Cermetek Microelectronics, Inc.

CH1786 Family of Ultra Small 2400bps Modems

2003 Cermetek Microelectronics, Inc.

Page 8

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

Table 2. CH1786 Pin Descriptions Continuation

PIN

NAME

TYPE

FUNCTION

18

HS

O

SPEED INDICATION. HIGH speed select output. A low on this pin indicates the

modem is operating at 2400bps.

19

VCC

-

POWER SUPPLY. 5 volts + 5% Note: Noise should be less than 50mV peak to

peak.

20

GND

-

GROUND. Note: Noise should be less than 50mV peak to peak.

21

RST

1

I

RESET. Active HIGH. This input must be asserted HIGH for at least 10 ms to reset

the modem. RESET is then returned LOW for normal operation. If no system reset

is available, let this pin float to enable internal reset.

22

NC

-

No connection required. For CH179X or CH2056 compatibility, connect to GND or

use as an active LOW input for request to send (RTS) functionality for CH179X/2056

products.

Note:

(1) If VCC has a slow power up ramp time, the internal reset may be ineffective.

(2) Do not drive LED’s directly from any output. Use Buffers.

Table 3. Summary of Result Codes and Meanings.

Result Codes

Status Message

Meaning

0

OK

Command Executed

1

CONNECT

Carrier detected at 300bps

2

RING

Ring Detected

3

NO CARRIER

Did not detect carrier

4

ERROR

Entry error

5

CONNECT 1200

Carrier detected at 1200bps

6

NO DIAL TONE

Off-hook, but no response after 5 seconds

7

BUSY

Busy signal detected

10

CONNECT 2400

Carrier detected at 2400bps

The escape sequence consists of a “guard time” (a

period where no characters are sent to the modem),

followed by 3 escape characters, followed by another

guard time. The guard time is set by register S12.

The escape character code is set in register S2. At

power-up, the guard time is set to 1 second minimum,

and the escape character is set as “+.” There is no

minimum delay between characters but the maximum

delay is set by register S12.

The CH1786 will stay off-hook with its carrier on after

the escape sequence is received. It will return an OK

status message when it is ready to accept

commands. The CH1786 may be put into data mode

by issuing the command ATO<CR>.

AT COMMAND APPLICATIONS

Dialing A Phone Number. The Dial command takes

the form Dn, where n is a string of characters. In the

simplest form, n will be only the digits of the phone

number to be dialed.

Example: Dial number.

Enter:

ATD1234567<CR>