PMR Signalling Processor
CMX881
1.5.4.4
Transmitting Selcall Tones
The Selcall tone to be generated is defined in the Tx Tone register ($C3). The tone level is set in the
Programming register (P1.0). The Selcall tone must be transmitted without other signals in the voice
band, so the host µC must disable the voice path prior to initiating transmission of a Selcall tone, and
restore the voice path after the Selcall tone transmission is complete. Table 5 shows valid Selcall tones,
together with the values for programming the Selcall bits of the Tx Tone register.
Custom Selcall tone frequencies are set in the program register ($C8) P1.2-18. See section 1.6.20.2 for
programming details.
1.5.4.5
Transmitting FFSK/MSK Signals
The FFSK/MSK encoding operates in accordance with the bit settings in the Mode Control register ($C1).
When enabled the MSK modulator will immediately begin transmitting data using the settings and values
in the Tx Data and General Control register(s) and block 0 of the Programming Register. Therefore,
these registers should be programmed to the required value before transmission is enabled.
The CMX881 generates it’s own internal data clock and converts the binary data into the appropriately
phased frequencies, as shown in Figure 11 and Table 7. The binary data is taken from register $CA,
most significant bit first. The following data words must be provided over the C-BUS within certain time
limits to ensure the selected baud rate is maintained. The time limits will be dependent on the data
coding being used, see Table 9.
Rx
SIGNAL I/P
2400 BAUD
LOGIC '0'
LOGIC '1'
LOGIC '0'
LOGIC '1'
Rx
SIGNAL I/P
1200 BAUD
LOGIC '0'
LOGIC '1'
LOGIC '0'
LOGIC '1'
Figure 11 Modulating Waveforms for 1200 and 2400 Baud FFSK/MSK Signals
The table below shows the combinations of frequencies and number of cycles to represent each bit of
data, for both baud rates.
Table 7 Data Frequencies for each Baud Rate
Baud Rate
1200 baud
Data
Frequency
1200Hz
1800Hz
1200Hz
2400Hz
Number of Cycles
1
0
1
0
one
one and a half
half
2400 baud
one
Note: FFSK may be transmitted in conjunction with a CTCSS or DCS sub-audio component.
2004 CML Microsystems Plc
25
D/881/7
CMLMICRO [ CML MICROCIRCUITS ]
