MT8841
edge corresponds to the centre of each DATA bit cell
(providing the incoming baud rate matches the DCLK
rate). 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. The CNIC 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 Central Office. This
output is typically used to interrupt a micro-controller.
The three outputs together, eliminate the need for a
UART
(Universal
Asynchronous
Receiver
Transmitter) or the high software overhead of
performing the UART function (asynchronous serial
data reception).
Note that the 3-pin interface may also output data
generated by voice since these frequencies are in
the input frequency detection band of the device.
The user may choose to ignore these outputs when
FSK data is not expected, or force the CNIC into its
powerdown mode.
Power Down Mode
For
applications
requiring
reduced
power
consumption, the CNIC can be forced into power
down when it is not needed to receive FSK data. This
is done by pulling the PWDN pin high. In powerdown
mode, the crystal oscillator, op-amp and internal
circuitry are all disabled and the CNIC will not react
to the input signal. DATA and DCLK are at logic high,
and DR and CD are at high impedance or at logic
high when pulled up with resistors.The CNIC can be
awakened for reception of the FSK signal by pulling
the PWDN pin to ground (see Figure 9).
Carrier Detect
The presence of the FSK signal is indicated by a
logic low at the carrier detect (CD) output. This
output has built in hysteresis to prevent toggling
when the received signal is shortly interrupted. Note
that the CD output is also activated by voice since
these frequencies are in the input frequency
detection band of the device. The user may choose
to ignore this output when FSK data is not expected,
or force the CNIC into its powerdown mode.
MT8841
OSC1 OSC2
MT8841
OSC1
OSC2
MT8841
OSC1
OSC2
3.579545 MHz
to the
next MT8841
Figure 5 - Common Crystal Connection
Crystal Oscillator
The CNIC uses a crystal oscillator as the master
timing source for filters and the FSK demodulator.
The crystal specification is as follows:
Frequency:
Frequency tolerance:
Resonance mode:
Load capacitance:
Maximum series resistance:
Maximum drive level (mW):
e.g. CTS MP036S
3.579545 MHz
±0.1%(-40°C+85°C)
Parallel
18 pF
150 ohms
2 mW
A number of MT8841 devices can be connected as
shown in Figure 5 such that only one crystal is
required. The connection between OSC2 and OSC1
can be D.C. coupled as shown, or A.C. coupled using
30pF capacitors. Alternatively, the OSC1 inputs on
all devices can be driven from a CMOS buffer (dc
coupled) with the OSC2 outputs left unconnected.
VRef and CAP Inputs
V
Ref
is the output of a low impedance voltage source
equal to V
DD/2
and is used to bias the input op-amp.
A 0.1µF capacitor is required between CAP and V
SS
to suppress noise on V
Ref.
5-14
ZARLINK [ ZARLINK SEMICONDUCTOR INC ]
