CMX808A View Datasheet(PDF) - CML Microcircuits
Part Name
Description
MFG CO.
'CMX808A' PDF : 23 Pages View PDF
Family Radio CTCSS 'Type 2' Encoder and Decoder
1.6 Application Notes
CMX808A
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:
A
=
fXTAL (Hz)
16 x fTONE (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 VBIAS or NOTONE without regard to the
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 VBIAS. Powersave is also achieved by disabling the Tx (Bit 7 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 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.
© 1999 Consumer Microcircuits Limited
14
D/808A/5
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