Rainbow Electronics RF12B - Manual (Page 26)

RF12B
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Polling Mode:
When nFFS signal is low the FIFO output is connected directly to the SDO pin and its content can be
clocked out by the SCK. Set the FIFO IT level to 1. In this case, as long as FFIT indicates received bits in
the FIFO, the controller may continue to take the bits away. When FFIT goes low, no more bits need to be
taken. An SPI read command is also available to read out the content of the FIFO.
RECOMMENDED PACKET STRUCTURES
Preamble Synchron word
(Can be network ID)
Payload CRC
Minimum length 4 - 8 bit (1010b or 0101b) D4h (programmable) ? 4 bit - 1 byte
Recommended
length
8 -12 bit (e.g. AAh or 55h) 2DD4h (D4 is
programmable)
? 2 byte
CRYSTAL SELECTION GUIDELINES
The crystal oscillator of the RF12B requires a 10 MHz parallel mode crystal. The circuit contains an
integrated load capacitor in order to minimize the external component count. The internal load
capacitance value is programmable from 8.5 pF to 16 pF in 0.5pF steps. With appropriate PCB layout,
the total load capacitance value can be 10 pF to 20 pF so a variety of crystal types can be used.
When the total load capacitance is not more than 20 pF and a worst case 7 pF shunt capacitance (C
0
)
value is expected for the crystal, the oscillator is able to start up with any crystal having less than 300
ohms ESR (equivalent series loss resistance). However, lower C
0
and ESR values guarantee faster
oscillator startup.
The crystal frequency is used as the reference of the PLL, which generates the local oscillator
frequency (f
LO
). Therefore f
LO
is directly proportional to the crystal frequency. The accuracy requirements
for production tolerance, temperature drift and aging can thus be determined from the maximum
allowable local oscillator frequency error.
Whenever a low frequency error is essential for the application, it is possible to “pull” the crystal to
the accurate frequency by changing the load capacitor value. The widest pulling range can be achieved if
the nominal required load capacitance of the crystal is in the “midrange”, for example 16 pF. The
“pull-ability” of the crystal is defined by its motional capacitance and C
0
.
Maximum XTAL Tolerances Including Temperature and Aging [ppm]
Bit Rate:
2.4 kbps Deviation [+/- kHz]
30 45 60 75 90 105 120
315 MHz 25 50 75 100 100 100 100
433 MHz 20 30 50 70 90 100 100
868 MHz 10 20 25 30 40 50 60
915 MHz 10 15 25 30 40 50 50
Bit Rate:
9.6 kbps Deviation [+/- kHz]
30 45 60 75 90 105 120
315 MHz 20 50 70 75 100 100 100