AN238

DS00238B-page 2

 2004 Microchip Technology Inc.

Sensor/Transmitter (S/TX) Device

There are typically five S/TX units per vehicle, one per

wheel, and the spare tire. Each unit has a unique serial

number enabling the system to distinguish between

each tire. When mounted within a vehicle tire, the S/TX

periodically measures internal tire pressure, tempera-

ture and battery condition. It then sends a RF signal

composed of the measured information to a central

receiver. The device described in this document is

based on Microchip's rfPIC12F675 and the pressure

and temperature sensing is performed by the Sensonor

SP-13, a sensor IC (www.sensonor.com). The unit is

also equipped with a LF receiver unit, used to commu-

nicate to the S/TX device and to enable it from a Sleep

state.

RF Receiver Module

A central RF receiver module receives transmissions

from the individual S/TX devices. The receiver can also

be used as a remote keyless entry receiver, saving on

overall system cost. The design of the RF receiver

module falls beyond the intent of this document. A

functional RF receiver module is assumed.

LF Commander Device

The LF commander is designed to send specific

commands to the S/TX unit via a 125 kHz ASK modu-

lated signal. The LF link communicates over a short

distance (1 meter or less), thus making it capable of

communicating with the wheel in its immediate range.

LF magnetic communications is well suited for sending

commands to the S/TX devices. These commands,

when received by the S/TX device, instruct it to carry

out specific tasks.

Control Unit

The control unit is responsible for initiating communica-

tions, interpreting received data and reporting the

relevant information back to the vehicle. The unit will

only be treated from a system overview perspective.

Pressure Vessel

The pressure vessels (tires) are the measurement

subjects, with pressure and temperature values

measured and reported.

TPM Sensor/Transmitter

TECHNICAL SPECIFICATIONS

• Modulation Format: ASK

• Operating Voltage: 2.5-3.6V

• Low Voltage Alert Threshold: 2.5V

• Quiescent Current: TBD

RF Specific:

• Transmit Frequency: 315 MHz

• Transmit Interval: 60, 15 or 5 seconds (LF

selectable)

• Power Output: +9 dBm into 50

Ω load

• Operating Current – Transmit: 12.5 mA at max RF

power

LF Specific:

• Input Frequency: 125 kHz

• Input Sensitivity: TBD

Pressure Sensor Specific:

• Pressure Sensor Range: 50-637 kPa absolute

• Temperature Sensor Range: -40–125

°C

The schematic for the TPM S/TX is shown in Appendix

A: “Schematics”.

THEORY OF OPERATION

The S/TX device comprises two integrated circuits:

• Microchip’s rfPIC12F675 MCU/RF transmitter IC

• Sensonor SP-13 (pressure, temperature and

low-voltage sensor IC)

In addition, the S/TX also includes LF input circuitry.

This circuitry allows the S/TX device to receive special

commands via the LF link. Refer to Appendix A:

“Schematics” for additional circuit detail.

rfPIC12F675 Transmitter IC

The rfPIC

the heart of the S/TX for several reasons. First, the

PIC12FXXX series of microcontrollers are widely used

microcontroller devices are currently used in transmit-

ter applications today. Second, this device features an

internal RC oscillator, thereby reducing the external

component count which, directly reduces module cost

as well as circuit board size. Third, this device includes

the RF transmitter circuitry, which again reduces exter-

nal component count, cost and overall size of the circuit

board. The rfPIC12F675 also has an internal compara-

tor which plays an important role in decoding the infor-

mation from the LF link. The internal comparator helps

reduce overall part count, thereby further reducing

module cost and circuit board size. Lastly, the

rfPIC12F675

features

a

10-bit

Analog-to-Digital

converter, allowing the designer to use analog output

pressure sensors.

The rfPIC performs three functions. It monitors the data

line from the SP-13 sensor IC and from the LF input,

and assembles and transmits a RF message at

periodic intervals.