SI4438 Datasheet, PDF (25/44 Pages) Silicon Laboratories – Low active power consumption

Si4438
5.3.1.2. Automatic RX Hopping and Hop Table
The transceiver supports an automatic hopping feature that can be fully configured through the API. This is
intended for RX hopping where the device has to hop from channel to channel and look for packets. Once the
device is put into the RX state, it automatically starts hopping through the hop table if the feature is enabled.
The hop table can hold up to 64 entries and is maintained in firmware. Each entry is a channel number; so, the hop
table can hold 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 non 0xFF entry.
There are three conditions that can be used to determine whether to continue hopping or to stay on a particular
channel. These conditions are:
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, 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.
Note that the RSSI threshold is not an absolute RSSI value; instead, it is a relative value and should be verified on
the bench to find an optimal threshold for the application.
The turnaround time from RX to RX on a different channel using this method is 115 µs. 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.
The following are example steps for using Auto Hop:
1. Set the base frequency (inte + frac) and channel step size.
2. Define the number of entries in the hop table (RX_HOP_TABLE_SIZE).
3. Write the channels to the hop table (RX_HOP_TABLE_ENTRY_n)
4. Configure the hop condition and enable auto hopping- RSSI, preamble, or sync (RX_HOP_CONTROL).
5. Set preamble and sync parameters if enabled.
6. Program the RSSI threshold property in the modem using “MODEM_RSSI_THRESH”.
7. Set the preamble threshold using “PREAMBLE_CONFIG_STD_1”.
8. Program the preamble timeout property using “PREAMBLE_CONFIG_STD_2”.
9. Set the sync detection parameters if enabled.
10. If needed, use “GPIO_PIN_CFG” to configure a GPIO to toggle on hop and hop table wrap.
11. Use the “START_RX” API with channel number set to the first valid entry in the hop table (i.e., the first non
0xFF entry).
12. Device should now be in auto hop mode.
5.3.1.3. Manual RX Hopping
The RX_HOP command provides the fastest method for hopping from RX to RX but it requires more overhead and
management by the host MCU. Using the RX_HOP command, the turn-around time is 75 µs. The timing is faster
with this method than Start_RX or RX hopping because one of the calculations required for the synthesizer
calibrations is offloaded to the host and must be calculated/stored by the host, VCO_CNT0. For information about
using fast manual hopping, contact customer support.
Rev 1.0
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