CTCSS Signalling Processor
FX818
1.6.4 Receiver (Fast/Predictive Detector)
This is used for detecting, in the fastest possible time, that sub-audio tones are present on the Rx channel.
Response time is optimised for speed at the expense of frequency resolution.
It can operate in parallel to the Rx decoder. It is enabled using Bit 5 of the "SUB-AUDIO CONTROL" register
($80). It has an IRQ which may be unmasked with Bit 2 of the "IRQ MASK" register ($8E). The "FAST
CTCSS MODE DETECT/PREDICTIVE" Bit 3 in the "SUB-AUDIO SET UP" register ($82) allows for one of two
alternatives in the FAST mode. In DETECT mode it will detect any periodic tone in the sub-audio band and
when in PREDICTIVE mode it will detect specific tones determined by the frequency set in the "CTCSS
TX/FAST RX" register ($83) and the fixed PREDICTIVE mode bandwidth. Successful detection is indicated by
the CTCSS FAST IRQ FLAG Bit 2 in the IRQ FLAG register ($8F), and the CTCSS FAST TONE Bit 6 in the
SUB-AUDIO STATUS register ($81).
1.6.5 General Purpose Timer (GPT)
This may be used in conjunction with the Rx Decoder to form part of the decode algorithm or as a timer for any
other purpose. It has an 8-bit value register "GENERAL PURPOSE TIMER" register ($8B) set in units of
1msec, an IRQ FLAG Bit 6 of the "IRQ FLAG" register ($8F) and an IRQ MASK Bit 6 "IRQ MASK" register
($8E).
1.6.6 Tx / Fast Rx Tone Table
The following table lists the commonly used CTCSS tones and the corresponding values for programming the
transmitter frequency / fast predictive frequency register (Hex address $83).
Freq.
(Hz)
67.0
69.3
71.9
74.4
77.0
79.7
82.5
85.4
88.5
91.5
94.8
97.4
100.0
103.5
107.2
110.9
Byte 1 Byte 2
Freq.
(Hz)
Byte 1 Byte 2
Freq.
(Hz)
Byte 1 Byte 2
(hex)
E
E
(hex)
B1
34
(hex)
8
(hex)
93
(hex)
5
(hex)
49
114.8
118.8
123.0
127.3
131.8
136.5
141.3
146.2
151.4
156.7
159.8
162.2
167.9
173.8
179.9
183.5
186.2
189.9
192.8
196.6
199.5
203.5
206.5
210.7
218.1
225.7
229.1
233.6
241.8
250.3
254.1
8
49
5
5
5
2F
1B
2
D
D
C
C
B
B1
3B
C9
5A
EF
87
8
1
7
7
BC
78
4
4
4
4
EF
D6
C4
AC
83
7
36
6
6
6
F7
BC
80
B
B
A
A
1F
C2
62
4
6
48
4
4
4
5D
4C
37
6
29
A
9
9
1B
D8
83
6
12
5
5
5
DD
AA
79
4
12
3
3
EF
E0
9
8
2F
E0
5
5D
ã 1997 Consumer Microcircuits Limited
19
D/818/4
CMLMICRO [ CML MICROCIRCUITS ]
