 2004 Microchip Technology Inc.

DS00238B-page 3

AN238

After application of power, the rfPIC executes an initial-

ization procedure and goes into a Sleep mode until a

state change is detected on either the SP-13 data line

or the LF input. Either of these inputs generates a

wake-up, causing the rfPIC to transition into the Run

mode. If the wake-up was generated by the SP-13, the

rfPIC reads the incoming data, assembles the data into

an appropriate message, and transmits the message

via the RF transmitter. Once the RF message is sent,

the rfPIC reenters the Sleep mode. If the wake-up was

generated by the LF input, the rfPIC interprets the LF

message, executes the command and then reenters

the Sleep mode.

RF Circuitry

The PLL style transmitter within the rfPIC requires

minimum external components to complete the RF

transmitter. The fundamental frequency of the transmit-

ter is determined by Y1. To derive the appropriate

crystal frequency, simply divide the desired transmit

frequency by 32. For example, if the desired transmit

frequency is 315 MHz, the crystal frequency is

9.84375 MHz.

Loop antenna L3 is matched to the single-ended RF

driver via C3 and C8, which also form the resonant tank.

Devices” Application Note AN831 (DS00831) and

“Designing Loop Antennas for the rfPIC12F675”

Application Note AN868 (DS00868) for additional

technical detail on selecting the appropriate component

values for your RF application.

Capacitor C4 is selected to provide decoupling for the

3V supply. Be sure the components selected for your

application have a self-resonant frequency well above

the desired transmit frequency. The filter formed by L2

and R6 further help decouple the high frequency

energy from the rest of the circuitry. The R6 also de-Q’s

the antenna.

The output power of the transmitter circuit can be

adjusted via R8, maximum power is obtained when it is

left an open circuit. The transmit power can be changed

per the “Power Select Resistor Select” table located in

the “rfPIC12F675” Data Sheet (DS70091). This is also

useful when trying to certify a product to FCC

regulations.

FIGURE 2:

RF CIRCUITRY

ANT

PS

Y1

C5

100 pF

R6

220

Ω

Loop Antenna

L2

120 nH

C4

270 pF

C8

22 pF

C3

5pF

R8

N. I.

XTAL

L3