Baseband Processor For Mixed Mode Land Mobile Radio
CMX880
Transmitting FFSK/MSK Signals
The FFSK/MSK transmit operates continuously in a free-flow mode, so preamble and frame sync need to be
programmed as normal data bytes. The CMX880 generates the data clock and converts the binary data stream
into the appropriately phased frequencies, as shown in Table 8 and Figure 11. The binary data stream is taken
from a 16-bit data buffer, most significant bit first. The next 16-bit data word must be provided over the “C-BUS”
(or FSB) within 13.2ms (at 1200baud), or 6.6ms (at 2400baud) of notification (interrupt) of start of transmission
of the previous data word, to ensure the selected baud rate is maintained.
The FFSK/MSK signal can be pre-emphasis filtered, by setting the required filter options in the ‘TRANSMIT
MODE CONTROL’ register ($C1). FFSK/MSK modes can be transmitted in conjunction with a sub-audio tone or
code.
Transmit Mode Associated Control and Status
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Reset.
Microphone select.
Microphone gain.
Microphone destination - internal; “C-BUS”; FSB.
Modulators drive source -
direct from microphone, (no external processing);
from “C-BUS”;
from FSB.
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Transmit mode select:
Voice band (select analogue channel width, 12.5kHz or 25kHz)
Selcall
DTMF
C4FM - voice or data
FFSK/MSK @ 1200 baud.
FFSK/MSK @ 2400 baud.
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Signal levels.
C4FM unit +1 symbol level value.
Modulator and audio output amplifiers control - independent enable and gain control.
Modulators and audio outputs - ramp all outputs to/from bias (1) prior to switching between outputs or (2) at
beginning or end of a transmission.
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Define output ramp rate.
Enable CTCSS Tone generation and add to voice band signal.
Enable DCS Code generation and add to voice band signal.
Transmit Enable
ö mutually exclusive modes.
ø
Transmit Data Buffer, 1x16-bit word.
Data transmission start flag - indicates that new data is required within the time specified in Table 17
Maximum Data Transfer Latency, to ensure transmission continues at correct rate.
Interrupt masks - Data word/buffer transmission complete, as above.
Interrupt flag - Data word/buffer transmission complete. Flags set on change of associated status and
interrupt issued if appropriate mask bit is set.
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Interrupt status - Data word/buffer transmission complete
ã 2001 Consumer Microcircuits Limited
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D/880/1
CMLMICRO [ CML MICROCIRCUITS ]
