NXP NTM88 User guide

Brand: NXP

Model: NTM88

Type: User guide

UM11363

NTM88 Dual-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 XZ dual-axis software version 2.0 designed to run on NXP's TPMS part

NTM88H13, which includes dual-axis accelerometers X and Z. 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 so 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. User code can specify one, two, three or four target location

angles and, depending on the wheel rotation speed, up to four RF data packets can be

transmitted during one full wheel rotation. Initially, the wheel rotation speed is estimated

based on the accelerometer Z centripetal acceleration reading. All subsequent TPMS

location angle processing is based on accelerometer X data.

This document describes the software that implements the various algorithms used in

the project. The algorithms are documented in the complementary NTM88 Dual-Axis

Location Angle Project – Algorithms user manual.

The NTM88_LocAngle_XZ project was designed as a reference project with limited

performance verification. Users are encouraged to experiment and modify the algorithms

and the project code to suit their application requirements.

2 Software requirements

The NTM88 Dual-Axis Location 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_xz_lib.lib.

• 10090 bytes of MCU flash memory

• 325 bytes of MCU RAM

3 Project organization

The NTM88 Dual-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 NTM88 device family.)

All project components are provided in the zip file NTM88_LocAngle_XZ_vxxx.zip.

This file contains the full CodeWarrior project with all required files in their appropriate

directories. After the project has been unzipped and then imported into a CodeWarrior

workspace and compiled without errors, executable code is ready to be programmed

into the TPMS NTM88 device's flash memory. If a compilation error occurs during this

process, execute the CodeWarrior menu command ‘Project\Clean…’.

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

• directory Lib:

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

– ntm88_axes_xz_lib.lib – the library file containing all dual-axis TPMS NTM88

firmware functions

– NTM88_LIB.c – the CodeWarrior project file, which contains the TPMS NTM88

device register definitions

• directory Project_Headers:

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

– LocAngle.h – the header file associated with the 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 TPMS

Application Library for NTM88

– ntm88_axis_xz_lib.h – the header file with library firmware function declarations

– NTM88_LIB.h – the CodeWarrior file, which 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 functions source code

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

4 Overview of the TPMS Location Angle algorithm

The purpose of the 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 each packet's

transmission ends when the TPMS is at a pre-defined location angle selected by the

user. The algorithm's operation is based only on accelerometers X and Z data. The

sequence of events is described below.

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

randomly read accelerometer Z data to a threshold of 5.9 g. An accelerometer Z reading

below the threshold indicates that the car is parked. A reading above the threshold

indicates that the car is moving. The threshold is equivalent to a car with 195/65R15 tires

moving with a speed of approximately 20 km/h.

If the car is moving, function u8fFindWheelDir() can be called, when required, to

determine the wheel rotation direction—either clockwise (CW) or counter-clockwise

(CCW). Function u8fFindWheelDir() is enabled by defining the software compilation

switch DIR_FIND_ENABLE. If finding the wheel rotation direction is not required, the

switch DIR_FIND_ENABLE should be commented out. Wheel rotation direction is found

in function u8fFindWheelDir() using data sequences from both accelerometers X and Z.

The function u8fXGetRotPeriod() is called next and the accelerometer X sampling

sequence is started to determine the wheel rotation period and the ±1 g sine wave 0-

crossing moment to be used as a starting point for TPMS location angle tracking.

When accelerometer X sampling is started, the function determines whether the slope of

the sample sequence sine wave is rising or falling. Depending on the result, execution

then follows one of two paths.

If a rising slope was detected first, the function waits for the sine wave maximum MAX1

count value and then waits for the sine wave count level to fall X_PRD_LVL_CNT below

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the count at MAX1. At this moment, the time count is recorded by reading the current

Free Running Counter (FRC) value. This value determines the point in time when the

sine wave period measurement begins. Next, the count level at the following sine wave

minimum MIN1 is recorded and the function waits for the next sine wave maximum

MAX2. When the sine wave level count drops by X_PRD_LVL_CNT below MAX2,

the FRC is read again and its count at this time is considered the ending point of the

sine wave period measurement. The difference between the two saved FRC counts

represents the wheel rotation period. The average value of counts at MAX1, MIN1 and

MAX2 is calculated and used as a count level to determine the sine wave positive-to-

negative 0-crossing point, which in this case coincides with 0° of the TPMS location

angle.

