SI4X55-C Datasheet, PDF (19/39 Pages) Silicon Laboratories – Preamble sense mode

Si4x55-C
4.2.3. Frequency Deviation
If FSK or GFSK modulation is selected, then a frequency deviation will also need to be selected. The frequency
deviation is the maximum instantaneous difference between the FM modulated frequency and the nominal carrier
frequency. The Si4x55 can operate across a wide range of data rates and frequency deviations. If a frequency
deviation needs to be selected, the following guideline might be helpful to build a robust link. A proper frequency
deviation is linked to the frequency error between transmitter and receiver. The frequency error can be calculated
using the crystal tolerance parameters and the RF operating frequency: (ppm_tx+ppm_rx)*Frf/1E-6. For frequency
errors below 50 kHz, the deviation can be about the same as the frequency error. For frequency errors exceeding
50 kHz, the frequency deviation can be set to about 0.75 times the frequency error. It is advised to position the
modulation index (= 2*freq_dev/data_rate) into a range between 1 and 100 for Packet Handling mode and 2 to 100
for direct mode (non-standard preamble). For example, when in Packet Handling mode and the frequency error is
smaller than data_rate/2, the frequency deviation is set to about data_rate/2. When the frequency error exceeds
100xdata_rate/2, the frequency deviation is preferred to be set to 100xdata_rate/2.
4.2.4. Channel Bandwidth
The channel bandwidth sets the bandwidth for the receiver. Since the receiver bandwidth is directly proportional to
the noise allowed in the system, this will normally be set as low as possible. The specific channel bandwidth used
will usually be determined based upon the precision of the oscillator and the frequency deviation of the transmitted
signal. The RCA can provide the recommended channel bandwidth based upon these two parameters to help
optimize the system.
4.2.5. Preamble Length
A preamble is a defined simple bit sequence used to notify the receiver that a data transmission is imminent. The
length of this preamble will normally be set as short as possible to minimize power while insuring that it will be
reliably detected given the receiver characteristics, such as duty cycling and packet error rate performance. The
Si4x55 allows the preamble length to be set between 0 to 255 bytes in length with a default length of 4 bytes. The
preamble pattern for the Si4x55 will always be 55h with a first bit of “0” if the packet handler capability is used.
4.2.6. Sync Word Length and Pattern
The sync word follows the preamble in the packet structure and is used to identify the start of the payload data and
to synchronize the receiver to the transmitted bit stream. The Si4x55 allows for sync word lengths of 1 to 4 bytes
and the specific pattern can be set within the RCA program. The default is a 2 byte length 2d d4 pattern.
4.2.7. Cyclic Redundancy Check
Cyclic Redundancy Check (CRC) is used to verify that no errors have occurred during transmission and the
received packet has exactly the same data as it did when transmitted. If this function is enabled in the Si4x55, the
last byte of transmitted data must include the CRC generated by the transmitter. The Si4x55 then performs a CRC
calculation on the received packet and compares that to the transmitted CRC. If these two values are the same, the
Si4455 will set an interrupt indicating a valid packet has been received and is waiting in the Rx FIFO. If these two
CRC values differ, the Si4455 will flag an interrupt indicating that a packet error occurred. The Si4x55 uses
CRC(16)-IBM: x16+x15+x2+1 with a seed of 0xFFFF as well as a 16-bit ITU-T CRC as specified in the IEEE
802.15.4g standard.
4.2.8. Preamble Sense Mode
This mode of operation is suitable for extremely low power applications where power consumption is important.
The preamble sense mode (PSM) takes advantage of the Digital Signal Arrival detector (DSA), which can detect a
preamble within eight bit times with no sensitivity degradation. This fast detection of an incoming signal can be
combined with duty cycling of the receiver during the time the device is searching or sniffing for packets over the
air. The average receive current is lowered significantly when using this mode. In applications where the timing of
the incoming signal is unknown, the amount of power savings is primarily dependent on the data rate and preamble
length as the Rx inactive time is determined by these factors. In applications where the sleep time is fixed and the
timing of the incoming signal is known, the average current also depends on the sleep time. The PSM mode is
similar to the low duty cycle mode but has the benefit of faster signal detection and autonomous duty cycling of the
receiver to achieve even lower average receive currents.
Rev 1.0
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