Preliminary Information
Note that signals such as dual tone alert signal,
speech and DTMF tones lie in the same frequency
band as FSK. They will, therefore, be demodulated
and as a result, false data will be generated. To avoid
demodulation of false data, an FSKen pin is provided
so that the FSK demodulator may be disabled when
FSK signal is not expected.
The FSK characteristics described in Table 2 have
been specified in BT and Bellcore specifications. The
BT signal frequencies correspond to CCITT V.23. The
Bellcore frequencies correspond to Bell 202. CTA
requires that the TE be able to receive both CCITT
V.23 and Bell 202, as specified in the BT and Bellcore
specifications. CNIC2 is compatible with both formats
with no external intervention.
•
3-wire User Interface
MT8843
The MT8843 provides a powerful dual mode 3-wire
interface so that the 8-bit data words in the
demodulated FSK bit stream can be extracted
without the need either for an external UART
(Universal Asynchronous Receiver Transmitter) or
for the TE/CPE’s microcontroller to perform the
UART function in software (asynchronous serial data
reception). The interface is specifically designed for
the 1200 baud rate and is comprised of the DATA,
DCLK (data clock) and DR (data ready) pins. Two
modes (modes 0 and 1) are selectable via control of
the device’s MODE pin: in mode 0, data transfer is
initiated by the CNIC2; in mode 1, data transfer is
initiated by the external microcontroller.
Mode 0
This mode is selected when the MODE pin is low. It
is the CNIC (MT8841) compatible mode where data
transfer is initiated by the device.
In this mode, CNIC2 receives the FSK signal,
demodulates it, and outputs the extracted data to the
DATA pin (refer to Figure 12). For each received stop
and start bit sequence, the CNIC2 outputs a fixed
frequency clock string of 8 pulses at the DCLK pin.
Each clock rising edge occurs in the centre of each
DATA bit cell. DCLK is not generated for the stop and
start bits. Consequently, DCLK will clock only valid
data into a peripheral device such as a serial to
parallel shift register or a micro-controller. CNIC2
also outputs an end of word pulse (data ready) at the
DR pin. The data ready signal indicates the reception
of every 10-bit word sent from the network to the TE/
CPE. This DR signal is typically used to interrupt a
micro-controller.
Mode 1
This mode is selected when the MODE pin is high. In
this mode, the microcontroller supplies read pulses
(DCLK) to shift the 8-bit data words out of the
MT8843, onto the DATA pin. CNIC2 asserts DR to
denote the word boundary and indicate to the
microprocessor that a new word has become
available (refer to Figure 14).
Internally, the MT8843’s demodulated data bits are
sampled and stored. After the 8th bit, the word is
parallel loaded into an 8 bit shift register and DR
goes low. The shift register’s contents are shifted out
to the DATA pin on DCLK’s rising edge in the order
they were received.
If DCLK begins while DR is low, DR will return to high
upon the first DCLK. This feature allows the
associated interrupt (see section on "Interrupt") to be
Item
Mark
frequency
(logic 1)
Space
frequency
(logic 0)
Received
signal level -
mark
Received
signal level -
space
Signal level
differential
(twist)
Unwanted
signals
Transmission
rate
Word format
BT
1300Hz
± 1.5%
2100Hz
± 1.5%
-8dBV to
-40dBV
(-5.78dBm to
-37.78dBm)
-8dBV to
-40dBV
up to 6dB
Bellcore
1200Hz
± 1%
2200Hz
± 1%
-12dBm
a
to
-32dBm
-12dBm to
-36dBm
up to 10dB
b
<= -20dB
(300-3400Hz)
1200 baud
± 1%
1 start bit (logic
0), 8 bit word
(LSB first), 1 to
10 stop bits
(logic 1)
<= -25dB
(200-3200Hz)
c
1200 baud
± 1%
1 start bit (logic
0), 8 bit word
(LSB first),
1 stop bit
(logic 1)
d
Table 2. FSK Characteristics
a. The signal power is expressed in dBm referenced to 600 ohm
at the CPE tip/ring (A/B) interface.
b. TR-NWT-000030, Bulletin No. 1
c. The frequency range is specified in TR-NWT-000030.
d. Up to 20 marks may be inserted in specific places in a single
or multiple data message.
CNIC2 will meet these characteristics with its input
op-amp at unity gain.
5-27
MITEL [ MITEL NETWORKS CORPORATION ]
