CTCSS Signalling Processor
FX818
1.6
Application Notes
1.6.1 General
The FX818 is intended for use in radio systems where sub-audio signalling is required for functions such as
trunking, control, selective calling or group calling.
The CTCSS fast/predictive detector is useful for the detection of occupied channels indicating either the
presence of any sub-audio tone, or range of tones, depending if it is set in fast detect or predictive mode. This
will increase the efficiency of scanning and trunking systems, reducing the average time allocated to assessing
each channel.
The facility to decode any of up to 15 programmed tones allows the use of tones for various signalling functions
such as masking a free channel or identifying sub groups within a user's groups.
Adjustable decoder bandwidths permits certainty and signal to noise performance to be traded when
congestion or range limits the system performance.
1.6.2 Transmitter
The transmitter is enabled with Bit 7 in the "SUB-AUDIO CONTROL" register ($80).
The Tx frequency is set using Bit 0 to Bit 12 in the CTCSS TX/FAST RX register ($83) using the formula below:
fXTAL (Hz)
16 xfTONE (Hz)
A =
where A is the binary number programmed into the 13 bits.
When Bit 7 (in the first 8 bits) is set to "1" the tone output is set at V
or NOTONE without regard to the
BIAS
number "A" programmed. When Bit 7 is "0" the programmed tone is set on the output. Programming the bits 0
to 12 to "0" puts the Tx into powersave and the output goes to V . Powersave is also achieved by disabling
BIAS
the Tx and the FAST DETECT (Bits 7 and 5 in the "SUB-AUDIO CONTROL" register $80).
1.6.3 Receiver (Decode)
The CTCSS Receiver (Decoder) should first be set up according to the desired characteristics. This entails
setting the CTCSS decoder bandwidth in the "SUB-AUDIO SET-UP" register ($82), also programming the
centre frequencies of the desired tones in the "CTCSS RX PROGRAM" register ($84). (It can hold up to 15
different tones). Any tone can be in any location. During operation when the device is receiving, the tones are
scanned in the sequence of their location, i.e. $0 first and $E last and once a tone is detected the remaining
tones are not checked. Therefore if two tones are close enough in frequency for their bandwidths to overlap
then the one in the lowest location will be detected.
The CTCSS IRQ MASK in the "IRQ MASK" register ($8E) should also be set as required.
The CTCSS DECODER ENABLE in the "SUB-AUDIO CONTROL" register ($80) should then be set to "1".
Whilst in the Decode mode the FAST DETECT may be enabled (see below). (Bit 5 in the SUB-AUDIO
CONTROL register $80).
When the receiver detects a change in its present state an IRQ will be generated and Bit 3 of the IRQ FLAG
register ($8F) will indicate this.
The change that occurred can be read from Bit 4 of the SUB-AUDIO STATUS register ($81) and if a tone is
indicated by these bits then the number of that tone can be read from Bits 3, 2, 1 and 0 of the same register.
ã 1997 Consumer Microcircuits Limited
18
D/818/4
CMLMICRO [ CML MICROCIRCUITS ]
