Baseband Processor For Mixed Mode Land Mobile Radio
CMX880
Receiving FFSK Signals
The device can decode incoming FFSK signals at either 1200 or 2400 baud data transfer rates. It can achieve
this by deriving the baud rate from the received signal, or a control word may set the baud rate and the device
only responds to signals operating at that rate. The form of FFSK signals for these baud rates, excluding noise,
is shown in Figure 11.
The received signal is filtered and data is extracted. A PLL is used to extract the clock from the recovered serial
data stream. The recovered data is grouped into 16-bit words and an interrupt issued to indicate received data
is ready. Data can be transferred over the “C-BUS” or the FSB, controlled by mC instructions. If this data is not
read within 16 bit periods it will be overwritten. The FFSK clock is not directly output on either external bus. It is
up to the user to ensure that the data is transferred at an adequate rate following data ready being flagged.
The extracted data is compared with the up to 3 pre-programmed frame sync patterns, SYNC, SYNT and SYND.
The SYNC and SYND patterns are user programmable. SYNT is the bit complement of SYNC. SYNC and
SYND are preset to $C4D7 following a reset. The user can individually enable any combination of the 3 patterns.
An interrupt will be flagged when a frame sync pattern is detected.
Rx
SIGNAL I/P
2400 BAUD
LOGIC '0'
LOGIC '0'
LOGIC '1'
LOGIC '1'
Rx
SIGNAL I/P
1200 BAUD
LOGIC '1'
LOGIC '0'
LOGIC '1'
LOGIC '0'
Figure 11 Modulating Waveforms for 1200 and 2400 Baud FFSK Signals
Table 8 shows the combinations of frequencies and number of cycles to represent ‘bits’ of data, for both baud
rates.
Table 8 Data Frequencies For Each Baud Rate
Baud Rate
1200 baud
Data
Frequency
Number of Cycles
1
0
1
0
1200Hz
1800Hz
1200Hz
2400Hz
one
one and a half
half
2400 baud
one
FFSK/MSK may be transmitted in conjunction with a CTCSS or DCS sub-audio component. The device will
handle the sub-audio signals as already described. If a sub-audio signal turns off during reception of
FFSK/MSK, it is up to the mC to turn off the FFSK/MSK decoding. The device will continue receiving and
processing the incoming signal until commanded otherwise by the µC.
Associated Control and Status
·
·
·
Reset.
Enable FFSK signal reception.
Select baud rate: 1200 or 2400. Both rates can be enabled at the same time. The CMX880 will identify the
baud rate from the received signal.
·
·
Set access port, “C-BUS” or FSB.
Select frame sync patterns to be recognised (SYNC, SYNT and SYND); set SYND pattern.
ã 2001 Consumer Microcircuits Limited
25
D/880/1
CMLMICRO [ CML MICROCIRCUITS ]