If a falling sine wave slope was detected first, a similar procedure is executed, but the

sequence of events is as follows. The sequence begins by waiting for the MIN1 value to

use as the starting point for measuring the rotation period. The program then waits for

the MAX1 value and, after finding the next minimum MIN2, uses this data to calculate the

period. Based on the average of counts MIN1, MAX1 and MIN2, the negative-to-positive

0-crossing level is determined. In this case the 0-crossing level coincides with 180° of the

TPMS location angle.

The RF packet transmission time, the wheel rotation period, the 0-crossing time, and

the fact that the 0-crossing happens when the TPMS device is at the wheel’s 0° or 180°

position are used to determine when the first RF packet transmission must begin.The

time delay from the 0-crossing point to the moment when the first RF packet transmission

should start is calculated so that the packet transmission ends when the TPMS device

reaches the required predefined first location angle on the rotating wheel. User code can

define from one to four location angles in an array of up to four elements. Immediately

after starting the transmission of the first RF packet, the time delay to the next RF packet

transmission start is calculated. After processing all location angle entries defined in the

array, the cycle of rotation period measurement and RF packet transmissions ends and

the MCU enters STOP1 mode. The cycle repeats based on a predefined time period

initiated by the PWU timer.

When the TPMS is mounted on a wheel with a 15" rim diameter, the RF packets are

transmitted in the wheel rotation speed range of 166 RPM to 1200 RPM, which are

equivalent to car speeds of 20 km/h and 144 km/h respectively. The lower limit of 166

RPM is determined by the centripetal acceleration of a 5.9 g Z accelerometer threshold

to decide whether the car is parked or moving. The upper limit is set by the minimum

allowed rotation period of 50 ms.

The maximum time required for successful completion of the period measurement

algorithm and the start of the first RF packet transmission is approximately 1.5 wheel

rotations. All RF packet transmissions are finished during the following single wheel

rotation.

The test equipment used to verify the algorithm's operation consisted of a rotating wheel

simulator with an MCU-controlled motor and an RF packets data receiver. A GPIO output

pin from the receiver signaled the motor-controller MCU when a new RF packet was

received. The motor-controller MCU recorded the angular position of the motor at the

instant the RF packet arrived. This data was then transmitted through a USB port to a PC

and its GUI software was used to create a plot of RF packet arrival angles vs. time.

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5 Description of project source code

5.1 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.

Parameter LOC_ANGLE_RUN_ERR_MAX, which initializes the loop counter

u16LoopCounter, determines the maximum number of times the 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, then 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.

5.2 Code in interrupts.c

This file contains the source code for the interrupt service routines and the interrupt

vector table.

5.3 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.

5.3.1 Function vfnProcessLocAngle()

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

functions to calculate the compensated values of voltage, temperature, pressure and

acceleration Z. Next, the calculated acceleration Z is compared to the threshold of

5.9 g to determine if the car is moving or parked. If the car is parked (acceleration Z

≤ 5.9 g), the function exits with status code LA_STS_CAR_NOT_MOVING. If the car

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is moving (acceleration Z > 5.9 g), the rotation direction finding and location angle

algorithms are executed. In this case, function RF_CreatePacket() is called to fill out

the constant elements of the RF data packet in RFData[] buffer. Next, if enabled, the

wheel rotation direction finding function u8fFindWheelDir() is called. The rotation direction

result is returned in variable u8dWheelDir. Function u8fXGetRotPeriod() is then called

to execute a state sequencer that monitors the accelerometer X samples sequence

to measure the wheel rotation period. The u8fXGetRotPeriod() function returns status

code LA_STS_OK=0 when the rotation period was measured successfully and the

acceleration X sine wave 0-crossing was detected. If so, a call is made to function

Xmit_RF_Packets_Loop(), which transmits RF packets so that each packet ends at a

target location angle defined in array cai16TargetAngle[]. If the status code returned by

u8fXGetRotPeriod() is anything other than LA_STS_OK, no RF packets are transmitted

and function vfnProcessLocAngle() exits immediately.

