Up (i.e., 911 PPU) feature
allows the CH1788 to give Off-Hook priority to any
telephone sharing the PSTN line with the CH1788.
Setup.
The 911 Priority Pick-Up feature requires
external circuitry. See Schematic 6.
Operation.
If a parallel connected phone instrument
goes Off-Hook while the CH1788 is Off-Hook, the
CH1788 will immediately disconnect to give priority to
the phone instrument.
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