NXP NTM88 User guide

Brand: NXP

Model: NTM88

Type: User guide

UM11361

NTM88 Single-Axis Location Angle Project − Software

Rev. 1 — 27 March 2020 User manual

1 Introduction

This document describes the Tire Pressure Management System (TPMS) NTM88

Location Angle software version 2.0 designed to run on NXP's TPMS part NTM88H05,

which includes a single-axis accelerometer X. The TPMS application software was

developed using the CodeWarrior IDE. The purpose of the software is to monitor

the angular position of a TPMS device attached to a rotating wheel and to start the

transmission of a fixed length RF data packet at a given location such that the end of the

packet arrives at the RF receiver when the TPMS device is at a pre-defined target angle

location on the wheel. One, two, three or four target location angles can be specified in

the user code and, depending on the wheel rotation speed, up to four RF data packets

can be transmitted during one full wheel rotation. All TPMS location angle processing is

based on accelerometer X data.

The NTM88 Single-Axis project is closely related to the NTM88 Dual-Axis project. The

only differences between the two projects are:

• Wheel rotation detection – In the Dual-Axis project, the compensated value of

accelerometer Z and the centripetal acceleration are used to determine if the car is

moving. In the Single-Axis project, function u8fWheelRotDet() determines if the car is

moving by analyzing the compensated acceleration readings of accelerometer X.

• Low Pass Filters (LPF) implementation – In the Dual-Axis project there are four

LPFs: LPF1, LPF2, LPF3 and LPF4. In the Single-Axis project there are three LPFs:

LPF_4HZ, LPF_9HZ and LPF_20HZ. The bandwidths of the LPFs used in the two

projects are similar but slightly different.

• LPF selection – For optimal performance when measuring the rotation period, LPF

selection must be based on the wheel rotation speed. In the Dual-Axis project,

accelerometer Z centripetal acceleration is used to estimate the rotation speed and

select the LPF. In the Single-Axis project, a sequence of calls must be made to the

period measurement function u8fXGetRotPeriod(LPFSel), with parameter LFPSel

selecting one of the three LPFs. The sequence of calls is described in Section 5.

The processing of accelerometer X sine waves to measure the rotation period and to

calculate target angles is the same in both projects. The description of accelerometer

X sine wave processing presented in the Dual-Axis project document NTM88 Dual-

Axis Location Angle Project – Algorithms User Manual can be used as a reference for

analyzing NTM88 Single-Axis project functions.

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2 Software Requirements

The NTM88 Single-Axis Location Angle project requires the following elements:

• CodeWarrior v11.0 or higher with the NTM88_LIB patch installed

• The NTM88 Application Library in file NTM88_AppLib_v2-2.lib

• The library version of NTM88 firmware provided in file ntm88_axis_x_lib.lib.

• 8558 bytes of MCU flash memory

• 293 bytes of MCU RAM

3 Project Organization

The NTM88 Single_Axis Location Angle project was developed using CodeWarrior v11.0

with the NTM88_LIB firmware library patch installed. (The firmware library patch provides

support for the NTM88 device family.)

The project components are provided in the zip file NTM88_LocAngle_vxxx.zip. This file

contains the full CodeWarrior project with all required files in their appropriate directories.

After the project has been unzipped and imported to a CodeWarrior workspace and

compiled without errors, executable code is ready to be programmed into a TPMS

NTM88 device's flash memory. If a compilation error occurs during this process, execute

the CodeWarrior menu command ‘Project\Clean…’.

The organization of project directories and source code files is as follows:

• directory Lib:

– NTM88_AppLib_v2-2.lib – the file containing theTPMS Application Library for NTM88

– ntm88_axis_x_lib.lib – the library file containing all TPMS NTM88 firmware functions

– NTM88_LIB.c – the CodeWarrior project file. In this project, the NTM88_LIB.c file is

modified as a workaround for the CodeWarrior default project file. It contains TPMS

NTM88 device register definitions.

• directory Project_Headers:

– derivative.h – the CodeWarrior definition of project device type

– LocAngle.h – the header file associated with location angle source code in

LocAngle.c

– main.h – the header file associated with the project main.c source file

– NTM88_AppLib_v2-2.h – the header file with function declarations for the NTM88

TPMS Application Library

– ntm88_axis_x_lib.h – the header file with library firmware function declarations

– NTM88_LIB.h – the CodeWarrior project file with the NTM88_LIB.h file modified

as a workaround for the CodeWarrior default project file. NTM88_LIB.h contains

declarations of device registers and their bit fields,

• directory Sources:

– interrupts.c – the TPMS interrupt service routines and interrupt vector table

– LocAngle.c – the location angle function source code

– main.c – the TPMS application project main function source code

4 Overview of the TPMS Location Angle algorithm

The purpose of the Location Angle algorithm is to monitor the angular location of the

TPMS device on a rotating wheel and to start the transmission of RF packets so that

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each packet's transmission ends when the TPMS is at pre-defined location angle

selected by the user. The algorithm's operation is based only on the accelerometer's X

data.The sequence of events is described below.

