20-09-2012, 02:50 PM
Tire Pressure Monitoring (TPM) System
[attachment=34168]
INTRODUCTION
This document explains a typical tire pressure monitoring
(TPM) system specifically intended for automotive
use. It serves as a reference to design a real-world
system based on various Microchip products. A TPM
system primarily monitors the internal temperature and
pressure of an automobile’s tire. There is a variety of
system approaches to follow, although this one is a
rather comprehensive auto-location system.
SYSTEM COMPONENTS
The TPM system consists of the following major
component.
• Sensor/Transmitter Device
• RF Receiver Module
• Low-Frequency (LF) Commander Device
• Control Unit
• Pressure Vessel (Tire)
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, temperature
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 communicate
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 modulated
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.
RF Circuitry
The PLL style transmitter within the rfPIC microcontroller
requires minimum external components to complete
the RF transmitter. The fundamental frequency of
the transmitter 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.
LF Input Circuitry
The LF input is designed to receive and demodulate a
125 kHz signal and transform the received data into a
specific command. The LF input circuit makes use of
the internal comparator of the rfPIC microcontroller,
thereby reducing cost, module size and quiescent current.
The LF input circuitry features a LC tank circuit
that is tuned to 125 kHz. The LF sensing input comprises
L1 and C11. L1 is specially designed by Coilcraft
for this type of application. It provides good sensitivity
in a compact package. A conventional coil could be
used in its place, but overall circuit sensitivity or range
would be reduced. Schottky diode D3 is used to clamp
the voltage developed across the LC tank to safe levels.
The output of the LC tank circuit, after passing
through current limiting resistor R5, is fed into the rfPIC
microcontroller comparator’s negative input. The comparator’s
positive input is configured as VREF through
the rfPIC12F675 VREF module. The output of the comparator
is then fed into an envelope detector consisting
of Schottky diode D2, capacitor C9 and resistor R3. C9
and R3 are selected to provide adequate filtering of the
LF frequency without rounding the edges of the desired
data signal. The output of the envelope detector is then
fed directly into a port pin on the rfPIC microcontroller
and used to process the LF data.
[attachment=34168]
INTRODUCTION
This document explains a typical tire pressure monitoring
(TPM) system specifically intended for automotive
use. It serves as a reference to design a real-world
system based on various Microchip products. A TPM
system primarily monitors the internal temperature and
pressure of an automobile’s tire. There is a variety of
system approaches to follow, although this one is a
rather comprehensive auto-location system.
SYSTEM COMPONENTS
The TPM system consists of the following major
component.
• Sensor/Transmitter Device
• RF Receiver Module
• Low-Frequency (LF) Commander Device
• Control Unit
• Pressure Vessel (Tire)
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, temperature
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 communicate
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 modulated
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.
RF Circuitry
The PLL style transmitter within the rfPIC microcontroller
requires minimum external components to complete
the RF transmitter. The fundamental frequency of
the transmitter 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.
LF Input Circuitry
The LF input is designed to receive and demodulate a
125 kHz signal and transform the received data into a
specific command. The LF input circuit makes use of
the internal comparator of the rfPIC microcontroller,
thereby reducing cost, module size and quiescent current.
The LF input circuitry features a LC tank circuit
that is tuned to 125 kHz. The LF sensing input comprises
L1 and C11. L1 is specially designed by Coilcraft
for this type of application. It provides good sensitivity
in a compact package. A conventional coil could be
used in its place, but overall circuit sensitivity or range
would be reduced. Schottky diode D3 is used to clamp
the voltage developed across the LC tank to safe levels.
The output of the LC tank circuit, after passing
through current limiting resistor R5, is fed into the rfPIC
microcontroller comparator’s negative input. The comparator’s
positive input is configured as VREF through
the rfPIC12F675 VREF module. The output of the comparator
is then fed into an envelope detector consisting
of Schottky diode D2, capacitor C9 and resistor R3. C9
and R3 are selected to provide adequate filtering of the
LF frequency without rounding the edges of the desired
data signal. The output of the envelope detector is then
fed directly into a port pin on the rfPIC microcontroller
and used to process the LF data.