Communications Controller
CMX850
1.5.13 CAS Tone and FSK Detector for CLI/CIDCW
The CAS (CPE alerting signal) tone and FSK detector circuit is driven from the CMX850’s line input
amplifier. It provides the ability to detect CAS tones (a burst of simultaneous 2130Hz and 2750Hz signals)
or to receive FSK signals and to convert them to either bits or bytes of data. The detector operates with low
power consumption and with improved Tone Alert performance in the presence of near-end signals, making
it particularly suitable for off-hook CIDCW (Calling Identity on Call Waiting) detection.
Note that the CAS/FSK detector uses the same VBIAS generator and line input amplifier as the modem, but
is otherwise independent of the modem operation; both can be enabled simultaneously. This may be useful
in certain applications, for example when it is necessary to simultaneously look for both DTMF and FSK
signals.
The CAS/FSK detector is accessed through the CASDET and FSKBUF SFRs.
1.5.13.1
CAS Tone Detector
The detector will function as a CAS Tone Alert detector when the Mode bit (CASDET b6) is set to 0. When
in this mode, the Detect bit (CASDET b0) will be set high while a Tone Alert signal is detected. If the Detect
signal stays high for a time within the CAS qualifying time TqCAS (see section 1.7.1.3), then on the falling
edge of the Detect signal the interrupt status bit in the CASDET SFR will go high. This also asserts the Int2
signal to the 8051 µC. The action of writing a 1 to the CAS Tone interrupt clear bit in CASDET generates a
short pulse that clears the interrupt status bit. The TqCAS timing is selectable, allowing detection of up to
135 ms Tone Alert (CAS) signals, but is mainly optimised for detection of 75 to 85 ms Tone Alert (CAS)
signals used in off-hook applications.
Note that Tton added to TqCAS determines the range of times for which the tones should be present for a
CAS interrupt to be generated. The TqCAS time range is selected through the Tone Detect Window Control
bits in the CASDET SFR. See section 1.7.1.3 for definitions of Tdon and Tdoff and TqCAS.
Figure 7a CAS Tone Alert Detector Operation
1.5.13.2
FSK Detector
The detector will function as a 1200 baud FSK receiver when the Mode bit (CASDET b6) is set to a 1. When
in this mode, the detect bit (CASDET b0) will be set high when the FSK signal has exceeded a preset
threshold for sufficient time. Amplitude and time hysteresis are used to reduce chattering on the detect bit in
marginal conditions.
Note that in FSK receive mode the detector may also respond to non-FSK signals such as speech.
© 2003 CML Microsystems Plc
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