The program is designed to run periodically every LOC_ANGLE_RUN_PERIOD seconds

(default = 4), as defined in main.c. Between runs, the TPMS MCU remains in STOP1

mode. After starting each run, the program determines if the wheel is rotating and, if

so, measures the rotation period. Then an RF data packet is transmitted such that its

transmission ends at one of the predefined location angles.

Initially, the software determines if the car is parked or moving by comparing the

accelerometer X compensated value saved during the previous run with compensated

accelerometer X readings taken in this run. The number of acceleration X readings and

the time delays between them can be configured by the user in the source code. If all

acceleration X data points read during this run are within a narrow window around the

acceleration saved in the previous run, acceleration X is assumed to remain constant,

which means the wheel is not rotating. In such cases, program execution ends and the

MCU enters STOP1 mode. If, on the other hand, any of the newly read acceleration X

data points is outside the window, the wheel is assumed to be rotating. In this case the

program starts monitoring the sequence of raw accelerometer X sine wave samples

to measure the rotation period. Detection of wheel rotation is performed in function

u8fWheelRotDet(), which returns the status byte that indicates whether or not the wheel

is rotating.

The function u8fXGetRotPeriod() measures the wheel rotation period and returns a

status byte which is used to decide whether or not the RF packets should be transmitted.

If the rotation period was determined successfully and the car's speed is within the

predefined speed range, the RF packets are transmitted. In this case, the program

generates the appropriate time delays and transmits the predefined number of RF

packets, and then exits to STOP1 mode. If the car's speed is too slow or too fast or if the

program was not able to determine the rotation period (for example, due to a high level

of noise in the accelerometer X samples sequence), the program exits to STOP1 mode

without transmitting any RF packets.

To help the higher level user application program cope with noise and distortions in

the accelerometer X signal, the function measuring the wheel rotation period accepts a

parameter selecting the bandwidth of the low-pass filter (LPF) to be used for processing

accelerometer X signals.

5 Low Pass Filter filtering accelerometer X signal

The LPF implemented in function RunLPF_IIR16x16() is used to filter noise out of the

accelerometer X signal. One set of digital LPF coefficients is used, and by adjusting

the signal sampling frequency, different LPF filter bandwidths can be selected (which is

equivalent to having different LPFs). Three possible LPFs are available: 4Hz, 9Hz and

20Hz, each of which can be selected by passing the appropriate parameter to function

vfnProcessLocAngle(…): LPF_4HZ, LPF_9HZ or LPF_20HZ. The bandwidth of each

LPF determines the maximum car speed at which it can be used and the parameters

of the three LPFs, including the maximum wheel RPM and car speed for test tire size

P205/65R16, are shown in Table 1. The digital LPF is IIR 4-pole type.

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Table 1. LPF parameters – data for tire size 205/65R16

LPF bandwidth in LPFSel

LPF_4HZ LPF_9HZ LPF_20HZ

Sampling frequency

[Hz]

73 153 333

Sampling period [ms] 13.7 6.55 3.0

Maximum RPM [RPM] 260 550 1200

Maximum car speed

[km/h]

33 70 152

When the MCU comes out of STOP1 mode, the program starts running and function

u8fWheelRotDet() returns the status indicating that the car is moving and the car speed

is unknown. The user program should call the u8fXGetRotPeriod() function up to three

times with parameters increasing the LPF bandwidth in the following sequence:

1. Call status=u8fXGetRotPeriod(LPF_4HZ) with the narrow bandwidth 4 Hz LPF.

2. If the returned status is LA_STS_MOVING_TOO_SLOW=3, the car is moving too

slowly. Go to exit.

3. If the returned status is LA_STS_OK=0, the narrow bandwidth LPF is usable. Go to

transmit RF packets, then go to exit.

4. If the returned status is LA_STS_NO_OK=1, then call status= u8fXGetRotPeriod

(LPF_9HZ) with the medium bandwidth 9 Hz LPF.

5. If the returned status is LA_STS_OK=0, the medium bandwidth LPF is useable. Go to

transmit RF packets, then go to exit.

6. if the returned status is LA_STS_NO_OK=1, then call status= u8fXGetRotPeriod

(LPF_20HZ) with the wide bandwidth 20 Hz LPF.

