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RSSI dBm = --------------------------------- – 130
Si4455
RSSI_value
2
The value of 130 in the above formula is based on bench characterization of the EZRadio RF Pico boards
(evaluation boards). The RSSI value is latched at sync word detection and can be read via the fast response
register. The latched value of RSSI is available until the device re-enters Rx mode. In addition, the current value of
RSSI can be read out using the GET_MODEM_STATUS command. This can be used to implement CCA (clear
channel assessment) functionality. The user can set up an RSSI threshold value using the WDS Radio
Configuration Application GUI.
3.3. Synthesizer
The Si4455 includes an integrated Sigma Delta ( ?? ) Fractional-N PLL synthesizer capable of operating over the
bands from 283–350, 425–525, and 850–960 MHz. The synthesizer has many advantages; it provides flexibility in
choosing data rate, deviation, channel frequency, and channel spacing. The transmit modulation is applied directly
to the loop in the digital domain through the fractional divider, which results in very precise accuracy and control
over the transmit deviation. The frequency resolution is (2/3)Freq_xo/(2 19 ) for 283–350 MHz, Freq_xo/(2 19 ) for
425–525 MHz, and Freq_xo/(2 18 ) for 850–960 MHz. The nominal reference frequency to the PLL is 30 MHz, but
any XTAL frequency from 25 to 32 MHz may be used. The modem configuration calculator in WDS will
automatically account for the XTAL frequency being used. The PLL utilizes a differential LC VCO with integrated
on-chip inductors. The output of the VCO is followed by a configurable divider, which will divide the signal down to
the desired output frequency band.
3.3.1. Synthesizer Frequency Control
The frequency is set by changing the integer and fractional settings to the synthesizer. The WDS calculator will
automatically provide these settings, but the synthesizer equation is shown below for convenience. Initial
frequency settings are configured in the EZConfig setup and can also be modified using the API commands:
FREQ_CONTROL_INTE, FREQ_CONTROL_FRAC2,
FREQ_CONTROL_FRAC1, and
RF frequency = ? fc_inte + ------------------ ? ? ------------------------------- ? Hz ?
? 19 ? outdiv
FREQ_CONTROL_FRAC0.
fc_frac 4 ? freq_xo
2
Note: The fc_frac/2 19 value in the above formula must be a number between 1 and 2. The LSB of fc_frac must be "1".
Table 10. Output Divider (Outdiv) Values
Outdiv
12
8
4
Lower (MHz)
284
425
850
Upper (MHz)
350
525
960
3.3.1.1. EZ Frequency Programming
EZ frequency programming allows for easily changing radio frequency using a single API command. The base
frequency is first set using the EZConfig setup. This base frequency will correspond to channel 0. Next, a channel
step size is also programmed within the EZConfig setup. The resulting frequency will be:
RF Frequency = Base Frequency + Channel ? Step Size
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, 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
previous value. The initial value of CHANNEL is 0 and so will be set to the base frequency if this argument is never
used.
Rev 1.1
15
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