SI4460-C2A-GM Datasheet, PDF (32/53 Pages) Silicon Laboratories – HIGH-PERFORMANCE, LOW-CURRENT TRANSCEIVER

Si4463/61/60-C
5.3.1.1. EZ Frequency Programming
In applications that utilize multiple frequencies or channels, it may not be desirable to write four API registers each
time a frequency change is required. EZ frequency programming is provided so that only a single register write
(channel number) is required to change frequency. A base frequency is first set by first programming the integer
and fractional components of the synthesizer. This base frequency will correspond to channel 0. Next, a channel
step size is programmed into the FREQ_CONTROL_CHANNEL_STEP_SIZE_1 and
FREQ_CONTROL_CHANNEL_STEP_SIZE_0 API registers. The resulting frequency will be:
RF Frequency = Base Frequency + Channel  Stepsize
The second argument of the START_RX or START_TX is CHANNEL, which sets the channel number for EZ
frequency programming. For example, if the channel step size is set to 1 MHz, the base frequency is set to
900 MHz with the FREQ_CONTROL_INTE and FREQ_CONTROL_FRAC API properties, and a CHANNEL
number of 5 is programmed during the START_TX command, the resulting frequency will be 905 MHz. If no
CHANNEL argument is written as part of the START_RX/TX command, it will default to the previously-programmed
value. The initial value of CHANNEL is 0; so, if no CHANNEL value is written, it will result in the programmed base
frequency.
5.3.1.2. Automatic RX Hopping and Hop Table
The transceiver supports an automatic RX hopping feature that can be fully configured through the API. This
functionality is useful in applications where it is desired to look for packets but to hop to the next channel if a packet
is not found. The sequence of channel numbers that are visited are specified by entries in a hop table. If this
feature is enabled, the device will automatically start hopping through the channels listed in the hop table as soon
as the chip enters RX mode.
The hop table can hold up to 64 entries and is maintained in firmware inside the RFIC. Each entry is a channel
number, allowing construction of a frequency plan of up to 64 channels. The number of entries in the table is set by
RX HOP TABLE_SIZE API. The specified channels correspond to the EZ frequency programming method for
programming the frequency. The receiver starts at the base channel and hops in sequence from the top of the hop
table to the bottom. The table will wrap around to the base channel once it reaches the end of the table. An entry of
0xFF in the table indicates that the entry should be skipped. The device will hop to the next entry in the table that
contains a non-0xFF value.
There are three conditions that can be used to determine whether to continue hopping or to stay on a particular
channel. These conditions are as follows:
RSSI threshold
Preamble timeout (invalid preamble pattern)
Sync word timeout (invalid or no sync word detected after preamble)
These conditions can be used individually, or they can be enabled all together by configuring the
RX_HOP_CONTROL API. However, the firmware will make a decision on whether or not to hop based on the first
condition that is met.
The RSSI that is monitored is the current RSSI value. This is compared to the threshold value set in the
MODEM_RSSI_THRESH API property, and, if it is above the threshold value, it will stay on the channel. If the
RSSI is below the threshold, it will continue hopping. There is no averaging of RSSI done during the automatic
hopping from channel to channel. Since the preamble timeout and the sync word timeout are features that require
packet handling, the RSSI threshold is the only condition that can be used if the user is in “direct” or “RAW” mode
where packet handling features are not used.
The RSSI threshold value may be converted to an approximate equivalent RF input power level through the
equation shown in "5.2.4. Received Signal Strength Indicator" on page 30. However, performance should be
verified on the bench to optimize the threshold setting for a given application.
The time spent in receive mode will be determined by the configuration of the hop conditions. Manual RX hopping
will have the fastest turn-around time but will require more overhead and management by the host MCU.
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