CTCSS Signalling Processor

FX818

© 1997 Consumer Microcircuits Limited

12

D/818/4

number "A" programmed. When Bit 7 is "0" the programmed tone is set on the output. Programming the bits 0

the Tx and the FAST DETECT.

CTCSS RX PROGRAM Register (Hex address $84)

This is a 16-bit register. Byte (1) is sent first. The two bytes are used to program the centre frequencies of up

to 15 tones in the sub-audio band that will be decoded by the receiver.

Each tone is identified by its address in bits 7, 6, 5 and 4 of byte (1). The remaining 12 bits contain the data

representing the tone frequency according to the formula below. If a tone is not required the 12 bits should be

set to zero.

Byte 1

Byte 2

Bit

7

Bit

6

Bit

5

Bit

4

Bit

3

Bit

2

Bit

1

Bit

0

Bit

7

Bit

6

Bit

5

Bit

4

Bit

3

Bit

2

Bit

1

Bit

0

0

0

0

0

<-------------------- N -------------------- >

<-------------------- R ------------------->

0

0

0

1

0

0

1

0

0

0

1

1

0

1

0

0

0

1

0

1

N is the binary representation of the

R is the nearest 6-bit binary

0

1

1

0

following decimal number (n):

representation of (r), where:

0

1

1

1

1

0

0

0

1

0

0

1

1

0

1

0

1

0

1

1

1

1

0

0

1

1

0

1

1

1

1

0

Example: To program 100Hz when using the recommended 4.032MHz Xtal.

n =

INT (948982 x 100 / 4.032 x 10^6)

=

INT (23.536) = 23

N =

010111 (binary)

r

=

((237245 / 4.032 x 10^6) - (23 / (4 x 100))) x 8400

=

11.26 (round up if exactly halfway)

R =

11

=

001011 (binary)

Thus the 12-bit code is 010111001011

The Hex address represented by bits 7, 6, 5 and 4 in byte (1) is used as the code to indicate which tone has

been decoded. This code appears in bits 3, 2, 1 and 0 of the SUB-AUDIO STATUS Register (Hex address

$81). The 15 programmed tones use Hex addresses $0 - $E.