Low Power V.22 bis Modem
CMX868
In all modes the transmitted bit and baud rates are the nominal rates for the selected modem type, with
an accuracy determined by the XTAL frequency accuracy, however for QAM and DPSK modes V.14
requires that Start-Stop characters can be transmitted at up to 1% overspeed (basic signalling rate range)
or 2.3% overspeed (extended signalling rate range) by deleting a Stop bit from no more than one out of
every 8 (basic range) or 4 (extended range) consecutive transmitted characters.
To accommodate the V.14 requirement the Tx Data Register has been given two C-BUS addresses, $E3
and $E4. Data should normally be written to $E3.
In QAM or DPSK Start-Stop modes if data is written to $E4 then the programmed number of Stop bits will
be reduced by one for that character. In this way the µC can delete transmitted Stop bits as needed.
In FSK Start-Stop modes, data written to $E4 will be transmitted with a 12.5% reduction in the length of
the Stop bit at the end of that character.
In all Synchronous Data modes data written to $E4 will be treated as though it had been written to $E3.
The underspeed transmission requirement of V.14 is automatically met by the CMX868 as in Start-Stop
mode it automatically inserts extra Stop bit(s) if it has to wait for new data to be loaded into the C-BUS
Tx Data Register.
The optional V.22/V.22 bis compatible data scrambler can be programmed to invert the next input bit in
the event of 64 consecutive ones appearing at its input. It uses the generating polynomial:
-14
-17
1 + x
+ x
1.5.2 FSK and QAM/DPSK Modulators
Serial data from the USART is fed via the optional scrambler to the FSK modulator if V.21, V.23, Bell
103 or Bell 202 mode has been selected or to the QAM/DPSK modulator for V.22, V.22 bis and Bell 212A
modes.
The FSK modulator generates one of two frequencies according to the transmit mode and the value of
current transmit data bit.
The QAM/DPSK modulator generates a carrier of 1200Hz (Low Band, Calling modem) or 2400Hz (High
Band, Answering modem) which is modulated at 600 symbols/sec as described below:
600bps V.22 signals are transmitted as a +90° carrier phase change for a ‘0’ bit, +270° for ‘1’.
For V.22 and Bell 212A 1200bps DPSK the transmit data stream is divided into groups of two
consecutive bits (dibits) which are encoded as a carrier phase change:
Dibit
(left-hand bit is the
Phase change
first of the pair)
00
01
11
10
+90°
0°
+270°
+180°
ã 2004 CML Microsystems Plc
12
D/868/9
CMLMICRO [ CML MICROCIRCUITS ]