7. if the returned status is LA_STS_OK=0, the wide bandwidth LPF is useable. Go to

transmit RF packets, then go to exit.

8. if the returned status is LA_STS_NO_OK=1 then exit with error – wheel rotation

period cannot be determined.

6 Project source code

6.1 Code in Main.c

The main() function in Sources\main.c initializes the MCU after it comes out of reset. If

MCU code execution was triggered by the first application of power, the PWU timer is

initialized with parameter LOC_ANGLE_RUN_PERIOD, which determines how often the

location angle algorithm will be run.

Next, the program checks if the ending of an RF module packet transmission brought the

MCU out of STOP1 mode. If this is the case, the RF module is disabled and the MCU is

put back into STOP1 mode.

After calibrating the PWU timer with a call to firmware function TPMS_LFOCAL(),

the program checks if the PWU timer brought the MCU out of STOP1 mode. If so,

the loop counter u16LoopCounter is initialized and a do-while() loop is entered. The

loop code decrements the loop counter u16LoopCounter, issues a call to function

vfnProcessLocAngle() and checks the status code returned by the function. The

vfnProcessLocAngle() function does all the processing of the TPMS device's position on

the wheel and the transmission of RF packets at the proper time.

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LPF_20HZ is the default LPF used for demonstration purposes and passed in variable

u8LPF_BW as a parameter to function vfnProcessLocAngle(u8LPF_BW). The user must

modify the code of this function as required and implement the sequence presented in

Section 5 for optimal performance in the full range of car speeds.

Parameter LOC_ANGLE_RUN_ERR_MAX, which initializes the loop counter

u16LoopCounter, determines the maximum number of times that function

vfnProcessLocAngle() may be called when it keeps returning an error code. The status

codes returned by vfnProcessLocAngle() are defined in the file Project_Headers

\LocAngle.h. The status code is LA_STS_OK=0 if the car is moving, the target angles

were successfully determined by the location angle algorithm and RF packets were

transmitted. If other than LA_STS_OK status code values are returned, either an error

occurred while monitoring the wheel rotation or the car's speed was determined to be too

low or too high to transmit RF packets. RF packets were transmitted only if the returned

status code is LA_STS_OK.

After exiting the do-while() loop the program enters STOP1 mode and is brought to RUN

mode again by the PWU timer after its time delay expires.

The file main.c also contains functions to configure the MCU, set up the PWU timer,

define MCU STOP modes and configure GPIO pins.

6.2 Code in interrupts.c

This file contains the source code for the interrupt service routines (ISR) and the interrupt

vector table.

6.3 Code in LocAngle.c

This file contains the source code for the vfnProcessLocAngle() function called from

main() and all the remaining code implementing the Location Angle algorithm.

6.3.1 Function FRC_COMP_DLY(UINT16 FRCCmpCnt)

This function executes the delay passed as a parameter in FRCCmpCnt. The value of

the requested delay is in milliseconds and the valid range of delays is 4ms to 65535ms.

This function uses Free Running Counter (FRC) to execute the delay. After configuring

the FRC registers, the MCU enters STOP4 mode to save power during the delay. Before

this function is called, the MCU STOP mode must be configured to be STOP4. The

MCU resumes code execution when the FRC compare interrupt is generated at the end

of the delay time. The code in this function works together with the FRC compare ISR

vfnFRCIsr() function in the file interrupts.c.

6.3.2 Function u8fWheelRotDet()

Code in this function detects if the wheel is rotating based on compensated

accelerometer X readings. First, the compensated X value u16CompAccelXPrev

saved in the previous program run is used to calculate the window's low and high

thresholds in variables i16ThrLow and i16ThrHi. Next, up to ROT_DET_POINTS=3

readings of acceleration X are taken and checked to see if the readings are within the

window's i16ThrLow and i16ThrHi thresholds. If any of the readings is outside of the

window, it means that the wheel is rotating and the function exits returning status 0. If

all the acceleration X readings were within the window, the status returned is 1, which

means the wheel is not rotating. Time delays between the readings are defined in array

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cau16RotDetDly[]. The delays are specified in milliseconds. The delays are executed by

calling function FRC_COMP_DLY(…) with MCU in STOP4 mode during the delay.

It is highly unlikely but possible that this function might return the incorrect status. This

could happen at one wheel rotation speed if acceleration X data points are taken at

the moment their values fall within the window between i16ThrLow and i16ThrHi. If, in

addition, the sum of both delays in cau16RotDetDly[] array happens to be equal to the

wheel rotation period, the function would return status 1 (wheel is not rotating).

