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MT88E46AS Datasheet(PDF) 8 Page - Zarlink Semiconductor Inc |
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MT88E46AS Datasheet(HTML) 8 Page - Zarlink Semiconductor Inc |
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8 / 23 page  MT88E46 Advance Information 8 In a telephone adjunct box application where there is no hybrid or speech IC, if the GS2 gain with respect to Tip/Ring is to be set to the same as that of GS1, the GS2 op-amp can be connected as a voltage follower to the GS1 op-amp output as shown in Figure 5. Figure 5 - GS2 Op-Amp Connected as GS1 Voltage Follower CAS Detection When CAS detection is selected, the dual purpose DR/DET pin is the DET output signal. DET goes low momentarily (416 µsec nominal) after the end of CAS to indicate that CAS has been detected, as shown in Figure 13. The CAS detector operates in ‘off hook mode’ or ‘on hook mode’ as selected by the CB1 and CB2 pins (see Table 1). ‘On hook mode’ and ‘off hook mode’ use different algorithms optimized for the CPE states. Normally DET goes low after the end of CAS. However, because of interference from speech or music, DET may go low before the end of CAS. The on hook and off hook mode algorithms ensure that DET will occur no earlier than tDET1 after the beginning of CAS (see Figure 13). Similarly, speech interference can cause DET to go low later than tDET2 after the end of CAS (see Figure 13). In off hook mode, the detection algorithm ensures that DET will occur no later than 35ms after the end of CAS even with speech interference, as required in TIA/EIA-777. In on hook mode, although the detection algorithm does not limit the detection delay from the end of CAS, talkdown typically meets the SR-TSV-002476 talkdown condition 1 (the average case) when only the detections which occur from the beginning of CAS to 35ms after the end of CAS are counted. Off Hook Mode CAS Detection The ‘off hook mode’ is Bellcore talkdown and talkoff compliant when the near end speech is attenuated 8dB or better, such as provided by the telephone hybrid or speech IC. When near end speech is not attenuated, such as from a parallel off hook CPE or when the application is a telephone adjunct box, talkoff is close to compliant while talkdown is close to condition 1 (the average case). This mode is intended for the off hook CPE situation and is available at the GS2 (4-wire side) input op-amp only. On Hook Mode CAS Detection The ‘on hook mode’ is optimized for talkdown only. It typically meets talkdown condition 1 without near end speech attenuation. It is intended for the MEI on hook CPE situation and must not be used when the CPE itself is off hook. The input is the GS1(Tip/Ring) op-amp because in most CPE designs, to detect CAS when the CPE itself is on hook, the signal must come from the Tip/Ring connection since either the telephone hybrid / speech IC is not operational or the 4-wire side signal level is severely attenuated. In the MEI protocol, the ACK-Sender and Backup ACK-Sender can be on hook or off hook. Therefore, if the on hook state CPE is the ACK-Sender or the Backup ACK-Sender, it must be able to detect CAS when the line is in use. Additionally, an on hook state MEI CPE must be able to detect CAS during a call so that it can listen in on the FSK to keep its call log consistent with the off hook CPEs, as required in TIA/EIA-777. The MT88E46 on hook mode algorithm has been optimized for talkdown because of the way the MEI protocol works. In MEI, the following events (described in TIA/EIA-777) occur when CAS is detected: • Each off hook CPE shall proceed to the on hook state not earlier than 25ms and no later than 60ms after the end of CAS as measured on Tip/ Ring. (The 25ms delay is necessary to prevent the on hook transition from corrupting the CAS for other CPEs that may not have completely qualified the signal. The additional 35ms that defines the 60ms upper limit allows for variation in CAS detection delay.) • After detecting a line HIGH state (the line voltage when the line is not terminated by any CPE), the ACK-Sender shall go off hook. The ACK-Sender shall allow the line to remain in the HIGH state for at least 5ms but not more than 8ms. If no line HIGH state is detected within 100ms after going on hook, all previously off hook CPE shall return to the off hook state. • Following a CAS the Backup ACK-Sender shall monitor the line for a line HIGH state lasting a minimum of 15ms. Once this condition has been detected, the Backup ACK-Sender shall IN2+ IN2- GS1 GS2 GS1 op-amp configured as in Figure 3 or 4 |
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