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参数资料
| 型号： | APGAC031 |
| 厂商： | Microchip Technology |
| 文件页数： | 6/12页 |
| 文件大小： | 0K |
| 描述： | KIT ACC TIRE PRESSURE MON SYSTEM |
| 标准包装： | 1 |
| 类型： | 汽车 - 轮胎压力监视器 |
| 频率： | 315MHz |
| 适用于相关产品： | APGRD003 |
| 已供物品： | 轮胎传感器节点 |
AN238
LF Commander
FIGURE 6:
LF TRANSMISSION
ENCODING METHOD
THEORY OF OPERATION
The system proposed in this document is based on an
auto-location system, enabling it to detect the position
of a specific S/TX device. This requires a LF
commander device at each wheel arc and possibly at
the spare tire mounting position.
Having a handheld LF commander unit can enable a
lower cost system. Although, this would require that the
system be manually relearned after a tire rotation, the
S/TX device is able to detect tire rotation or some other
system to engage data transmission. The LF
commander device is capable of sending commands to
the S/TX device via a LF transmission such as:
Logic ‘1’
T E T E T E
Preamble
Command
Logic ‘ 0 ’
T E T E T E
Bits
16
8
?
?
Enable RF transmissions
Disable RF transmissions
Sum Check
8
?
?
?
?
Transmit an immediate message
Transmit at 60-second intervals
Transmit at 15-second intervals
Transmit at 5-second intervals
Preamble : The preamble is a series of 15 logic ‘ 1 ’ bits
and 1 logic ‘ 0 ’ bit. This reduces the chance of the S/TX
receiving erroneous data from electronic devices that
generate strong 125 kHz fields. Computer CRTs and
The LF commander unit is based on the PIC16F628
MCU device. Communication between the LF
commander and the S/TX is accomplished via
magnetic field. When the LF commander is transmitting
a message, it is essentially creating a magnetic field by
exciting a series LC circuit. The LC circuit is excited by
the PIC ? microcontroller PWM port. This port is set up
switching power supplies are examples of such
devices.
Command : These 8 bits of data contains the specific
command that the S/TX is being asked to perform.
Table 1 illustrates the various commands (MSB is left
most column).
to generate a 50% duty cycle at 125 kHz. The com-
mand data is then modulated on this 125 kHz signal in
the form of ASK modulation. Functionally, this is
accomplished by instructing the PIC microcontroller to
toggle the PWM port between 0% and 50% duty cycle
at the rate of the data.
LF TRANSMISSION FORMAT
The encoding format used in the LF link is a
TABLE 1:
Bits
01101101
10010010
10101010
10110110
11001100
BIT FUNCTIONS
Function
Enable RF transmissions
Disable RF transmissions
Transmit an immediate RF message
Transmit at 60-second interval
Transmit at 15-second interval
1/3-2/3 PWM format with 400 μ s T E (basic pulse
element). Selecting 400 μ s T E or greater ensures the
magnetic field generated by the series LC circuit has
adequate time to rise and decay, without requiring too
much wave shaping of the recovered square wave in
the S/TX circuitry.
The transmission data format for the LF link is shown in
Figure 6.
DS00238C-page 6
11010011 Transmit at 5-second interval
Sum Check : Calculated by adding contents of the
command byte with ‘ 10101010 ’.
SUMMARY
TPM use in the automotive industry is growing, driven
by customer demand, improved safety, and possible
compulsory-usage legislation. Microchip’s low-cost
rfPIC devices are well suited for the application and
help reduce the overall TPM system cost.
The use of rfPIC microcontroller-based S/TX makes for
a flexible solution that allows for the merging with exist-
ing systems such as security, PKE, RKE and more.
? 2009 Microchip Technology Inc.
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