Cermetek Microelectronics, Inc.
CH1788 Surface Mount PLCC 2400bps Modem
2006 Cermetek Microelectronics, Inc.
Page 8
Document No. 607-0018 Revision A6 (01/06)
INITIALIZING THE MODEM
The CH1788 must be initialized before the CH1788
will recognize commands. Initialization is a two step
activity consisting of:
Table 4.
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.
THE COMMAND FORMAT
Typical commands consist of three elements: the
attention sequence, the commands themselves, and
a terminating carriage return.
AT[commands]<CR>.
Where:
AT
represents attention sequence.
[Commands]
represents command strings.
<CR>
represents carriage return (ASCII 13
or HEX 0D).
Training the Modem.
The CH1788 must be trained
to set the DTE speed (i.e., to match its host’s speed
and parity) so that the CH1788 is able to recognize
serial asynchronous commands sent to it by the
host’s UART. The host must retrain the CH1788
each time a reset is performed.
Modem Training Command Sequence.
The
CH1788 is trained by sending it the
AT
sequence:
Enter:
AT<CR>
Result:
OK
Where:
A
and
T
may be either upper or lower case
but must be the same case.
<CR>
represents carriage return
The
AT
sequence is referred to as the attention
sequence. When received, the CH1788 will respond
with one of the following status messages, depending
on whether the CH1788 is optioned for Terse
(abbreviated) or Verbose (English) status messages.
Result:
0<CR>
(Terse)
or
Result:
<CR><LF>OK<CR><LF>
(Verbose)
Where:
<CR>
represents carriage return (ASCII 13
or HEX 0D).
<LF>
represents line feed (ASCII 10 or
HEX 0A).
After responding, the CH1788 will be in command
mode and ready to accept additional commands. An
alternate attention sequence
A/
may be sent. This
command causes the previously entered command
string specified with the
AT
prefix to be executed.
AT Command Set.
The available commands are
summarized in Table 5. Note that each command
may have several options and the options may be
influenced by various S-Register values. It is
recommended that the designer review Cermetek
publication CH1788 AT Commands and S-Registers
Reference Guide.
AT Command Structure.
When issuing commands
to the CH1788, the backspace character-control-H
(ASCII 8 or HEX 08) may be used to perform
character editing.
Multiple commands may be placed on the command
line as long as the command line does not exceed 63
characters, excluding the letters
AT
. By way of
example, the command string below instructs the
CH1788 to configure itself to suppress character
echoing when in command mode
E0
and then put
itself in the answer state
A
.
Enter:
ATE0A<CR>
Result:
OK
IMPORTANT NOTE
Allow a minimum of 125msec delay between
subsequent
AT
commands.
MODEM STATES
The CH1788 is in either of two states: command
mode or data mode. The CH1788 will only respond
to
AT
commands when it is in command mode.
When the modem is idle, by default it is in command
mode.
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