Si3220/Si3225
Decimation
Filter
ADC
12/16 kHz
Bandpass
Peak Detector
PMAMPTH
–
+
+
IBUF
ZA
DAC
+
PMRAMP
Pulse
+
+
±
x
Metering
DAC
Pulse
Volume
8 kHz
Clip
Metering
Oscillator
Logic
7FFF
or 0
Figure 38. Pulse Metering Generation Block Diagram
The detection process occurs twice within the 45 ms
minimum tone time. A digit must be detected on two
consecutive tests after a pause to be recognized as a
new digit. If all tests pass, an interrupt is generated, and
the DTMF digit value is loaded into the DTMF register
according to Table 38. If tones occur at the maximum
rate of 100 ms per digit, the interrupt must be serviced
within 85 ms so that the current digit is not overwritten
by a new one. There is no buffering of the digit
information.
DTMF Detection
On-chip DTMF detection, also known as touch tone, is
available in the Si3220 and Si3225.
It is an in-band signaling system that replaces the pulse-
dial signaling standard. In DTMF, two tones generate a
DTMF digit. One tone is chosen from the four possible
row tones, and one tone is chosen from the four
possible column tones. The sum of these tones
constitutes one of 16 possible DTMF digits. The row
and column tones and corresponding digits are shown
in Table 38.
Table 38. DTMF Row/Column Tones
DTMF detection is performed using a modified Goertzel
algorithm to compute the DFT for each of the eight 697 Hz
DTMF frequencies and their second harmonics. At the
1
2
5
8
0
3
6
9
#
A
B
C
D
770 Hz
852 Hz
941 Hz
4
end of the DFT computation, the squared magnitudes of
the DFT results for the 8 DTMF fundamental tones are
computed. The row results are sorted to determine the
strongest row frequency, and the column frequencies
are sorted as well. Upon completion of this process,
checks are made to determine if the strongest row and
column tones constitute a DTMF digit.
7
*
1209 Hz
1336 Hz 1477 Hz 1633 Hz
Rev. 1.0
65
SILICONIMAGE [ Silicon image ]
