CTCSS/DCS/SELCALL Processor

FX828

© 2009 CML Microsystems Plc

7

D/828/4

1.5

General Description

The FX828 is a signalling encoder/decoder for use in land mobile radio equipment, see Figure 1.

The transmitter section of this device has independently controllable tone generators for sub-

audio (CTCSS) and inband (Selcall) signalling. It also features a DCS code generator, which may

be used in place of the CTCSS tone generator.

The receiver section of the FX828 has a fast/predictive CTCSS tone detector which operates in

parallel with a DCS decoder and a CTCSS/Selcall tone decoder. The latter is switchable to

perform either CTCSS or Selcall tone decoding of a user-programmable set of up to 15 tones. In

the CTCSS mode it performs a more accurate (but slower) analysis of the tones detected by the

fast/predictive CTCSS tone detector, which is a single detector that is switchable to provide either

a fast response to any CTCSS tone (FAST DETECT mode) or a fast response to a single user-

programmed CTCSS tone (PREDICTIVE mode).

Other functions on the FX828 are a comparator with programmable threshold level, a general

purpose timer and a summing amplifier with two adjustable gain blocks, which may be used for

two point modulation, for example. All FX828 functions are controlled by an external µC over the

C-BUS interface, a serial interface designed to reduce interference levels in radio equipment.

1.5.1

Software Description

Address/Commands

Instructions and data are transferred, via "C-BUS", in accordance with the timing information

given in Figure 4.

Instruction and data transactions to and from the FX828 consist of an Address/Command (A/C)

byte followed by either:

(i)

a further instruction or data (1 or 2 bytes) or

(ii)

a status or Rx data reply (1 byte)