 2004 Microchip Technology Inc.

DS00238B-page 5

AN238

FIGURE 4:

LF CIRCUITRY

Details of the LF transmission format and the specific

commands can be found in the Section, "LF Com-

mander".

RF Transmission Format

The encoding method used for this demonstration

element) time of 400

1.2 ms.

FIGURE 5:

RF TRANSMISSION

ENCODING METHOD

Preamble: The preamble is a series of 31 logic ‘1’ bits

followed by a single logic ‘0’ bit. The preamble allows

the receiver to recognize the RF transmission as a valid

S/TX message. The preamble also allows the receiver

to synchronize to the RF message, thereby compen-

sating for any oscillator inaccuracies within the

transmitter. The system designer may vary the number

of preamble bits based on system requirements.

Longer preamble bit lengths may be appropriate where

receiver quiescent current is an issue. Shorter

preamble bit lengths may be appropriate where S/TX

battery usage is a concern. In either case, it is purely a

trade-off between receiver quiescent current and

battery power consumed by the S/TX device.

Transmitter ID: The 32 transmitter ID bits are used to

uniquely identify each S/TX. A frame of 32 bits insures

that there is a very low probability that any two S/TXs

will have the same ID.

Pressure: The pressure in kPa is obtained by multiply-

ing the unsigned binary value of this byte by 2.5.

Temperature: The temperature in degrees C is

obtained by subtracting 40 from the unsigned value of

byte 8.

Battery: Bit 7 of this byte indicates the battery

condition. A logic ‘1’ is considered normal while a logic

‘0’ indicates a low battery voltage.

Status: This status byte contains the following informa-

tion:

Bits 0 and 1: Indicate operating state of sensor IC

00

= Initial or Storage mode

01

= Normal mode

10

= Pressure Alert mode

11

= Temperature Alert mode

CRC (2 bytes): Implement according to CCITT

standards.

L1

D3

C11

Schottky

R5

10 k

Ω

rfPIC12F675_SSOP

R3

51 k

Ω

C9

1000 pF

LF Out

LF In

COUT

GP3

CIN-

D2

Schottky

220 pF

Inductor

Bits

Preamble

32

Transmitter ID

32

Pressure

8

Temperature

8

Battery

8

Status

8

CRC

16

Logic ‘1’

Logic ‘0’