Digital Line to POTS Interface
CMX605
1.5.7 DTMF Tone Decoder
This block is enabled or disabled by bit 5 of the SETUP register. If disabled, bit 0 to bit 5 of the STATUS
Register are set to ‘0’ and no interrupts are generated. When enabled (set to ‘1’), a status change of the
decoder will generate an interrupt and bit 5 of the STATUS Register will be set to ‘1’. The validity of the
data is indicated by bit 4 of the STATUS Register. The decode truth table is shown below:
STATUS Register Bits 0 - 3
DTMF Tone Pairs
Bit 3
(D3)
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
Bit 2
(D2)
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
Bit 1
(D1)
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
Bit 0
(D0)
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
Lower Frequency Upper Frequency
Keypad
Legend
(Hz)
941
697
697
697
770
770
770
852
852
852
941
941
941
697
770
852
(Hz)
1633
1209
1336
1477
1209
1336
1477
1209
1336
1477
1336
1209
1477
1633
1633
1633
D
1
2
3
4
5
6
7
8
9
0
*
#
A
B
C
1
1
1
1
A status change of the decoder and the generation of an interrupt will occur both when a tone is first
decoded and also when a tone, which was previously present, is no longer decoded. In the latter case,
Bit 4 of the STATUS Register will be set to “0” to indicate that no tone was detected.
ã 2001 Consumer Microcircuits Limited
12
D/605/6
CMLMICRO [ CML MICROCIRCUITS ]
