Datenblatt-Suchmaschine für elektronische Bauteile |
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MC145443DW Datenblatt(PDF) 5 Page - Motorola, Inc |
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MC145443DW Datenblatt(HTML) 5 Page - Motorola, Inc |
5 / 12 page MC145442 •MC145443 MOTOROLA 5 RxA2, RxA1 Receive Carrier (Pins 15, 16) The receive carrier is the FSK input to the demodulator through the receive band–pass filter. RxA1 is the non–invert- ing input and RxA2 is the inverting input of the receive hybrid (duplexer) operational amplifier. LB Analog Loopback (Pin 2) When a high level is applied to this pin (SQT must be low), the analog loopback test is enabled. The analog loopback test connects the TxA pin to the RxA2 pin and the RxA1 to analog ground. In loopback, the demodulator frequencies are switched to the modulation frequencies for the selected mode. (See Tables 1 and 2 and Figures 4c and 4d.) When LB is connected to analog ground (VAG), the modu- lator generates an echo cancellation tone of 2100 Hz for MC145442 CCITT V.21 and 2225 Hz for MC145443 Bell 103 systems. For normal operation, this pin should be at a logic low level (VSS). The power–down mode is enabled when both LB and SQT are connected to a logic high level (see Table 2). Table 2. Functional Table MODE Pin 13 SQT Pin 14 LB Pin 2 Operating Mode 1 0 0 Originate Mode 0 0 0 Answer Mode X 0 VAG (VDD/2) Echo Tone X 0 1 Analog Loopback X 1 0 Squelch Mode X 1 VAG (VDD/2) Squelch Mode X 1 1 Power Down MODE Mode (Pin 13) This input selects the pair of transmit and frequencies used during modulation and demodulation. When a logic high level is placed on this input, originate (Bell) or channel 1 (CCITT) is selected. When a low level is placed on this input, answer (Bell) or channel 2 (CCITT) is selected. (See Tables 1 and 2 and Figure 4.) CDT Carrier Detect Timing (Pin 4) A capacitor on this pin to VSS sets the amount of time the carrier must be present before CD goes low (see Applica- tions Information for the capacitor values). CD Carrier Detect Output (Pin 3) This output is used to indicate when a carrier has been sensed by the carrier detect circuit. This output goes to a logic low level when a valid signal above the maximum threshold level (defined by CDA, Pin 7) is maintained on the input to the hybrid circuit longer then the response (defined by CDT, Pin 4). This pin is held at the logic low level until the signal falls below the maximum threshold level for longer than the turn off time. (See Applications Information and Figure 5.) CDA Carrier Detect Adjust (Pin 7) An external voltage may be applied to this pin to adjust the carrier detect threshold. The threshold hysteresis is internally fixed at 3 dB (see Applications Information). Xout, Xin Crystal Oscillator (Pins 8, 9) A crystal reference oscillator is formed when a 3.579 MHz crystal is connected between these two pins. Xout (Pin 8) is the output of the oscillator circuit, and Xin (Pin 9) is the input to the oscillator circuit. When using an external clock, apply the clock to the Xin (Pin 9) pin and leave Xout (Pin 8) open. An internal 10 M Ω resistor and internal capacitors, typically 10 pF on Xin and 16 pF on Xout, allow the crystal to be con- nected without any other external components. Printed cir- cuit board layout should keep external stray capacitance to a minimum. FB Filter Bias (Pin 10) This is the negative input to the ac amplifier. In normal op- eration, this pin is connected to analog ground through a 0.1 µF bypass capacitor in order to cancel the input offset voltage of the limiter. It has a nominal input impedance of 16 k Ω. (see Figure 3). SQT Transmit Squelch (Pin 14) When this input pin is at a logic high level, the modulator is disabled. The line driver remains active if LB is at a logic low level (see Table 2) . When both LB and SQT are connected to a logic high level, see Table 2, the entire chip is in a power down state and all circuitry except the crystal oscillator is disabled. Total power supply current decreases from 10 mA (Max) to 300 µA (Max). 0.1 µF FB 16 k Ω 10 490 k Ω + – FROM BAND–PASS FILTER TO CARRIER DETECT CIRCUIT AND DEMODULATOR Figure 3. ac Amplifier Circuit |
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Ähnliche Beschreibung - MC145443DW |
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