Depending on application software requirements the user can customize this function

operation by modifying these parameters:

• ROT_DET_POINTS – The number of acceleration X data points to consider. With the

value 1, the function executes the fastest but the probability of returning incorrect status

is the highest. With the value 4 and properly selected time delays, wheel rotation is

always detected correctly.

• ACCEL_X_MOVE_THR – The width of the window around u16CompAccelXPrev used

to calculate thresholds i16ThrLow and i16ThrHi.

• UINT16 cau16RotDetDly[] – The array with time delays between reading acceleration X

data points.

The values of these parameters used in the demo project are a reasonable compromise

between rotation detection reliability and MCU power consumption.

6.3.3 Function vfnProcessLocAngle(UINT8 LPFSel)

The vfnProcessLocAngle() function first executes a sequence of calls to firmware

functions to read the values of voltage, temperature, pressure and acceleration

X. Next, a call is made to function u8fWheelRotDet(), which detects whether the

wheel is rotating. When the status returned by u8fWheelRotDet() is 0, the car is

moving and if the status is 1, the car is not moving. If the car is not moving, the

vfnProcessLocAngle() function exits with status code LA_STS_CAR_NOT_MOVING.

If the car is moving, the location angle algorithm is executed. In this case, function

RF_CreatePacket() is called to fill out the constant elements of the RF data packet in the

RFData[] buffer. Function u8fXGetRotPeriod(LPFSel) is then called, which executes the

state sequencer monitoring accelerometer X samples sequence to measure the wheel

rotation period. The LPF to be used is passed as a parameter in the LPFSel variable.

The u8fXGetRotPeriod(LPFSel) function returns the status code LA_STS_OK=0 when

the rotation period was measured successfully and the acceleration X sine wave 0-

crossing was detected. If status returned by u8fXGetRotPeriod(LPFSel) is anything other

than LA_STS_OK=0, no RF packets are transmitted and the function exits immediately

returning the status value to main(). The possible reasons that the status could be

different than LA_STS_OK=0 are:

• LA_STS_NO_OK=1 – An LPF with too narrow a bandwidth for the given car speed

was selected or the accelerometer X sine wave is so distorted that it was impossible to

measure its period.

• LA_STS_MOVING_TOO_FAST=2 – The car is moving too fast to transmit RF packets.

• LA_STS_MOVING_TOO_SLOW=3 – The car is moving too slowly.

• LA_STS_TIMEOUT=4 – A timeout occurred because the car is moving very slowly.

If the status returned by u8fXGetRotPeriod(LPFSel) is LA_STS_OK=0, the function

Xmit_RF_Packets_Loop() is executed, transmitting RF packets at proper times

such that packet transmissions end at the target location angles defined in array

cai16TargetAngle[]. Control is then returned to main().

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6.3.4 Function u8fXGetRotPeriod(UINT8 LPFSel)

The u8fXGetRotPeriod(LPFSel) function is the main function where the accelerometer

X data sequence is continuously read, filtered and processed to determine the wheel

rotation period and the sine wave sequence 0-crossing moment in preparation for

calculating the first RF packet transmission start time.

The function accepts parameter LPFSel, which selects one of the three LPF filters

used for filtering the accelerometer X samples sequence to attenuate noise and

signal distortions. A method of selecting the appropriate LPF filter for optimal software

performance is described in Section 5.

At the beginning of the function, a call is made to TPMS_E_FRC_CALIB() to calibrate the

FRC frequency. The calibration value, returned in the variable u16usPerPrd, represents

the duration of one LFO period expressed in microseconds. The code that follows

measures the wheel rotation period in terms of the FRC period count and uses the value

in u16usPerPrd to convert the wheel rotation period from the FRC count in variable

u16FRC_Prd to milliseconds in variable u16XPrd_ms.

When a TPMS device with accelerometer X is installed on the wheel such that the X

sensitivity axis is perfectly tangential to the wheel, the sequence of samples should be

a sine wave with an average value equal to 0. In practice, when the accelerometer X

sensitivity axis is not perfectly tangential to the wheel, the resulting sine wave will have

an average value other than 0, representing X centripetal acceleration proportional to the

car's speed.

Following the two dummy reads of accelerometer X to stabilize its ADC input level, the

centripetal acceleration of X is estimated by averaging four consecutive samples of

accelerometer X in variable i16AccXCnt_cpt. In the following while() loop, the centripetal

offset value in i16AccXCnt_cpt is subtracted from all accelerometer X samples.

After variable initialization, the control enters the while() loop. It remains there until the

wheel rotation period and 0-crossing rotation angle 0° or 180° are determined or until an

abnormal condition occurs that prevents the algorithm from continuing and results in the

function exiting with an error code.

