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MT8880CC Datasheet(PDF) 5 Page - Mitel Networks Corporation |
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MT8880CC Datasheet(HTML) 5 Page - Mitel Networks Corporation |
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5 / 18 page  ISO2-CMOS MT8880C/MT8880C-1 4-37 Increasing tREC improves talk-off performance since it reduces the probability that tones simulated by speech will maintain a valid signal condition long enough to be registered. Alternatively, a relatively short tREC with a long tDO would be appropriate for extremely noisy environments where fast acquisition time and immunity to tone drop-outs are required. Design information for guard time adjustment is shown in Figure 6. The receiver timing is shown in Figure 9 with a description of the events in Figure 11. Call Progress Filter A call progress mode, using the MT8880C/C-1, can be selected allowing the detection of various tones which identify the progress of a telephone call on the network. The call progress tone input and DTMF input are common, however, call progress tones can only be detected when CP mode has been selected. DTMF signals cannot be detected if CP mode has been selected (see Table 5). Figure 8 indicates the useful detect bandwidth of the call progress filter. Frequencies presented to the input, which are within the ‘accept’ bandwidth limits of the filter, are hard- limited by a high gain comparator with the IRQ/CP pin serving as the output. The squarewave output obtained from the schmitt trigger can be analyzed by a microprocessor or counter arrangement to determine the nature of the call progress tone being detected. Frequencies which are in the ‘reject’ area will not be detected and consequently the IRQ/CP pin will remain low. DTMF Generator The DTMF transmitter employed in the MT8880C/C- 1 is capable of generating all sixteen standard DTMF tone pairs with low distortion and high accuracy. All frequencies are derived from an external 3.579545 MHz crystal. The sinusoidal waveforms for the individual tones are digitally synthesized using row and column programmable dividers and switched capacitor D/A converters. The row and column tones are mixed and filtered providing a DTMF signal with low total harmonic distortion and high accuracy. To specify a DTMF signal, data conforming to the encoding format shown in Figure 7 must be written to the transmit Data Register. Note that this is the same as the receiver output code. The individual tones which are generated (fLOW and fHIGH) are referred to as Low Group and High Group tones. As seen from the table, the low group frequencies are 697, 770, 852 and 941 Hz. The high group frequencies are 1209, 1336, 1477 and 1633 Hz. Typically, the high group to low group amplitude ratio (pre-emphasis) is 2dB to compensate for high group attenuation on long loops. 0= LOGIC LOW, 1= LOGIC HIGH Figure 7 - Functional Encode/Decode Table Figure 8 - Call Progress Response The period of each tone consists of 32 equal time segments. The period of a tone is controlled by varying the length of these time segments. During write operations to the Transmit Data Register the 4 bit data on the bus is latched and converted to 2 of 8 coding for use by the programmable divider circuitry. This code is used to specify a time segment length which will ultimately determine the frequency of the tone. When the divider reaches the appropriate count, as determined by the input code, a reset pulse is issued and the counter starts again. The number FLOW FHIGH DIGIT D3 D2 D1 D0 697 1209 1 0001 697 1336 2 0010 697 1477 3 0011 770 1209 4 0100 770 1336 5 0101 770 1477 6 0110 852 1209 7 0111 852 1336 8 1000 852 1477 9 1001 941 1336 0 1010 941 1209 * 1011 941 1477 # 1100 697 1633 A 1101 770 1633 B 1110 852 1633 C 1111 941 1633 D 0000 LEVEL (dBm) FREQUENCY (Hz) -25 0 250 500 750 = Reject = May Accept = Accept |
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Similar Description - MT8880CC |
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