3
IA4420
Crystal Oscillator
The IA4420 has a single-pin crystal oscillator circuit, which provides
a 10 MHz reference signal for the PLL. To reduce external parts and
simplify design, the crystal load capacitor is internal and
programmable. Guidelines for selecting the appropriate crystal can
be found later in this datasheet.
The transceiver can supply the clock signal for the microcontroller;
so accurate timing is possible without the need for a second crystal.
Interface and Controller
An SPI compatible serial interface lets the user select the frequency
band, center frequency of the synthesizer, and the bandwidth of the
baseband signal path. Division ratio for the microcontroller clock,
wake-up timer period, and low supply voltage detector threshold are
also programmable. Any of these auxiliary functions can be disabled
when not needed. All parameters are set to default after power-on;
the programmed values are retained during sleep mode. The interface
supports the read-out of a status register, providing detailed
information about the status of the transceiver and the received
data.
The transmitter block is equipped with an 8 bit wide TX data register.
It is possible to write 8 bits into the register in burst mode and the
internal bit rate generator transmits the bits out with the predefined
rate.
It is also possible to store the received data bits into a FIFO register
and read them out in a buffered mode.
By using an integrated Automatic Frequency Control (AFC) feature,
the receiver can minimize the TX/RX offset in discrete steps, allowing
the use of:
·
Inexpensive, low accuracy crystals
·
Narrower receiver bandwidth (i.e. increased sensitivity)
·
Higher data rate
AFC
P1
-65 dBm
1300 mV
P2
-65 dBm
1000 mV
P3
-100 dBm
600 mV
P4
-100 dBm
300 mV
Input Power [dBm]
RSSI
voltage
[V]
P4
P2
P1
P3
Data Validity Blocks
R S S I
R S S I
R S S I
R S S I
R S S I
A digital RSSI output is provided to monitor the input signal level. It
goes high if the received signal strength exceeds a given
preprogrammed level. An analog RSSI signal is also available. The
RSSI settling time depends on the external filter capacitor. Pin 15 is
used as analog RSSI output. The digital RSSI can be can be monitored
by reading the status register.
Low Battery Voltage Detector
The low battery detector circuit monitors the supply voltage and
generates an interrupt if it falls below a programmable threshold
level. The detector circuit has 50 mV hysteresis.
Wake-Up Timer
The wake-up timer has very low current consumption (1.5 uA typical)
and can be programmed from 1 ms to several days with an accuracy
of ±5%.
It calibrates itself to the crystal oscillator at every startup, and then
at every 30 seconds. When the crystal oscillator is switched off, the
calibration circuit switches it back on only long enough for a quick
calibration (a few milliseconds) to facilitate accurate wake-up timing.
Event Handling
In order to minimize current consumption, the transceiver supports
different power saving modes. Active mode can be initiated by several
wake-up events (negative logical pulse on nINT input, wake-up timer
timeout, low supply voltage detection, on-chip FIFO filled up or
receiving a request through the serial interface).
If any wake-up event occurs, the wake-up logic generates an interrupt
signal, which can be used to wake up the microcontroller, effectively
reducing the period the microcontroller has to be active. The source
of the interrupt can be read out from the transceiver by the
microcontroller through the SDO pin.
D Q D
D Q D
D Q D
D Q D
D Q D
The Data Quality Detector is based on counting the spikes on the
unfiltered received data. For correct operation, the “DQD threshold”
parameter must be filled in by using the Data Filter Command.
Analog RSSI Voltage vs. RF Input Power
When the microcontroller turns the crystal oscillator off by clearing
the appropriate bit using the Configuration Setting Command, the
chip provides a fixed number (196) of further clock pulses (“clock
tail”) for the microcontroller to let it go to idle or sleep mode.
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