V23 / Bell 202 Modem
CMX624
Bell 202 mode (bit 7 of SETUP register = ‘1’):
FSK MODE Reg
Bit 1
Bit 0
0
1
1
x
0
1
FSK / DTMF Modulator block output
(Bit 7 of TX TONES = ‘0’)
Disabled (o/p held at V
DD
/ 2)
150bps FSK
1200bps FSK
FSK Signal Frequency
‘0’ (Space)
‘1’ (Mark)
-
487Hz
2200Hz
-
387Hz
1200Hz
When bit 7 of the TX TONES Register is set to ‘1’, the block generates DTMF tone pairs or single tones
from the DTMF range as shown in the table below. Bit 6 of the TX TONES Register is then used to enable
or disable the block’s output to the Tx filter.
TX DATA Register Bits
0-3
D3
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
1.5.7
D2
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
D1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
D0
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
DTMF Tone Pairs
(TX TONES Register Bit 4 = ‘0’)
Lower
Upper
Keypad
Frequency (Hz)
Frequency (Hz)
Legend
941
1633
D
697
1209
1
697
1336
2
697
1477
3
770
1209
4
770
1336
5
770
1477
6
852
1209
7
852
1336
8
852
1477
9
941
1336
0
941
1209
*
941
1477
#
697
1633
A
770
1633
B
852
1633
C
Single Tone
(Bit 4 = ‘1’)
Single Tone
Frequency (Hz)
1633
1209
1336
1477
1209
1336
1477
1209
852
852
941
941
941
697
770
852
Transmit Filter
This stage attenuates out of band signals present at the output of the FSK/DTMF modulator and also
includes a programmable 3dB level switch, selected by bit 2 of the FSK MODE Register.
The nominal output levels at the TOP pin when V
DD
= 5.0V are as shown below.
FSK MODE
Register bit 2
0 (low level)
1 (high level)
0dB = 775mVrms
FSK Signal
-6dB
-3dB
DTMF Tone
(Low group)
-5dB
-2dB
DTMF Tone
(High group)
-3dB
0dB
These levels are proportional to V
DD
, and the actual transmit signal levels present on the 2- or 4-wire line
will depend on the external circuitry as described in Section 1.6.1. Using the external components
recommended in Section 1.6.1 for a nominal FSK transmit level of -9dBm, DTMF tone levels of -8dBm and
-6dBm, then the out of band energy sent to the line will be within the limits shown in Figure 5 for both FSK
and DTMF signals.
2003 CML Microsystems Plc
9
D/624/7
CMLMICRO [ CML MICROCIRCUITS ]