The first operation within the while() loop is to check for a timeout. If a timeout is

detected, control exits the loop with the error code LA_STS_TIMEOUT.

A single pass of the loop has a fixed period of execution time made up of two main

components: 1.35 ms is consumed in reading the accelerometer X sample and 1.65

ms is consumbed in executing the LPF function RunLPF_IIR16x16(). The execution

time of the remaining code instructions in the loop is short and negligible compared

with the above two main components. So, the minimum signal sampling period in

the loop is 1.35 ms + 1.65 ms = 3.00 ms. This is the sampling period used with the

widest bandwidth filter LPF_20HZ. For other LPFs, their lower bandwidth is obtained by

proportionally increasing the signal sampling period. For LPF_4HZ the total sampling

period is 13.65 ms and for LPF_9HZ the sampling period is 6.55 ms. The additional

delay above the minimum 3.00 ms loop delay is implemented by making dummy

calls to the TPMS_E_READ_ACCEL_X() firmware function with a precisely selected

execution time. This way of implementing the additional required delays allows execution

of temperature stable time delays and saves battery power because the function

TPMS_E_READ_ACCEL_X() spends most of the time in STOP4 mode.

In the loop code, after checking for a timeout, a sequence of calls is made to the

TPMS_E_READ_ACCEL_X() function with different values of the delay parameter

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SAMP_DLY_INIT_xxxxUS depending on the selected LPF and the required total signal

sampling period.

Then a new accelerometer X sample is read into the u16UUMA[UUMA_X] array element.

Next, the centripetal offset in i16AccXCnt_cpt is subtracted from the new accelerometer

sample and the new sample count in i16XSamp_new is verified to be in the valid

count range between -1023 and +1023. After multiplying the new sample count by 16

to properly scale it, i16XSamp_new becomes the input sample to the low pass filter

function RunLPF_IIR16x16(). The sine wave peak-to-peak amplitude count in variable

i16XSamp_new on LPF input is 3456, which is equivalent to the acceleration change ±1

g. The LPF output sample is returned in variable i16XLPF_out. LPF gain is 1, so at signal

frequencies in the LPF passband, the peak-to-peak sine wave count on the LPF output in

variable i16XLPF_out is also 3456.

Control is then transferred to the main part of the while() loop, which is the state

sequencer implemented with the switch() statement. The state sequencer state number

in the range 0 to 15 is held in variable u8MoveState and the functions performed in each

state are described below.

• u8MoveState = 0 – Introduces a delay of 16 samples to fill out internal variables of LPF

and stabilize its output signal.

• u8MoveState = 1 – Waits for detection of the sine wave rising or falling slope. If the

rising slope is detected first, control proceeds to states 2, 3, 4, 5. If the falling slope is

detected first, control is transferred to states 12, 13, 14, 15.

• u8MoveState = 2 – If the sine wave rising slope was detected, waits for sine wave

maximum MAX1. Saves the count at MAX1 in i16XSamp_max1 and waits for the sine

wave LPF output level count to drop X_PRD_LVL_CNT=812 below the maximum.

At this point, the FRC counter is saved in u16FRC_PrdStart as the starting time for

measuring the sine wave period. The interpolation is applied between the current and

the previous samples to improve the precision of the period measurement starting time.

Control is transferred to state 3.

• u8MoveState = 3 – Waits for sine wave minimum MIN1. Saves the count at MIN1

in i16XSamp_min1 and waits for the sine wave LPF output level count to rise

X_PRD_LVL_CNT above the minimum. Control is transferred to state 4.

• u8MoveState = 4 – Waits for sine wave maximum MAX2. Saves the count at MAX2

in i16XSamp_max2 and waits for the sine wave LPF output level count to drop

X_PRD_LVL_CNT below the maximum. When that happens, the peak-to-peak

count of the sine wave is validated to eliminate the possibility of measuring an invalid

period caused by an excessively distorted sine wave. Also, the FRC counter is

saved in u16FRC_PrdEnd as the ending time for measuring the sine wave period.

Interpolation is applied between the last two samples to improve the precision of the

period measurement ending time. Calculates the rotation period as the difference

between u16FRC_PrdEnd and u16FRC_PrdStart and stores the result in terms of the

FRC count in variable u16FRC_Prd. Converts the wheel rotation period from the FRC

count to milliseconds in u16XPrd_ms using the LFO calibrated period in u16usPerPrd.

