PMR Signalling Processor
CMX881
1.5.4.6
Transmitting DTMF Tones
The DTMF signals to be generated are defined in the TX TONE register ($C3). Single tones and twist
(lower frequency tone reduced by 2dB) can be enabled by setting the appropriate bit in the $C3 register
to '1'. The DTMF level is set in programming register P1.0. The DTMF tones must be transmitted on
their own within the voice band, the host µC must disable other voice band signals prior to initiating
transmission of the DTMF tones, and (if required) restore the voice band signals after the DTMF
transmission is complete. Table 8 shows the DTMF tone pairs, together with the values for programming
the ‘Tone Pair’ field of the TX TONE register.
Table 8 DTMF Tone Pairs and Corresponding Tx Programming Codes
Tone Pair
Key Pad
Low Tone
(Hz)
697
697
697
770
770
770
852
852
852
941
941
941
697
770
852
941
High Tone
(Hz)
Code (Hex)
Position
1
2
3
4
5
6
7
8
9
A
B
C
D
E
F
0
1
2
3
4
5
6
7
8
9
0
*
1209
1336
1477
1209
1336
1477
1209
1336
1477
1336
1209
1477
1633
1633
1633
1633
#
A
B
C
D
Note: Only the underlined tone is generated when the 'Single Tone' bit is enabled.
1.5.5
FFSK/MSK Data packeting
The CMX881 has a built in 15 bit CRC and 1 bit parity generator / checker to ease host processing during
transmission and reception of data packets. The CRC / parity function can be used with any length
message in both Tx and Rx modes. In Tx the host may reset, add to or send the 2 byte checksum at any
byte boundary in the data sequence. In Rx the host may reset the checking circuit at any byte boundary
and the CMX881 will indicate for each subsequent byte if the preceding bytes satisfied the CRC and
parity requirements.
Tx frame example:
Write Mode reg 2
MSK transfer flag
Read Status reg 2
Write Tx Data reg 2
Enable CRC bit 1
Tx CRC bit 1
BS
BS
FS
FS
0
1
2
3
0
1
2
…
BS
BS
FS
FS
0
1
2
3
CS
CS
0
1
2
…
Tx over air data
BS = Bit sync
FS = Frame sync
0, 1, … Data bytes CS = Checksum
Notes: 1 The Tx CRC and Enable CRC bits are controlled by writing to the Tx Data register
2 Actions requiring a C-BUS transfer
2004 CML Microsystems Plc
26
D/881/7
CMLMICRO [ CML MICROCIRCUITS ]
