MT88E45
FSK Data Interface
The MT88E45 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.
The interface is specifically designed for the 1200
baud rate and is consisted of 3 pins: DATA, DCLK
(Data Clock) and DR (Data Ready). DR/STD is a
dual purpose output pin. When FSK is selected it is
DR.
Two 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 desired,
the CB0 pin can be hardwired to Vdd. If mode 0 is
desired and full chip power down is not required, the
CB0 pin can be hardwired 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 draft TIA Type 2 standard
has proposed that the CPE must 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.
Advance Information
bit, it can also be used to read the stop bit to check
for framing error.
Alternatively, DCLK and DATA may occupy 2 bits of a
microcontroller’s input port. The microcontroller polls
the input port and saves the DATA bit whenever
DCLK changes from low to high. When DR goes low,
the word may then be assembled from the last 8
saved bits.
DATA may also be connected to a personal
computer’s serial communication port after
conversion from CMOS to RS-232 voltage levels.
Mode 1 - Buffer Mode
This mode is selected when the CB0 pin is high. In
this mode the received byte is stored on chip. At the
end of a byte DR goes low to indicate that a new byte
has become available. The microcontroller applies
DCLK pulses to read the register contents serially
out of the DATA pin (see Figure 14).
Internal to the MT88E45, the start bit is stripped off,
the data bits and the trailing stop bit are sampled and
stored. Midway through the stop bit, the 8 data bits
and the stop bit are parallel loaded into a 9 bit shift
register and DR goes low. The register’s contents are
shifted out to the DATA pin on the supplied DCLK’s
rising edges in the order they were received. The last
bit must be shifted out and DCLK returned to low
before the next DR. DCLK must be low for t
DDS
before
DR goes low and must remain low for t
DDH
after DR
has gone low (see Figure 14).
If DCLK begins while DR is low, DR will return to high
upon the first DCLK rising edge. If DR interrupts a
microcontroller then this feature allows the interrupt
to be cleared by the first read pulse. Otherwise DR is
low for half a nominal bit time (1/2400 sec).
Reading the stop bit allows the software to check for
framing errors. When framing error is not checked
the microcontroller only needs to send 8 DCLK
pulses to shift the data byte out.
•
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 pins are timing signal outputs
(see Figure 13).
For each received stop and start bit sequence, the
MT88E45 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 cell. 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 a
microcontroller. The MT88E45 also outputs an end
of word pulse (Data Ready) at the DR pin. 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. Since
the DR rising edge occurs in the middle of the stop
10
Carrier Detect
The carrier detector provides an indication of the
presence of a signal in the FSK frequency band. It
detects the presence of a signal of sufficient
amplitude at the output of the FSK bandpass filter.
The signal is qualified by a digital algorithm before
the CD output is set low to indicate carrier detection.
A 10ms hysteresis is provided to allow for
momentary signal dropout once CD has been
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