Figure 1 illustrates the vfnProcessLocAngle() flow.

5.3.2 Function u8fXGetRotPeriod ()

In function u8fXGetRotPeriod(), 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.

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

the FRC frequency. The calibration value, which represents the duration of one LFO

period expressed in microseconds, is returned in variable u16usPerPrd. The function

then measures the wheel rotation period in terms of the FRC period count (in variable

u16FRC_Prd). Using the LFO period duration in variable u16usPerPrd, the wheel

rotation period from the FRC period count (in variable u16FRC_Prd) is converted into

milliseconds and stored in variable u16XPrd_ms.

Next, the function SelectLPF() is called to select which of four possible LPF filters is

to be used for filtering the accelerometer X sample sequence to attenuate noise and

signal distortions. The selection of an LPF is made depending on the car’s speed and is

based on the compensated value of the accelerometer Z reading performed in function

vfnProcessLocAngle() and saved in variables u8AccelOffsetZ and u16CompAccelZ. The

compensated value of acceleration Z represents the Z centripetal acceleration, which is

used as an estimate of the wheel rotation speed.

The NTM88 Dual-Axis Location Angle Project – Algorithms user manual lists the

acceleration Z thresholds and the equivalent car speeds used to decide which LPF to

use.

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

car's speed.

Following the LPF selection in the code and 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.

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After variable initialization, 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 conditions 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 consumed in reading the accelerometer X sample and 1.65

ms consumed 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 LPF4. For other LPFs, their lower bandwidth is obtained by

proportionally increasing the signal sampling period. For LPF1 the total sampling period

is 13.68 ms, for LPF2 the sampling period is 7.24 ms and for LPF3 it is 4.14 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. Implementing the additional required delays in this way allows

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

function TPMS_E_READ_ACCEL_X() spends most of its time in STOP4 mode.

Within 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

SAMP_DLY_INIT_xxxxUS depending on the selected LPF and the required total signal

sampling period.

A new accelerometer X sample is then read into the u16UUMA[UUMA_X] array

element. The centripetal offset in i16AccXCnt_cpt is subtracted from the 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 3360, 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 3360.

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 the sine wave rising or falling slope detection. 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=800 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.

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• u8MoveState = 3 – Waits for the 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 the 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, calls function SetLPFDelay() 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. Transfers

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

• 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 top where the rotation angle is 0°, so u8InitAng_0cr is set

to 0. Saves 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().

Figure 2 and Figure 3 illustrate the u8fXGetRotPeriod () flow.

5.3.3 Function u8fFindWheelDir()

This function determines the wheel rotation direction by analyzing the phase shift

between ±1 g sine waves made up by sequences of samples read from accelerometers

X and Z. The rotation direction result is encoded in bit 0 and bit 1 of variable

u8dWheelDir as follows:

bit 0 = 0 – rotation direction is clockwise (CW)

bit 0 = 1 – rotation direction is counter-clockwise (CCW)

bit 1 = 1 – bit 0 has valid rotation direction information

bit 1 = 0 – it was impossible to determine rotation direction and bit 0 is invalid

The user can configure the project to include or not include the function

u8fFindWheelDir(). If u8fFindWheelDir() is included, the project must be configured to

select whether 2-pole or 4-pole LPFs are used for filtering X and Z sequences. When

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the software switch DIR_FIND_ENABLE is defined, the function code is included in the

project. If the software switch DIR_LPF_4POLE is defined, 4-pole LPFs are selected. If

DIR_LPF_4POLE is not defined, the default 2-pole LPFs are used.

To make wheel rotation detection more reliable, the algorithm can be configured

to run multiple times. Variable u8dDirCnt is the counter for the number of times the

algorithm runs. In the demo project, u8dDirCnt is initialized with D_REPEAT_CNT=3.

After each run, direction detection is performed and, depending on the result, the

appropriate direction counter is incremented: u8dCWCnt if direction was found to be CW

or u8dCCWCnt if the direction was found as CCW. At the end of the algorithm the two

direction counters are compared and the greater is used as the result of the direction

finding algorithm and that value is stored in the variable u8dWheelDir

In the u8fFindWheelDir() function code, the LFO calibration function

TPMS_E_FRC_CALIB() is called to determine the precise LFO period in u16usPerPrd.

