AN238

DS00238B-page 6

 2004 Microchip Technology Inc.

LF Commander

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:

• Enable RF transmissions

• Disable RF transmissions

• Transmit an immediate message

• Transmit at 60-second intervals

• Transmit at 15-second intervals

• Transmit at 5-second intervals

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 PICmicro's PWM port. This port is set up to gener-

ate a 50% duty cycle at 125 kHz. The command data is

then modulated on this 125 kHz signal in the form of

ASK modulation. Functionally, this is accomplished by

instructing the PICmicro 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

1/3 - 2/3 PWM format with 400

element). Selecting 400

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.

FIGURE 6:

LF TRANSMISSION

ENCODING METHOD

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 CRT's and

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).

TABLE 1:

BIT FUNCTIONS

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 based S/TX makes for a flexible

solution that allows for the merging with existing

systems such as security, PKE, RKE and more.

Bits

Function

01101101

Enable RF transmissions

10010010

Disable RF transmissions

10101010

Transmit an immediate RF message

10110110

Transmit at 60-second interval

11001100

Transmit at 15-second interval

11010011

Transmit at 5-second interval

Bits

Preamble

16

Command

8

Sum Check

8

Logic ‘1’

Logic ‘0’