RF96 datasheet(77/121 Pages) HOPE

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Page 77

RF96/97/98

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WIRELESS & SENSING

PRELIMINARY

DATASHEET

5.3.2. CLKOUT Output

The reference frequency, or a fraction of it, can be provided on DIO5 (pin 13) by modifying bits ClkOut in RegDioMapping2.

Two typical applications of the CLKOUT output include:

 To provide a clock output for a companion processor, thus saving the cost of an additional oscillator. CLKOUT can be

made available in any operation mode except Sleep mode and is automatically enabled at power on reset.

 To provide an oscillator reference output. Measurement of the CLKOUT signal enables simple software trimming of the

initial crystal tolerance.

Note

To minimize the current consumption of the RF96/97/98, please ensure that the CLKOUT signal is disabled

when not required.

5.3.3. PLL

The local oscillator of the RF96/97/98 is derived from two almost identical fractional-N PLLs that are referenced to the

crystal oscillator circuit. Both PLLs feature a programmable bandwidth setting where one of four discrete preset bandwidths

may be accessed.

The RF96/97/98 PLL uses a 19-bit sigma-delta modulator whose frequency resolution, constant over the whole

frequency range, is given by:

STE P

----------------

The carrier frequency is programmed through RegFrf, split across addresses 0x06 to 0x08:

R F = FSTEP

× Frf(23,0)

Note

The Frf setting is split across 3 bytes. A change in the center frequency will only be taken into account when the

least significant byte FrfLsb in RegFrfLsb is written. This allows the potential for user generation of m-ary FSK at

very low bit rates. This is possible where frequency modulation is achieved by direct programming of the

programmed RF centre frequency. To enable this functionality set the FastHopOn bit of register RegPllHop.

5.3.4. RC Oscillator

All timing operations in the low-power Sleep state of the Top Level Sequencer rely on the accuracy of the internal low-

power RC oscillator. This oscillator is automatically calibrated at the device power-up not requiring any user input.

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