AN746
Rev. 0.1
9
Figure 4. Transmit FIFO Status Signals
4.5. How to Measure the Battery Voltage
The Si4012 is able to measure its own supply voltage in two modes.
One mode of operation is to measure the supply voltage whenever the GET_BAT_STATUS command is invoked.
The supply voltage is measured after the main current-consuming blocks of the device are temporarily turned on.
These parts are turned off after the measurement is done. The measurement itself takes place after
(LoadWaitTime x 17 µs) of wait time after the GET_BAT_STATUS command is invoked. If the LoadWaitTime
parameter of the command is set to 0, the battery voltage is measured immediately without any load applied. The
command gives back the result in mV.
In the other mode used for battery measurement, the Si4012 measures the supply voltage periodically with no load
applied. If the battery voltage is below the LBD threshold, the Low Battery Detector Interrupt is raised. This
periodical measurement happens only if Si4012 is in sensor mode and low battery interrupt is enabled. The period
and threshold can be set in the LBD_CONFIG property.
4.6. Crystal Oscillator Tuning Capacitor
When crystal is used, the accuracy of the radio center frequency is determined by the parameters of the crystal
(such as load capacitance, crystal accuracy, etc.) and the parasitic of the PCB associated with the crystal circuit. To
reduce the impact of these crystal parameters and to guarantee safe startup of the oscillator, the user has to do the
following:
Check the crystal data sheet for the “Cload” capacitor value that should be placed across the crystal’s
terminals to oscillate at the correct frequency
If Cload > 14 pF, XoLowCap field of the XO_CONFIG property have to be set to 0. In this case, the input
capacitance of the XTAL pin of the Si4012 is 5 pF, so a (Cload – 5)pF capacitor should be placed externally
across the crystal terminals.
If Cload < 14 pF, XoLowCap field of the XO_CONFIG property have to be set to 1. In this case, the input
capacitance of the XTAL pin of the Si4012 is 2.5 pF, so the external capacitor placed across the crystal has
to be (Cload – 2.5) pF.
The example program does not use a crystal.
Summary of Contents for Si4012
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