CTCSS Signalling Processor

FX818

© 1997 Consumer Microcircuits Limited

18

D/818/4

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:

A

f

(Hz)

16 x f

(Hz)

XTAL

TONE

=

where A is the binary number programmed into the 13 bits.

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 (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.