Checks if the measured period in milliseconds in variable u16XPrd_ms is in a valid

range between ROT_PRD_MAX=400 and ROT_PRD_MIN=50 and if not, function

u8fXGetRotPeriod() exits returning an error code. Calculates the expected LPF output

level at the sine wave 0-crossing in i16XSamp_0crLvl as the average of counts at

MAX1, MIN1 and MAX2. Also, function SetLPFDelay() is called to fill out the variable

u16LPFDlyx10 with the LPF delay, depending on which LPF is selected in u8SelLPF.

The variable u16LPFDlyx10 is used later for target angle delay calculations. Control is

transferred to state 5 to wait for the 0-crossing level saved in i16XSamp_0crLvl.

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• u8MoveState = 5 – Waits for the sine wave positive-to-negative 0-crossing level in

variable i16XSamp_0crLvl. Interpolates the FRC count between two consecutive

samples to find the FRC count at the precise 0-crossing level. This is the 0-crossing

point at the top of the wheel where the rotation angle is 0°, so u8InitAng_0cr is set to 0.

Saves the 0-crossing FRC time in variable u16FRC_0cr and transfers control to state

21 in function Xmit_RF_Packtes_Loop(), which handles the RF packets transmission

timing.

If a falling slope is detected first in state 1, control transfers to states 12, 13, 14 and 15,

which are equivalent respectively to states 2, 3, 4 and 5 as described above. In state 15,

the 0-crossing happens at the bottom of the wheel where the rotation angle is 180°, so

u8InitAng_0cr is set to 180. After saving the time of 0-crossing in variable u16FRC_0cr,

control transfers to state 21 in function Xmit_RF_Packets_Loop().

6.3.5 Function Xmit_RF_Packets_Loop()

The Xmit_RF_Packets_Loop() function calculates RF packet transmission start times

so that each packet transmission ends at the required target angle. The number of

target angles at which the software attempts to transmit RF packets is defined as

TARGET_ANG_MAX and values of the angles in degrees are defined by the user in

the array cai16TargetAngle[]. The valid range of TARGET_ANG_MAX is between one

and four. In the demonstration project, TARGET_ANG_MAX=1. Angle values should be

listed in the cai16TargetAngle[] array in ascending order from 0° to 359°. All angles listed

in the array cai16TargetAngle[] are processed in sequence in a circular buffer manner.

After starting the current RF packet transmission, a delay to the next packet transmission

start time is calculated for the next target angle in the list. The calculation is based on

the wheel rotation period determined in function u8fXGetRotPeriod(). If the calculated

delay to start the next packet transmission is shorter than the packet transmission time,

the next packet will not be transmitted. Instead, a delay is calculated for the next listed

target angle. The code in Xmit_RF_Packets_Loop() is made up of two states which are

the continuation of the state sequencer started in function u8fXGetRptPeriod().

• u8MoveState = 21 – Control is transferred here from state 5 or 15. At the moment

state 21 executes, the TPMS device is at the bottom or top point of the rotating wheel,

depending on the value in u8InitAng_0cr. The delay to the start of the next RF packet is

calculated and control transfers to state 22 to wait for the transmission to start.

• u8MoveState = 22 – Control waits until the arrival of the designated start time for

the next RF packet transmission and then begins the packet transmission. Next,

based on the target angles list in the cai16TargetAngle[] array, a delay to the

next packet transmission start is calculated. Control remains in state 22 until all

TARGET_ANG_MAX target angles are processed.

After all RF packets are processed, function Xmit_RF_Packets_Loop() exits, control is

returned to the main() function and the MCU is put into STOP1 mode where it remains

until the next PWU timer wakeup interrupt.

6.3.6 Other functions in LocAngle.c

RF_Init() – Initializes the RF module to prepare for transmitting the next RF packet.

The transmission rate is 9600 baud, with a carrier frequency of 434.000 MHz and the

frequency deviation ±50 kHz.

RF_CreatePacket() – Fills out the RFData[] buffer with data packet elements that remain

constant for all packets transmitted during the current run

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RF_UpdatePacket() – Fills out the RFData[] buffer with data packet elements specific to

the next packet to be transmitted

RF_CreatePacket_DBG1() – Creates a debug version of the RF packet. The transmitted

packet length is 32 bytes and different variables can be transmitted in the packet to

debug/verify program operation.

Calculate_CRC8_CCITT() – Calculates the 8-bit checksum of the packet used in the

demo version

ComputeCrc8Byte() – Used by Calculate_CRC8_CCITT() to calculate the 8-bit packet

checksum

Calculate_CRC_MKW01() – Calculates the 16-bit checksum of the packet used in the

debug version of the project

ComputeCrc_MKW01() – Used by Calculate_CRC_MKW01() to calculate the 16-bit

packet checksum

i16fnMlts16xs16() – Implements multiplication of two signed 16-bit fixed-point numbers

and returns a 16-bit signed result. Used in LPF function RunLPF_IIR16x16().

