Family Radio CTCSS 'Type 2' Encoder and Decoder
CMX808A
1.6 Application Notes
1.6.1 General
The CMX808A is intended for use in radio systems where sub-audio signalling is required for functions
such as Family Radio Service (FRS) Handportables, Amateur Radio Equipment, General Mobile Radio
Service (GMRS) and Short Range Business Radio.
The facility to decode any of up to 7 programmed tones allows FRS designers to offer equipment which
can look for personal, family or open channel codes at the same time. Codes can be used as paging
codes, open chat mode codes as well as personal and family codes.
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 bits 0 to 12 in the CTCSS TX FREQUENCY register $83, using the formula
below:
fXTAL (Hz)
A =
16 xfTONE (Hz)
where A is the binary number programmed into the 13 bits.
When Bit 7 (in the first 8 bits) is set to "1" the tone output is set at V
or NOTONE without regard to the
BIAS
number "A" programmed. When Bit 7 is "0" the programmed tone is set on the output. Programming bits
0 to 12 to "0" sets the output to V
AUDIO CONTROL register $80).
. Powersave is also achieved by disabling the Tx (Bit 7 in the SUB-
BIAS
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 CONTROL register $80, also
programming the centre frequencies of the desired tones in the CTCSS RX PROGRAM register $84. (It
can hold up to 7 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 $6 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 SUB-AUDIO CONTROL register $80 should also be set as required.
The CTCSS DECODER ENABLE in the SUB-AUDIO CONTROL register $80 should then be set to "1".
The TONE CLONING register $9C should be set as required.
When the receiver detects a change in its present state an interrupt will be generated. The change that
occurred can be read from Bit 3 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 2, 1 and 0 of the same register. The
interrupt is cleared by reading the SUB-AUDIO STATUS register.
2003 CML Microsystems Plc
14
D/808A/6
CMLMICRO [ CML MICROCIRCUITS ]
