FRS Signalling Processor
CMX882
transmitted last. The CMX882 is able to encode and transmit either of the two DCS modulation modes
defined by TIA/EIA-603 and described in Table 4.
To signal the end of the DCS transmission, the host should set the special sub-audio bits in the Audio &
CTCSS Control register ($C2) to enable the DCS turn off tone for 150ms to 200ms. After this time period
has elapsed the host should then disable DCS in the Mode register ($C1). Do not enable CTCSS in the
Mode Control ($C1) register when transmitting the DCS turn off tone. To summarize, detection of DCS
turn off tone requires the CTCSS decoder to be enabled, whereas generation of the DCS turn off tone
requires the CTCSS encoder to be disabled.
1.5.4.4
Transmitting In-band Tones
The In-band tone to be generated is defined in the Tx Tone register ($C3). The tone level is set in the
Programming register (P1.4). The In-band 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 In-band tone, and
restore the voice path after the In-band tone transmission is complete. Table 6 shows valid In-band
tones, together with the values for programming the In-band bits of the Tx Tone register.
Custom In-band tone frequencies are set in the program register ($C8) P1.6-9. See section 1.6.20.2 for
programming details.
1.5.4.5
Transmitting FFSK/MSK Signals
The FFSK/MSK encoding operates in accordance with the bit settings in the Modem Control register
($C7). 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 CMX882 generates it’s own internal data clock and converts the binary data into the appropriately
phased frequencies, as shown in Figure 11 and Table 8. The binary data is taken from registers $CA and
$CB, 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
2004 CML Microsystems Plc
28
D/882/7
CMLMICRO [ CML MICROCIRCUITS ]