The period in u16usPerPrd is used to calculate a temperature independent timeout

threshold u16FRC_ToutThr.

Next, the readings of accelerometers X and Z are taken to find their offsets (stored in

u8AccelOffsetX and u8AccelOffsetZ) and the estimates of centripetal accelerations X and

Z are found in i16AccXCnt_cpt and i16AccZCnt_cpt.

After initializing variables, the program enters the direction monitoring state sequencer

while() loop. On each pass of the while() loop, the timeout is checked and, if it remains

below the threshold in u16FRC_ToutThr, the function continues by reading samples

of accelerometers X and Z. The new sample X is available in u16UUMA[UUMA_X]

and sample Z is in u16UUMA[UUMA_Z]. The centripetal acceleration components

are then removed and new LPF input samples are stored in i16XSamp_new and

i16ZSamp_new. The X channel LPF can be the 4-pole RunLPF_IIR16x16() or the 2-pole

RunLPF_IIR2P16x16(), depending on DIR_LPF_4POLE switch. Equivalent LPFs for Z

channel are 4-pole RunLPF_IIR16x16Z() and 2-pole RunLPF_IIR2P16x16Z(). The LPFs

return their output samples in variables i16XLPF_out and i16ZLPF_out. Then, one of the

direction state sequencer states is executed with state variable u8MoveState. The states

of the direction state sequencer are described below.

u8MoveState = 0 – Remain in state 0 for the number of loop passes in u8InitCnt to

stabilize the internal variables of the X and Z LPFs.

u8MoveState = 1 – Synchronize the state sequencer with the acceleration X sine wave. If

the sine wave slope is rising, initialize variables and transfer control to state 2 to wait for

the sine wave maximum. If the sine wave slope is falling, initialize variables and transfer

control to state 3 to wait for the sine wave minimum.

u8MoveState = 2 – Decide on the wheel rotation direction at the acceleration X sine

wave maximum. While monitoring the X sine wave values, save the counts of X and

Z sine waves at the X sine wave maximum point: X in variable i16XSamp_max1

and Z in i16ZSamp_Xmax1. When the X sine wave level drops D_X_DIR_CNT=400

count below its peak count saved in i16XSamp_max1, calculate the difference

between i16ZSamp_Xmax1 and the current Z count in i16ZLPF_out and save the

result in i16ZSamp_DirDiff. Then, make the direction decision based on the sign

and count value in i16ZSamp_DirDiff. If the difference value is positive and greater

than D_Z_DIR_CNT=200, increment the CW detection counter in u8dCWCnt. If the

difference value is negative and lower than -D_Z_DIR_CNT=-200, increment the CCW

detection counter in u8dCCWCnt . Then, update the counter of algorithm run repetitions

(u8dDirCnt) and check if the algorithm has already been run the required number of

times. If the algorithm has been repeated the required number of times, set the state

variable u8MoveState to 20 and exit the while() loop.

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u8MoveState = 3 – Decide on the wheel rotation direction at the acceleration X

sine wave minimum. While monitoring the X sine wave values, save the counts of

X and Z sine waves at X sine wave minimum point: X in variable i16XSamp_min1

and Z in i16ZSamp_Xmin1. When the X sine wave level rises D_X_DIR_CNT=400

count above its minimum count saved in i16XSamp_min1, calculate the difference

between i16ZSamp_Xmin1 and the current Z count in i16ZLPF_out and save the

result in i16ZSamp_DirDiff. Then make the direction decision based on the sign and

count value in i16ZSamp_DirDiff. If the difference value is negative and lower than

-D_Z_DIR_CNT=-200, increment the CW detection counter in u8dCWCnt. If the

difference value is positive and greater than D_Z_DIR_CNT=200, increment the CCW

detection counter in u8dCCWCnt. Then, update the counter of algorithm run repetitions

(u8dDirCnt) and check if the algorithm has already been run the required number of

times. If the algorithm has been repeated the required number of times, set the state

variable u8MoveState to 20 and exit the while() loop.

