CTCSS Signalling Processor

FX818

© 1997 Consumer Microcircuits Limited

19

D/818/4

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.

Byte 1

Byte 2

Freq.

Byte 1

Byte 2

Freq.

Byte 1

Byte 2

(Hz)

(hex)

(hex)

(Hz)

(hex)

(hex)

(Hz)

(hex)

(hex)

67.0

E

B1

114.8

8

93

186.2

5

49

69.3

E

34

118.8

8

49

189.9

5

2F

71.9

D

B1

123.0

8

1

192.8

5

1B

74.4

D

3B

127.3

7

BC

196.6

5

2

77.0

C

C9

131.8

7

78

199.5

4

EF

79.7

C

5A

136.5

7

36

203.5

4

D6

82.5

B

EF

141.3

6

F7

206.5

4

C4

85.4

B

87

146.2

6

BC

210.7

4

AC

88.5

B

1F

151.4

6

80

218.1

4

83

91.5

A

C2

156.7

6

48

225.7

4

5D

94.8

A

62

159.8

6

29

229.1

4

4C

97.4

A

1B

162.2

6

12

233.6

4

37

100.0

9

D8

167.9

5

DD

241.8

4

12

103.5

9

83

173.8

5

AA

250.3

3

EF

107.2

9

2F

179.9

5

79

254.1

3

E0

110.9

8

E0

183.5

5

5D