SI4468-7 Datasheet, PDF (42/57 Pages) Silicon Laboratories – Highly configurable packet handler

Si4468/7
8.3. Temperature, Battery Voltage, and Auxiliary ADC
The Si446x family contains an integrated auxiliary ADC for measuring internal battery voltage, an internal
temperature sensor, or an external component over a GPIO. The ADC utilizes a SAR architecture and achieves
11-bit resolution. The Effective Number of Bits (ENOB) is 9 bits. When measuring external components, the input
voltage range is 1 V, and the conversion rate is between 300 Hz to 2.44 kHz. The ADC value is read by first
sending the GET_ADC_READING command and enabling the inputs that are desired to be read: GPIO, battery, or
temp. The temperature sensor accuracy at 25 °C is typically ±2 °C. Refer to API documentation for details on the
command and reply stream.
8.4. Low Battery Detector
The low battery detector (LBD) is enabled and utilized as part of the wake-up-timer (WUT). The LBD function is not
available unless the WUT is enabled, but the host MCU can manually check the battery voltage anytime with the
auxiliary ADC. The LBD function is enabled in the GLOBAL_WUT_CONFIG API property. The battery voltage will
be compared against the threshold each time the WUT expires. The threshold for the LBD function is set in
GLOBAL_LOW_BATT_THRESH. The threshold steps are in increments of 50 mV, ranging from a minimum of
1.5 V up to 3.05 V. The accuracy of the LBD is ±3%. The LBD notification can be configured as an interrupt on the
nIRQ pin or enabled as a direct function on one of the GPIOs.
8.5. Antenna Diversity
To mitigate the problem of frequency-selective fading due to multipath propagation, some transceiver systems use
a scheme known as antenna diversity. In this scheme, two antennas are used. Each time the transceiver enters RX
mode the receive signal strength from each antenna is evaluated. This evaluation process takes place during the
preamble portion of the packet. The antenna with the strongest received signal is then used for the remainder of
that RX packet. The same antenna will also be used for the next corresponding TX packet. This chip fully supports
antenna diversity with an integrated antenna diversity control algorithm. The required signals needed to control an
external SPDT RF switch (such as a PIN diode or GaAs switch) are available on the GPIOx pins. The operation of
these GPIO signals is programmable to allow for different antenna diversity architectures and configurations. The
antdiv[2:0] bits are found in the MODEM_ANT_DIV_CONTROL API property descriptions and enable the antenna
diversity mode. The GPIO pins are capable of sourcing up to 5 mA of current; so, it may be used directly to
forward-bias a PIN diode if desired. The antenna diversity algorithm will automatically toggle back and forth
between the antennas until the packet starts to arrive. The recommended preamble length for optimal antenna
selection is 8 bytes.
8.6. 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. This mode can be used with the low power mode (LP)
which has an active RX current of 10 mA or with the high-performance (HP) mode which has an active RX current
of 13 mA.
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