After exiting the while() loop, if u8dStatus is equal to 0, the direction counters u8dCWCnt

and u8dCCWCnt contain valid data and voting on the most likely rotation direction can

be performed. If u8dStatus is not 0, an error occurred while reading a sensor and the

direction variable u8dWheelDir is cleared to 0x00, indicating that u8fFindWheelDir()

function was not able to determine the direction. Voting is done simply by comparing the

number of times that one direction was detected against the other direction. If u8dCWCnt

> u8dCCWCnt, the lowest two flag bits in u8dWheelDir are set to 0x02, indicating a valid

CW direction. If u8dCCWCnt > u8dCWCnt, the lowest two flag bits in u8dWheelDir are

set to 0x03, indicating a valid CCW direction. If u8dCWCnt=u8dCCWCnt, the return value

in u8dWheelDir is 0x00, indicating that the rotation direction was impossible to determine.

5.3.4 Function Xmit_RF_Packets_Loop()

The Xmit_RF_Packets_Loop() function calculates RF packets 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 packet 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 designated start time for the next RF packet

transmission, then begins the packet transmission. 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.

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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.

Figure 4 illustrates the Xmit_RF_Packets_Loop() flow.

5.3.5 Other functions in LocAngle.c

RF_Init() – Initializes the RF module to make it ready 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 the RFData[] buffer with data packet elements which remain

constant for all packets transmitted during the current run

RF_UpdatePacket() – Fills 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. 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 debug version of the

RF packet

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

packet checksum

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

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

RunLPF_IIR16x16() – Implements a 4-pole LPF filtering the accelerometer X signal. LPF

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

SelectLPF() – Uses the last read compensated acceleration Z value as the wheel

rotation speed estimator and selects the correct LPF to be used for filtering the

accelerometer X signal. The thresholds for LPF selection are shown below. Wheel

rotation speeds in RPM and equivalent car speeds are given for tire size 205/65R16.

• LPF1 – for Z acceleration greater than 5.9 g and lower than 13.6 g, car speed greater

than 20.5 km/h and lower than 31 km/h,

• LPF2 – for Z acceleration greater than 13.6 g and lower than 51 g, car speed greater

than 31 km/h and lower than 60 km/h,

• LPF3 – for Z acceleration greater than 51 g and lower than 155 g, car speed greater

than 60 km/h and lower than 105 km/h,

• LPF4 – for Z acceleration greater than 155 g, car speed greater than 105 km/h and

lower than 152 km/h.

SetLPFDelay() – Calculates 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 ms multiplied by 10. The delay value is calculated by

straight line approximation of the signal delay vs. the signal period characteristic of the

LPF selected in u8SelLPF.

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RunLPF_IIR16x16Z() – Implements a 4-pole LPF filtering accelerometer Z signal. Used

in rotation direction finding function

RunLPF_IIR2P16x16X() – Implements a 2-pole LPF filtering accelerometer X signal.

Used in the rotation direction finding function

RunLPF_IIR2P16x16Z() – Implements a 2-pole LPF filtering accelerometer Z signal.

Used in rotation direction finding function

6 RF data packet format

The RF data packet is made up of three sections. These are:

– 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 the

array cai16TargetAngle[],

#define PACKET_DEBUG1 – Used only for debugging purposes. The program transmits

one 32-byte long packet after a rotation period was measured. 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

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 byte

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byte 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

The debug packet is created in function RF_CreatePacket_DBG1(). Using this packet,

any program variables can be monitored by transmitting them in byte positions 5 through

29.

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7 TPMS NTM88 Location Angle flowcharts

aaa-036734

READ VOLTAGE, TEMPERATURE AND PRESSURE

RUN FUNCTION u8fFindWheelDir()

- FIND WHEEL ROTATION DIRECTION

RUN FUNCTION Xmit_RF_Packets_Loop()

- TRANSMIT RF PACKETS

RUN FUNCTION u8fXGetRotPeriod()

- MEASURE ROTATION PERIOD

- FIND STARTING REFERENCE ANGLE

READ COMPENSATED ACCEL Z

START

YesNo

IS COMPENSATED ACCEL Z > 5.9 g?

