Rx fifo buffered data read, Crystal selection guidelines, Rf12 – Rainbow Electronics RF12 User Manual

Page 25

RF12

RX FIFO BUFFERED DATA READ

In this operating mode, incoming data are clocked into a 16 bit FIFO buffer. The receiver starts to fill

up the FIFO when the Valid Data Indicator (VDI) bit and the synchron pattern recognition circuit indicates

potentially real incoming data. This prevents the FIFO from being filled with noise and overloading the

external microcontroller.

Polling Mode:

The nFFS signal selects the buffer directly and its content can be clocked out through pin SDO by

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.

Interrupt Controlled Mode:

The user can define the FIFO level (the number of received bits) which will generate the nFFIT when

exceeded. The status bits report the changed FIFO status in this case.

FIFO Read Example with FFIT Polling

During FIFO access f

SCK

cannot be higher than f

ref

/4, where f

ref

is the crystal oscillator frequency.

CRYSTAL SELECTION GUIDELINES

The crystal oscillator of the RF12 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 0.5 pF 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

)

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

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

). Therefore f

is directly proportional to the crystal frequency. The accuracy requirements

for production tolerance, temperature drift and aging can thus be determined from the maximum

Tel: +86-755-86096587 Fax: +86-755-86096602 E-mail: [email protected] http://www.hoperf.com