MT88E46_06 Datasheet, PDF (8/23 Pages) Texas Instruments – Bellcore Compliant Calling Number Identification Circuit

MT88E46
Data Sheet
GS1 _{GS1 op-amp configured}
as in Figure 3 or 4
IN2+
IN2-
GS2
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 t_DET1 after the beginning of CAS (see Figure 13).
Similarly, speech interference can cause DET to go low
later than t_DET2 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 35 ms 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 25 ms and no later than 60 ms
after the end of CAS as measured on Tip/Ring. (The
25 ms 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 35 ms that defines the 60 ms 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 5 ms 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 15 ms. Once this condition has been
detected, the Backup ACK-Sender shall
immediately become the ACK-Sender, go off hook
no later than 20 ms after the start of the line HIGH
state, complete the CAS-ACK handshake, and
remain as ACK-Sender for the remainder of the call.
This situation may happen if the designated ACK-
Sender is not MEI compliant.
• An MEI compliant CPE that is not the designated
ACK-Sender or the Backup ACK-Sender but which
is off hook at the time of the CAS, shall monitor the
line for a line HIGH state lasting a minimum of
30 ms. Once this condition has been detected, the
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