RFM22 Datasheet, PDF (34/150 Pages) –

RFM22
The Invalid Preamble Detector issues an interrupt when no valid preamble signal is found. After the receiver is enabled, the
Invalid Preamble Detector output is ignored for 16 Tb (Where Tb is the time of a bit duration) to allow the receiver to settle. The
Invalid Preamble Detect interrupt can be used to save power and speed-up search in receive mode. It is advised to mask the
invalid preamble interrupt when Antenna Diversity is enabled.
The Received Signal Strength Indicator (RSSI) provides a measure of the signal strength received on the tuned channel. The
resolution of the RSSI is 0.5 dB. This high resolution RSSI enables accurate channel power measurements for clear channel
assessment (CCA), carrier sense (CS), and listen before talk (LBT) functionality.
Frequency mistuning caused by crystal inaccuracies can be compensated by enabling the digital Automatic Frequency Control
(AFC) in receive mode.
A comprehensive programmable Packet Handler including key features is integrated to create a variety of communication
topologies ranging from peer-to-peer networks to mesh networks. The extensive programmability of the packet header allows for
advanced packet filtering which in turn enables a mix of broadcast,group, and point-to-point communication.
A wireless communication channel can be corrupted by noise and interference, and it is therefore important to know if the
received data is free of errors. A cyclic redundancy check (CRC) is used to detect the presence of erroneous bits in each packet.
A CRC is computed and appended at the tail of each transmitted packet and verified by the receiver to confirm that no errors have
occurred. The Packet Handler and CRC are extremely valuable features which can significantly reduce the load on the system
microcontroller allowing for a simpler and cheaper microcontroller.
The digital modem includes the TX Modulator which converts the TX Data bits into the corresponding stream of digital modulation
values to be summed with the fractional input to the sigma-delta modulator. This modulation approach results in highly accurate
resolution of the frequency deviation. A Gaussian filter is implemented to support GFSK, considerably reducing the energy in the
adjacent channels. The bandwidth-time product (BT) is 0.5 for all programmed data rates.
5.6. Synthesizer
An integrated Sigma Delta (ΣΔ) Fractional-N PLL synthesizer capable of operating from 240–930 MHz is provided on-chip.
Using a ΣΔ synthesizer has many advantages; it provides large amounts of flexibility in choosing data rate, deviation,
channel frequency, and channel spacing. The transmit modulation is applied directly to the loop in the digital domain
through the fractional divider which results in very precise accuracy and control over the transmit deviation.
The PLL and Δ-Σ modulator scheme is designed to support any desired frequency and channel spacing in the range from
240–930 MHz with a frequency resolution of 156.25 Hz (Low band) or 312.5 Hz (High band). The transmit data rate can be
programmed between 1–128 kbps, and the frequency deviation can be programmed between ±1–160 kHz. These
parameters may be adjusted via registers as shown in "3.6. Frequency Control".
Figure 15. PLL Synthesizer Block Diagram
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