Low Power V.22 bis Modem
CMX868
1.6
Application Notes
1.6.1 V.22 bis Calling Modem Application
This section describes how the CMX868 can be used in a V.22 bis Calling modem application, employing
V.25 automatic answering and the V.22 bis recommended handshake sequence. This attempts to
establish a 2400bps connection but may fall back to 1200bps if the answering modem is not capable of
2400bps operation.
1. Ensure that the CMX868 is powered up. Set the Tx Mode Register to DTMF/Tones mode (set to ‘No
Tone’ at this time), and the Rx Mode Register to Call Progress Detect mode.
2. Connect the line (go off hook) then dial the required number using the DTMF generator, monitoring for
call progress signals (dial tone, busy, etc). Change to Answer Tone Detect mode.
3. On detection of the 2100Hz answer tone wait for it to end then wait for the 2225Hz answer tone
detector to respond. (The ‘2225Hz’ answer tone detector will recognise unscrambled binary 1s at
1200bps High Band as well as 2225Hz). When unscrambled binary 1s or 2225Hz have been received
for 155ms set a 456ms timer.
4. When the 456ms timer expires check that the 2225Hz or unscrambled 1s is still being received, then
set the Tx Mode Register for V.22 1200bps Low Band transmission of S1 signal and set a 100ms
timer. Also set the Rx Mode register to V.22 1200bps High Band receive, descrambler enabled and
Rx USART disabled.
5. When the 100ms timer expires set the Tx Mode Register for V.22 1200bps Low Band transmission of
scrambled 1s (continuous 1s with the scrambler enabled) and look for received S1 signal.
6. If the S1 signal is not detected within 270ms then go to step 14 as the answering modem is not
capable of 2400bps operation.
7. If S1 signal is detected wait for it to end then set a 450ms timer.
8. When the 450ms timer expires set the Rx Mode Register to V.22 bis 2400bps High Band (this will
begin 16-way decisions) with the auto-equaliser and descrambler enabled. Start to monitor for Rx
scrambled 1s. Set a 150ms timer.
9. Once 32 consecutive bits of received scrambled 1s at 2400bps have been detected, enable the Rx
USART.
10. When the 150ms timer expires set the Tx Mode Register for V.22 bis 2400bps scrambled 1s, set a
200ms timer.
11. Load the Tx Data Register with the first data to be transmitted.
12. When the 200ms timer expires set the Tx Mode Register for Start-Stop or Synchronous transmission
of data from the Tx Data Buffer. This will start transmission of the data loaded in step 11.
13. A 2400bps data connection has now been established.
14. If the S1 signal had not been detected within 270ms after step 5 then monitor for scrambled 1s at
1200bps.
15. When scrambled 1s (at 1200bps) have been received for 270ms enable the Rx USART, set a 765ms
timer and load the Tx Data Register with the first data to be transmitted.
ã 2004 CML Microsystems Plc
36
D/868/9
CMLMICRO [ CML MICROCIRCUITS ]
