FXTH87xx pressure measurement

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FXTH87xx pressure measurement

Contributor I

I have purchased TPM sensor FXTH8705116T1- 450kpa.

I have installed it inside Tire and filled with an air pressure of 42 PSI.

The pressure value received from the sensor is 0x1FE (TPMS_COMP_PRESSURE).

From the data sheet,  0x1FE (TPMS_COMP_PRESSURE) represents pressure overflow(high). But 450kpa sensor has a range of 14 - 65 psi(100 – 450kpa).

Also when we gradually reduced the air level from tire, sensor pressure value also decreased gradually from 0x1FE (0x1EA , 0x1Ab etc.,).

For 42 psi , expected pressure value from the sensor should be around 0x122. But  receiving 0x1FE. Please help.

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NXP Employee
NXP Employee



Measuring the actual value of pressure is done in two steps:
1. TPMS_READ_PRESSURE(gu16UUMA, 1u) will store the 10-bit uncompensated value of pressure (output of the ADC) in UUMA.
2. TPMS_COMP_PRESSURE(&u16CompPress,gu16UUMA) performs a compensation of the pressure value stored in UUMA: the function linearizes the value and correct the error due to temperature and voltage (this is why this function must be called after the readings of pressure, temperature and voltage).
Both functions (READ and COMP) return a status to indicate if the pressure value is ok or if there has been a problem with the ADC or an underflow/overflow.
What is important to do is to check the returned status to know if the pressure value is ok or not. The returned status is explained in the firmware user guides. For example, below is the returned status of TPMS_COMP_PRESSURE:
Table 13.JPG
The Embedded Firmware User Guides are on FXTH87 Documentation webpage, under User Guides.
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