SI4468-7 Datasheet, PDF (26/57 Pages) Silicon Laboratories – Highly configurable packet handler

Si4468/7
4.3. Preamble Length
4.3.1. Digital Signal Arrival Detector (DSA)
Traditional preamble detection requires 20 preamble bits to detect a preamble. This device introduces a new
approach to signal detection that can detect a preamble pattern in as little as one byte. If AFC is enabled, a
preamble length of two bytes is sufficient to reliably detect signal arrival and settle a one-shot AFC. The impact of
this is significant for low-power solutions as it reduces the amount of time the receiver has to stay active to detect
the preamble. This feature is used with Preamble Sense Mode (see "8.6. Preamble Sense Mode" on page 42) and
the latest WMBus N modes as well as with features, such as frequency hopping, which may use the DSA as a
condition to hop. The traditional preamble detector is also available to maintain backward compatibility. Note that
the DSA is using the RSSI jump detector. When used for collision detection, the RSSI jump detector may need to
be reconfigured after preamble detection. Refer to the API documentation for details on how to configure the
device to use the signal arrival detector.
4.3.2. Traditional Preamble Detection
Optimal performance of the chip is obtained by qualifying reception of a valid Preamble pattern prior to continuing
with reception of the remainder of the packet (e.g., Sync Word and Payload). Reception of the Preamble is
considered valid when a minimum number of consecutive bits of 101010... pattern have been received; the
required threshold for preamble detection is specified by the RX_THRESH[6:0] field in the
PREAMBLE_CONFIG_STD_1 property. The appropriate value of the detection threshold depends upon the
system application and typically trades off speed of acquisition against the probability of false detection. If the
detection threshold is set too low, the chip may readily detect the short pattern within noise; the chip then proceeds
to attempt to detect the remainder of the non-existent packet, with the result that the arrival of an actual valid
packet may be missed. If the detection threshold is set too high, the required number of transmitted Preamble bits
must be increased accordingly, leading to longer packet lengths and shorter battery life. A preamble detection
threshold value of 20 bits is suitable for most applications. The total length of the transmitted Preamble field must
be at least equal to the receive preamble detection threshold, plus an additional number of bits to allow for
acquisition of bit timing and settling of the AFC algorithm. The recommended preamble detection thresholds and
preamble lengths for a variety of operational modes are listed in Table 13.
Configuration of the preamble detection threshold in the RX_THRESH[6:0] field is only required for reception of a
standard Preamble pattern (i.e., 101010... pattern). Reception of a repetitive but non-standard Preamble pattern is
also supported in the chip but is configured through the PREAMBLE_CONFIG_NSTD and PREAMBLE_PATTERN
properties.
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