Rainbow Electronics RF12B - Manual (Page 5)

RF12B
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The RF12B 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 a clock signal for the microcontroller; so accurate timing is possible
without the need for a second crystal.
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. If this clock output is not used, turn the output
buffer off by the Power Management Command.
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 50mV hysteresis.
Wake-Up Timer
The wake-up timer has very low current consumption (1.5 µA 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 even in case of changing
ambient temperature and supply voltage.
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.
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 two 8 bit wide TX data registers. 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. For further details, see the TX Register Buffered Data Transmission section.
It is also possible to store the received data bits into a FIFO register and read them out in a
buffered mode.