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PDF AN238 Data sheet ( Hoja de datos )
| Número de pieza | AN238 | |
| Descripción | Tire Pressure Monitoring (TPM) System | |
| Fabricantes | Microchip Technology | |
| Logotipo |  |
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AN238
Tire Pressure Monitoring (TPM) System
Authors: Ruan Lourens
Microchip Technology Inc.
Curtis Kell
Kell Laboratories
INTRODUCTION
This document explains a typical tire pressure monitor-
ing (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.
An auto-location system can dynamically detect the
position of a specific sensor, which is useful when tires
are rotated. The heart of the TPM system is the
Sensor/Transmitter (S/TX) device and it is based on
Microchip's rfPIC12F675.
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)
FIGURE 1:
TIRE PRESSURE MONITORING (TPM) SYSTEM
2004 Microchip Technology Inc.
DS00238B-page 1
1 page  
www.DataSheet4U.com
FIGURE 4:
LF CIRCUITRY
LF In
R5
10 kΩ
L1 C11
Inductor 220 pF
D3
Schottky
CIN-
COUT
GP3
rfPIC12F675_SSOP
AN238
LF Out
R3
51 kΩ
D2
C9 Schottky
1000 pF
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
system is the 1/3-2/3 PWM format with TE (basic pulse
element) time of 400 µs and a bit period of 3xTE or
1.2 ms.
FIGURE 5:
RF TRANSMISSION
ENCODING METHOD
Logic ‘1’
Logic ‘0’
TE TE TE
Preamble
Transmitter ID
Pressure
Temperature
Battery
Status
CRC
TE TE TE
Bits
32
32
8
8
8
8
16
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.
2004 Microchip Technology Inc.
DS00238B-page 5
5 Page 
www.DataSheet4U.com
Note the following details of the code protection feature on Microchip devices:
• Microchip products meet the specification contained in their particular Microchip Data Sheet.
• Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the
intended manner and under normal conditions.
• There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our
knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip's Data
Sheets. Most likely, the person doing so is engaged in theft of intellectual property.
• Microchip is willing to work with the customer who is concerned about the integrity of their code.
• Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not
mean that we are guaranteeing the product as “unbreakable.”
Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our
products. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts
allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act.
Information contained in this publication regarding device
applications and the like is intended through suggestion only
and may be superseded by updates. It is your responsibility to
ensure that your application meets with your specifications.
No representation or warranty is given and no liability is
assumed by Microchip Technology Incorporated with respect
to the accuracy or use of such information, or infringement of
patents or other intellectual property rights arising from such
use or otherwise. Use of Microchip’s products as critical
components in life support systems is not authorized except
with express written approval by Microchip. No licenses are
conveyed, implicitly or otherwise, under any intellectual
property rights.
Trademarks
The Microchip name and logo, the Microchip logo, Accuron,
dsPIC, KEELOQ, MPLAB, PIC, PICmicro, PICSTART,
PRO MATE and PowerSmart are registered trademarks of
Microchip Technology Incorporated in the U.S.A. and other
countries.
AmpLab, FilterLab, microID, MXDEV, MXLAB, PICMASTER,
SEEVAL, SmartShunt and The Embedded Control Solutions
Company are registered trademarks of Microchip Technology
Incorporated in the U.S.A.
Application Maestro, dsPICDEM, dsPICDEM.net,
dsPICworks, ECAN, ECONOMONITOR, FanSense,
FlexROM, fuzzyLAB, In-Circuit Serial Programming, ICSP,
ICEPIC, Migratable Memory, MPASM, MPLIB, MPLINK,
MPSIM, PICkit, PICDEM, PICDEM.net, PICtail, PowerCal,
PowerInfo, PowerMate, PowerTool, rfLAB, rfPIC, Select
Mode, SmartSensor, SmartTel and Total Endurance are
trademarks of Microchip Technology Incorporated in the
U.S.A. and other countries.
Serialized Quick Turn Programming (SQTP) is a service mark
of Microchip Technology Incorporated in the U.S.A.
All other trademarks mentioned herein are property of their
respective companies.
© 2004, Microchip Technology Incorporated, Printed in the
U.S.A., All Rights Reserved.
Printed on recycled paper.
2004 Microchip Technology Inc.
Microchip received ISO/TS-16949:2002 quality system certification for
its worldwide headquarters, design and wafer fabrication facilities in
Chandler and Tempe, Arizona and Mountain View, California in October
2003. The Company’s quality system processes and procedures are for
its PICmicro® 8-bit MCUs, KEELOQ® code hopping devices, Serial
EEPROMs, microperipherals, nonvolatile memory and analog
products. In addition, Microchip’s quality system for the design and
manufacture of development systems is ISO 9001:2000 certified.
DS00238B-page 11
11 Page | ||
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| PDF Descargar | [ Datasheet AN238.PDF ] | |
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