CMX969 Consumer Microcircuits Limited, CMX969 Datasheet - Page 29

• Manufacturer Part Number: CMX969
• Description: RD-LAP/MDC4800/Motient/ARDIS4-Level FSK Packet Data Modem
• Manufacturer: Consumer Microcircuits Limited

RD-LAP/MDC4800 Motient/ARDIS
1.6
1.6.1
In receive mode the CMX969 needs to know the received signal levels and symbol timing in order to
successfully extract the received data from the incoming signal. This can be done by using the AFSD function
to establish the levels and timing from the Frame Sync pattern at the start of a received message.
The AFSD function is enabled whenever bit 7 of the Command Register is 1 and operates in parallel with any
other receive task that may be running. It monitors the received signal for the presence of a Frame Sync
pattern without regard for any preconceived idea of the symbol timing or levels. When a Frame Sync pattern is
found the AFSD function then uses it to establish the optimum timing and levels for extracting the following
data.
This contrasts with the SFS and SFP (Search for Frame Sync and Search for Frame Preamble) tasks that rely
on previously established timing and level information and - when a Frame Sync pattern is detected - do not
change these timing and level settings.
On detecting a Frame Sync pattern the AFSD function sets the AFSDET and IRQ bits of the Status Register
and terminates any other current task.
AFSD should be enabled to look for a Frame Sync pattern whenever the CMX969 is switched to receive mode
or the radio is switched to a new receive channel or when the transmitter may have been switched off - in fact
in any circumstance where the correct levels and timing need to be re-established.
Once Frame Sync has been detected the AFSD function may be switched off if it is known that the transmitter
is sending continuous concatenated frames, in which case SFS or SFP tasks should be used to find
subsequent Frame Sync patterns. Alternatively, the AFSD function may be used instead of SFS to find all
received Frame Sync patterns.
If the system is operating in SFR (Single Frequency Re-use) mode and so has to be able to detect the start of
a new (stronger) transmission then AFSD should be left running so that it can detect the new transmitter and
automatically abort any current task and adjust the timing and level settings to suit the new signal.
2001 Consumer Microcircuits Limited
Application Notes
Autonomous Frame Sync Detect Function
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CMX969
D/969/5