CREATE RF PACKET

RETURN

Figure 1. Function vfnProcessLocAngle()

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aaa-036735

CALIBRATE FRC (LFO) PERIOD

READ ACCEL X CENTRIPETAL OFFSET

ESTIMATE INTO i16AccXCnt_cpt

RUN TIME DELAY APPROPRIATE

FOR SELECTED LPF

RUN LPF - NEW OUTPUT SAMPLE

IS IN i16XLPF_out

- READ NEXT ACCEL X RAW SAMPLE,

- SUBTRACT i16AccXCnt_cpt FROM THE RAW SAMPLE

- NEW X SAMPLE IS IN i16XSamp_new

SELECT LPF

INITIALIZE VARIABLES

CHECK TIMEOUT

START

Yes No

SINE WAVE RISING SLOPE DETECTED?

No Yes

SINE WAVE FALLING SLOPE DETECTED?

SINE WAVE RISING SLOPE 1st

SINE WAVE FALLING SLOPE 1st

Figure 2. Function u8fXGetRotPeriod() - 1

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aaa-036736

WAIT FOR MINIMUM MIN1

SINE WAVE RISING SLOPE 1st

- WAIT FOR MAXIMUM MAX1

- START PERIOD MEASUREMENT

WAIT FOR SINE WAVE

0-CROSSING MOMENT AT

ANGLE 0º (WHEEL TOP)

- WAIT FOR MAXIMUM MAX2

- END PERIOD MEASUREMENT

- CALCULATE PERIOD

go to state 21

RETURN

WAIT FOR MAXIMUM MAX1

SINE WAVE FALLING SLOPE 1st

- WAIT FOR MINIMUM MIN1

- START PERIOD MEASUREMENT

WAIT FOR SINE WAVE

0-CROSSING MOMENT AT

ANGLE 180º (WHEEL BOTTOM)

- WAIT FOR MINIMUM MIN2

- END PERIOD MEASUREMENT

- CALCULATE PERIOD

go to state 21

RETURN

Figure 3. Function u8fxGetRotPeriod() - 2

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aaa-036737

- FIND NEXT TARGET ANGLE IN cai16TargetAngle[]

- CALCULATE DELAY TO START OF PACKET XMIT

WAIT UNTIL IT IS TIME TO

START NEXT PACKET XMIT

FIND NEXT TARGET ANGLE IN cai16TargetAngle[],

CALCULATE DELAY TO START OF NEXT PACKET XMIT

START NEXT PACKET XMIT

CHECK TIMEOUT

START

STATE 21

STATE 22

ALL TARGET ANGLES

IN cai16TargetAngle[]

PROCESSED?

No Yes

RETURN

Figure 4. Function Xmit_RF-Packets_Loop()

8 Revision history

Table 1. Revision history

Document ID Release

date

Description

UM11363 v.1 20200327 Initial release

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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.

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Tables

Tab. 1. Revision history ...............................................16

Figures

Fig. 1. Function vfnProcessLocAngle() .......................13

Fig. 2. Function u8fXGetRotPeriod() - 1 .....................14

Fig. 3. Function u8fxGetRotPeriod() - 2 ..................... 15

Fig. 4. Function Xmit_RF-Packets_Loop() ................. 16

NXP Semiconductors

UM11363

NTM88 Dual-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: UM11363

Contents

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

2 Software requirements ....................................... 1

3 Project organization ............................................1

4 Overview of the TPMS Location Angle

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

5 Description of project source code ...................4

5.1 Code in main.c .................................................. 4

5.2 Code in interrupts.c ........................................... 4

5.3 Code in LocAngle.c ........................................... 4

5.3.1 Function vfnProcessLocAngle() .........................4

5.3.2 Function u8fXGetRotPeriod () ........................... 5

5.3.3 Function u8fFindWheelDir() ............................... 7

5.3.4 Function Xmit_RF_Packets_Loop() ................... 9

5.3.5 Other functions in LocAngle.c ..........................10

6 RF data packet format ......................................11

7 TPMS NTM88 Location Angle flowcharts ........13

8 Revision history ................................................ 16

9 Legal information ..............................................17

NXP NTM88 User guide

NXP NTM88 User guide

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