Low Power V.32 bis Modem
CMX869
threshold. The response of the Call Progress filter, including the effect of external components of figures
4a and 4b, is shown in Figure 6.
10
0
-10
-20
dB
-30
-40
-50
-60
0
0.5
1
1.5
2
2.5
3
3.5
4
kHz
Figure 6 Response of Call Progress Filter
The Answer Tone Detector measures both amplitude and frequency of the received signal and sets bit 6
or bit 7 of the Status Register when a valid 2225Hz or 2100Hz signal is received.
5.7
Rx Modem Filterering and Demodulation
When the receive part of the CMX869 is operating as a modem, the received signal is fed through a
bandpass filter to attenuate unwanted signals. The characteristics of the filter are determined by the
chosen receive modem type and frequency band.
The output of the filter is fed to the appropriate FSK or QAM demodulator depending on the selected
modem type.
In FSK modem modes the signal level at the output of the Filter is also measured, compared to a
threshold value, and the result controls bit 10 of the Status Register.
In QAM modem modes, a V.32 bis/V.32 echo canceller is included, which will work with a round trip delay
of up to 1.25 seconds.
5.8
Rx Modem Pattern Detectors
In FSK modem modes the received bit stream is monitored for continuous 1’s, for continuous 0’s, and for
continuous alternating 1’s and 0’s. Bit 7, 8 or 9 of the Status Register will be set to 1 whenever 32 bits of
the appropriate pattern has been received and will then remain at 1 for 12 bit times after the end of the
detected pattern unless the receive operating mode is changed, in which case the pattern detectors are
reset within 2 milliseconds.
The demodulated data is passed through a de-scrambler according to the requirements of the receiver
operating mode. This function is enabled automatically, as required.
5.9
Rx USART
A flexible Rx USART is provided for all modem modes which, depending on the setting of the Rx Mode
Register will treat the received data bit stream as Synchronous data or as Start-Stop characters.
© 2004 CML Microsystems Plc
15
D/869/4
CMLMICRO [ CML MICROCIRCUITS ]
