Advance Information
immediately become the ACK-Sender, go off
hook no later than 20ms 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 30ms. Once this condition
has been detected, the CPE shall immediately
become the ACK-Sender, go off hook no later
than 35ms after the start of the line HIGH state,
complete the CAS-ACK handshake, and remain
as ACK-Sender for the duration of the call. This
situation can happen if the designated ACK-
Sender and the Backup ACK-Sender are not
MEI compliant.
After going off hook the ACK-Sender shall
begin transmission of the ACK no earlier than
30ms and no later than 40ms after the leading
edge of the line HIGH voltage transition.
MT88E46
The interface is specifically designed for the 1200
baud rate and is consisted of 3 signals: DATA, DCLK
(Data Clock) and DR (Data Ready). DATA is an
output pin. DCLK is an input output pin. DR uses the
dual purpose output pin DR/DET. When FSK is
selected it is the DR signal.
Two FSK interface modes (modes 0 and 1) are
selectable via the CB0 pin. In mode 0, the FSK bit
stream is output directly. In mode 1, the data byte
and the trailing stop bit are stored in a 9 bit buffer. If
mode 1 is used, the CB0 pin can be connected to
Vdd. If mode 0 is used and full chip power down is
not required, the CB0 pin can be connected to Vss.
In Bellcore’s off hook protocol, a Type 2 CPE should
restore the voicepath within 50ms after the end of
the FSK signal. Due to noise, end of carrier detection
is not always reliable. The TIA/EIA-777 standard
requires the CPE to detect the end of FSK when any
one of the following occurs:
• absence of carrier signal or,
• more than five framing errors (trailing stop bit a
0 instead of a 1) have been detected in the FSK
message or,
• more than 150ms of continuous mark signal or
space signal has been detected.
FSK Data Interface Mode 0 - Bit Stream Mode
This mode is selected when the CB0 pin is low. In
this mode the FSK data is output directly to the DATA
pin. DCLK and DR are timing signal outputs (see
Figure 14).
For each received stop and start bit sequence, the
MT88E46 outputs a fixed frequency clock string of 8
pulses at the DCLK pin. Each DCLK rising edge
occurs in the middle of a DATA bit of the FSK byte.
DCLK is not generated for the start and stop bits.
Consequently, DCLK will clock only valid data into a
peripheral device such as a serial to parallel shift
register or into a microcontroller. The MT88E46 also
outputs an end of word pulse DR (Data Ready). DR
goes low for half a nominal bit time at the beginning
of the trailing stop bit. It can be used to interrupt a
microcontroller or cause a serial to parallel converter
to parallel load its data into the microcontroller. If a
shift register is not used, DCLK and DATA may
occupy 2 bits of a microcontroller’s input port. The
microcontroller polls the input port and saves the
DATA bit when DCLK changes from low to high.
When DR goes low, the word may then be
assembled from the last 8 saved bits.
•
•
After the ACK-Sender or Backup ACK-Sender
detected CAS, it must monitor the line for the line
HIGH state, which can happen only if all off hook
CPEs also detected CAS. Hence if the ACK-Sender
or Backup ACK-Sender is an on hook CPE, even if it
falsely detected CAS, talkoff can occur only if all off
hook CPEs also falsely detected CAS. Thus in the
situation where the ACK-Sender or Backup ACK-
Sender is an on hook CPE using the MT88E46 on
hook mode detection algorithm, talkoff protection is
provided by the off hook CPEs. The on hook mode
has been optimized to be more talkdown immune so
that in this situation the on hook CPE will be
successful in fulfilling its ACK-Sender or Backup
ACK-Sender responsibility.
FSK Demodulation
The FSK demodulator is compatible with Bellcore
SR-TSV-002476, TIA/EIA-716 and TIA/EIA-777
standards. It is capable of both Bell 202 and CCITT
V.23 formats transparently. FSK demodulation is
available at the GS1 input op-amp only.
FSK Data Interface
The MT88E46 provides a powerful dual mode 3-wire
interface so that the data bytes in the demodulated
FSK bit stream can be extracted without the need
either for an external UART or for the CPE’s
microcontroller to perform the function in software.
9
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