FRS Signalling Processor
CMX882
Table 9 Maximum Data Transfer Latency
Max time to read from
Data buffer
Data encoding type
or write to data buffer
size
1200b/s
2400b/s
0
1
2
3
4
5
13.3ms
N/A*
6.6ms
N/A*
2 bytes
4 bytes
2 bytes
2 bytes
4 bytes
4 bytes
13.3ms
20ms
40ms
40ms
6.6ms
10ms
20ms
20ms
* Type 1 message is an isolated Frame Head, there is no subsequent data to load (Tx) or read (Rx).
1.5.6
XTCSS Coding
The CMX882 allows addressed calling using a 4 tone In-band tone burst followed by an optional sub-
audio ‘XTCSS maintenance tone’ (at 64.7Hz). In transmit the CMX882 handles the transmission of the 4
tone sequence and the sub-audio tone. In receive the CMX882 will search for valid In-band tone
sequences containing the previously programmed address.
The over air signalling of XTCSS is shown below:
XTCSS sub-audio
A
A1 A0 S0 S1
In-band tones
Voice / Data
B
Notes:
•
To reduce 'cut on' time with XTCSS voice calls, the host can enable the receive audio path at 'B'
(as soon as the 4 tone sequence is available), before the sub-audio is detected.
XTCSS 4 tone sequences must be prefixed and suffixed with a silent 'no-tone' period of at least
the length of each tone. See also programming register P1.1.
•
In-band tones A1 and A0 are the BCD (binary coded decimal) representation of the Device Address bits
of $C2 register, the valid XTCSS address range is 01 to 99, A0 is the least significant digit. The XTCSS
address '40' is reserved for an all call address - regardless of the XTCSS address being searched for the
CMX882 will always indicate when a valid 4 tone set containing address '40' has been received.
In transmit the CMX882 will only generate the sub-audio maintenance tone when the CTCSS enable bit is
also set to '0'. The sub-audio tone (if enabled) will be automatically output after the 4th XTCSS tone has
been transmitted. An XTCSS interrupt is generated (if enabled) at point 'A' - see diagram above, the host
should then wait one tone period before enabling the audio path (or transmitting data) to ensure sufficient
no-tone suffix to the XTCSS 4 tone set. To summarize, detection of XTCSS sub-audio maintenance tone
requires the CTCSS decoder to be enabled, whereas generation of the XTCSS sub-audio maintenance
tone requires the CTCSS encoder to be disabled.
In-band tone S0 is selected from the normal tone range of $B - $D to maintain compatibility with HSC
type addressing. In-band tone S1 is selected from the normal tone range $0 - $9. The bit patterns for S0
and S1 indicate the type of information to follow according to the following tables:
2004 CML Microsystems Plc
32
D/882/7
CMLMICRO [ CML MICROCIRCUITS ]