RunLPF_IIR16x16() – Implements a 4-pole low pass filter filtering the accelerometer X

signal. LPF coefficients and input/output variables are 16-bit fixed-point numbers.

SetLPFDelay() – Calculates the signal delay through LPF into variable u16LPFDlyx10

to be used in calculating the starting time of RF packet transmissions. The delay value

returned in u16LPFDlyx10 is in milliseconds multiplied by 10.

7 RF Data Packet Format

The RF data packet is made up of three sections:

• preamble,

• data section,

• checksum byte or word.

The software can be configured to transmit one of two RF packet types. The packet

type is selected by defining one of two switches in file LocAngle.h used for conditional

compilation:

#define PACKET_NXP_WSIM – used in the demo version where the program attempts

to transmit TARGET_ANG_MAX 16-byte long packets at target angles defined in array

cai16TargetAngle[],

#define PACKET_DEBUG1 – used only for debugging purposes, the program transmits

one 32-byte long packet after measuring a rotation period. The data part can be filled

with different variables to verify their values while executing the state sequencer in

function u8fXGetRotPeriod().

Format of demo RF packet enabled by PACKET_NXP_WSIM switch:

byte 0: 0xFF - preamble byte 1

byte 1: 0xAA - preamble byte 2

byte 2: 0x9B - preamble byte 3

byte 3: 0xA1 - preamble byte 4

NXP Semiconductors

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byte 4: 0x?? - TPMS device ID byte 1

byte 5: 0x?? - TPMS device ID byte 2

byte 6: 0x?? - TPMS device ID byte 3

byte 7: 0x?? - TPMS device ID byte 4

byte 8: 0x?? - tire pressure, high byte

byte 9: 0x?? - tire pressure, low byte

byte 10: 0x?? - tire temperature

byte 11: 0x?? - target angle in degrees divided by 2

byte 12: 0x?? - rotation period in milliseconds, high byte

byte 13: 0x?? - rotation period in milliseconds, low byte

byte 14: 0x?? - packet counter bytebyte 15: 0x?? - CRC8 byte

Bytes 0 through 10 are filled out in function RF_CreatePacket() and bytes 11 through 15

are filled out in function RF_UpdatePacket().

Format of debug RF packet enabled by PACKET_DEBUG1 switch:

byte 0: 0x55 - preamble byte 1

byte 1: 0x55 - preamble byte 2

byte 2: 0x02 - preamble byte 3

byte 3: 0x02 - preamble byte 4

byte 4: 0x19 - length of packet data section, 25 bytes

byte 5: 0x?? - 1-st data byte

byte 29: 0x?? - 25-th data byte

byte 30: 0x?? - CRC16, high byte

byte 31: 0x?? - CRC16, low byte

Debug packet is created in function RF_CreatePacket_DBG1(). Using this packet, any

program variable can be monitored by transmitting it in byte positions 5 through 29.

8 Revision History

Document ID Release

date

Descriptions

UM11361 v.1 20200327 Initial release

NXP Semiconductors

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NTM88 Single-Axis Location Angle Project − Software

User manual Rev. 1 — 27 March 2020

12 / 14

9 Legal information

9.1 Definitions

Draft — The document is a draft version only. The content is still under

internal review and subject to formal approval, which may result in

modifications or additions. NXP Semiconductors does not give any

representations or warranties as to the accuracy or completeness of

information included herein and shall have no liability for the consequences

of use of such information.

9.2 Disclaimers

Limited warranty and liability — Information in this document is believed

to be accurate and reliable. However, NXP Semiconductors does not

give any representations or warranties, expressed or implied, as to the

accuracy or completeness of such information and shall have no liability

for the consequences of use of such information. NXP Semiconductors

takes no responsibility for the content in this document if provided by an

information source outside of NXP Semiconductors. In no event shall NXP

Semiconductors be liable for any indirect, incidental, punitive, special or

consequential damages (including - without limitation - lost profits, lost

savings, business interruption, costs related to the removal or replacement

of any products or rework charges) whether or not such damages are based

on tort (including negligence), warranty, breach of contract or any other

legal theory. Notwithstanding any damages that customer might incur for

any reason whatsoever, NXP Semiconductors’ aggregate and cumulative

liability towards customer for the products described herein shall be limited

in accordance with the Terms and conditions of commercial sale of NXP

Semiconductors.

Right to make changes — NXP Semiconductors reserves the right to

make changes to information published in this document, including without

limitation specifications and product descriptions, at any time and without

notice. This document supersedes and replaces all information supplied prior

to the publication hereof.

