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M-8888-01P bảng dữ liệu(PDF) 4 Page - Clare, Inc. |
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4 / 14 page www.clare.com 4 M-8888 Rev. 1 The value of t DP is a device parameter and tREC is the minimum signal duration to be recognized by the receiver. A value for C1 of 0.1 µF is recommended for most applications, leaving R1 to be selected by the designer. Different steering arrangements may be used to select independently the guard times for tone present (t GTP) and tone absent (tGTA). This may be nec- essary to meet system specifications that place both accept and reject limits on both tone duration and inter- digit pause. Guard time adjustment also allows the designer to tailor system parameters such as talkoff and noise immunity. Increasing t REC improves talkoff performance since it reduces the probability that tones simulated by speech will maintain signal condition long enough to be registered. Alternatively, a relatively short t REC with a long tDO would be appropriate for extreme- ly noisy environments where fast acquisition time and immunity to tone dropouts are required. Design infor- mation for guard time adjustment is shown in the Guard Time Adjustment above. Call Progress Filter A call progress (CP) mode can be selected, allowing the detection of various tones that 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 the CP mode has been selected. DTMF signals cannot be detected if the CP mode has been selected (see the Actual Frequencies vs Standard Requirements on page 5). The Call Progress Response above indicates the useful detect bandwidth of the call progress filter. Frequencies presented to the input (IN+ and IN-) that 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 square wave output obtained from the schmitt trigger can be ana- yzed by a microprocessor or counter arrangement to determine the nature of the call progress tone being detected. Frequencies in the reject area will not be detected, and consequently there will be no activity on IRQ/CP as a result of these frequencies. DTMF Generator The DTMF transmitter used in the M-8888 is capable of generating all 16 standard DTMF tone pairs with low distortion and high accuracy. All frequencies are derived from an external 3.58 MHz crystal. The sinu- soidal waveforms for the individual tones are digitally synthesized using row and column programmable dividers and switched capacitor digital-to-analog con- verters. The row and column tones are mixed and fil- tered, providing a DTMF signal with low total harmonic distortion and high accuracy. To specify a DTMF sig- nal, data conforming to the encoding format shown in the Tone Encoding/Decoding Table on page 3 must be written to the transmit data register. Note that this is the same as the receiver output code. The individual tones that are generated (f LOW and fHIGH) are referred to as low-group and high-group tones. Typically, the high- group to low-group amplitude ratio (twist) is 2 dB to compensate for high-group attenuation on long loops. Operation: During write operations to the transmit data register, 4- bit data on the bus is latched and converted to a 2 of 8 code for use by the programmable divider circuitry to specify a time segment length that will ultimately deter- mine the tone frequency. The number of time seg- ments is fixed at 32, but the frequency is varied by varying the segment length. When the divider reaches the appropriate count as determined by the input code, a reset pulse is issued and the counter starts again. Guard Time Adjustment Call Progress Response |
Số phần tương tự - M-8888-01P |
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Mô tả tương tự - M-8888-01P |
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