NTM88H0 Reading pressure measurements - wrong equation

[OzEl]

Hi,

I'm using the NTM88H065T, and I successfully read the value of the pressure sensor and the temperature sensor. The temperature sensor is pretty clear, the equation is: T °C = (1 °C / LSB × TCODE) – 55 °C
and the received values make sense.
My goal is to understand the pressure value that the host MCU receives.

In the TPMS, I read values of ~208 raw data while the external device measured 22.1 PSI. The equation below shows that this is equal to 259.745 [kPa] or 37.6 PSI, which is far from the actual value.
P kPa = (0.824 kPa / LSB × PCODE) + 88.353 kPa

What am I missing? what is the right equation?

About my project:

1. I'm using NTM88H065T 
2. The FW based on the example: "NTM88-KW38-BLE-BEACONS"
3. The above  equation is taken from the datasheet: NTM88Hxx5, Tire pressure monitor sensor - Data sheet section 6.10.1
   
   

Any help will be appreciated

[diazmarin09]

Hello,

I hope all is great with you. Thank you for using the NXP communities.

The execution time of sensor reading and compensation is indicated in the firmware user guide available at the link below. In this case, please review the TPMS_READ_PRESSURE or TPMS_COMP_PRESSURE functions.

https://www.nxp.com/webapp/Download?colCode=UM11145

The compensated measurements are expressed in counts and not directly in common units (volt, degree Celsius, kPa, and g). The formula to convert a measurement from compensated counts into common units is called the transfer function and is indicated in the data sheet of the device. Different part numbers have different transfer functions. Conversion examples are provided in each transfer function.

I hope this information helps.

Regards,

David

[diazmarin09]

Hello Oz,

Please accept my apologies for the delayed response.

The information that I shared, is based on the application note AN13614 from the link below:

AN13614, NTM88 Sensor Measurements - Application Note

You are correct, the correct transfer function to get the pressure value is:

P kPa = (0.824 kPa / LSB × PCODE) + 88.353 kPa

The current datasheet is Rev. 1 — 7 September 2022.

Regards,

David

[OzEl]

Hi David,

1. Thanks for your quick answer,
   I am unable to locate the datasheet that you quoted, so is it possible to add a link to the latest version of the datasheet? Among the other versions, the datasheet that you quoted seems to be missing. (By the way, you have far too much different datasheets!!!)
2. There seems to be a typo in the datasheet attached as a screenshot, the transfer function should be minus 88.353 or plus 88.353? Probably + 88.353 (Attach screenshot below)
3.  I'm not sure why did you mention the functions TPMS_READ_PRESSURE and TPMS_COMP_PRESSURE? As I write I'm using the example project "NTM88-KW38-BLE-BEACONS" which contains them in the function "vfnTakeSensorsMeasurements", I obviously used them.
4. Bottom line, it still not clear what is the correct transfer function? and how the data is 208 (after compensation) while the actual pressure is 22.1 PSI (1 PSI equal to 6.89476 kPa). So 208 equal to 259.745 kPa =0.824*208+88.353 which is far from 22.1 PSI.

What am I missing?

[OzEl]

Hi David,

The weekend delay gives me the opportunity to measure additional results and attach them here.

I change the transfer function (The +/- 88.353 was removed, attached screenshot below), and here are the results of 6 different NTM88 chips in the same tire:

A short explanation of the results:
Y-axis - Pressure in PSI units
The X-axis represents the number of measurements at every experiment
In each color, the pressure is different (measured by two different external devices).

Can you please explain why I changed the transfer function as described above and still (and only then) got good results? Naturally, I had bad results at very low pressures (on the air).

The primary question remains, what is the correct transfer function?