Applications — Applications that are described herein for any of these

products are for illustrative purposes only. NXP Semiconductors makes

no representation or warranty that such applications will be suitable

for the specified use without further testing or modification. Customers

are responsible for the design and operation of their applications and

products using NXP Semiconductors products, and NXP Semiconductors

accepts no liability for any assistance with applications or customer product

design. It is customer’s sole responsibility to determine whether the NXP

Semiconductors product is suitable and fit for the customer’s applications

and products planned, as well as for the planned application and use of

customer’s third party customer(s). Customers should provide appropriate

design and operating safeguards to minimize the risks associated with

their applications and products. NXP Semiconductors does not accept any

liability related to any default, damage, costs or problem which is based

on any weakness or default in the customer’s applications or products, or

the application or use by customer’s third party customer(s). Customer is

responsible for doing all necessary testing for the customer’s applications

and products using NXP Semiconductors products in order to avoid a

default of the applications and the products or of the application or use by

customer’s third party customer(s). NXP does not accept any liability in this

respect.

Limiting values — Stress above one or more limiting values (as defined in

the Absolute Maximum Ratings System of IEC 60134) will cause permanent

damage to the device. Limiting values are stress ratings only and (proper)

operation of the device at these or any other conditions above those

given in the Recommended operating conditions section (if present) or the

Characteristics sections of this document is not warranted. Constant or

repeated exposure to limiting values will permanently and irreversibly affect

the quality and reliability of the device.

Terms and conditions of commercial sale — NXP Semiconductors

products are sold subject to the general terms and conditions of commercial

sale, as published at http://www.nxp.com/profile/terms, unless otherwise

agreed in a valid written individual agreement. In case an individual

agreement is concluded only the terms and conditions of the respective

agreement shall apply. NXP Semiconductors hereby expressly objects to

applying the customer’s general terms and conditions with regard to the

purchase of NXP Semiconductors products by customer.

Suitability for use in automotive applications — This NXP

Semiconductors product has been qualified for use in automotive

applications. Unless otherwise agreed in writing, the product is not designed,

authorized or warranted to be suitable for use in life support, life-critical or

safety-critical systems or equipment, nor in applications where failure or

malfunction of an NXP Semiconductors product can reasonably be expected

to result in personal injury, death or severe property or environmental

damage. NXP Semiconductors and its suppliers accept no liability for

inclusion and/or use of NXP Semiconductors products in such equipment or

applications and therefore such inclusion and/or use is at the customer's own

risk.

Export control — This document as well as the item(s) described herein

may be subject to export control regulations. Export might require a prior

authorization from competent authorities.

Translations — A non-English (translated) version of a document is for

reference only. The English version shall prevail in case of any discrepancy

between the translated and English versions.

9.3 Trademarks

Notice: All referenced brands, product names, service names and

trademarks are the property of their respective owners.

NXP — is a trademark of NXP B.V.

NXP Semiconductors

UM11361

NTM88 Single-Axis Location Angle Project − Software

User manual Rev. 1 — 27 March 2020

13 / 14

Tables

Tab. 1. LPF parameters – data for tire size

205/65R16 .........................................................4

NXP Semiconductors

UM11361

NTM88 Single-Axis Location Angle Project − Software

Please be aware that important notices concerning this document and the product(s)

described herein, have been included in section 'Legal information'.

For more information, please visit: http://www.nxp.com

For sales office addresses, please send an email to: [email protected]

Date of release: 27 March 2020

Document identifier: UM11361

Contents

1 Introduction ......................................................... 1

2 Software Requirements ...................................... 2

3 Project Organization ........................................... 2

4 Overview of the TPMS Location Angle

algorithm .............................................................. 2

5 Low Pass Filter filtering accelerometer X

signal .................................................................... 3

6 Project source code ........................................... 4

6.1 Code in Main.c .................................................. 4

6.2 Code in interrupts.c ........................................... 5

6.3 Code in LocAngle.c ........................................... 5

6.3.1 Function FRC_COMP_DLY(UINT16

FRCCmpCnt) ..................................................... 5

6.3.2 Function u8fWheelRotDet() ............................... 5

6.3.3 Function vfnProcessLocAngle(UINT8

LPFSel) .............................................................. 6

6.3.4 Function u8fXGetRotPeriod(UINT8 LPFSel) ......7

6.3.5 Function Xmit_RF_Packets_Loop() ................... 9

6.3.6 Other functions in LocAngle.c ............................9

7 RF Data Packet Format .................................... 10

8 Revision History ................................................11

9 Legal information ..............................................12

NXP NTM88 User guide

NXP NTM88 User guide

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