NXP Semiconductors FXTH870 D Series, Manual

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FXTH870xD
Sensors
80 Freescale Semiconductor, Inc.
10 Other MCU Resources
It is not intended that physical parameter measurements be made during the time that LFR may be actively receiving/decoding
LF signals; or during the time that the RFM may be actively powered up and/or transmitting RF data. The resulting interactions
will degrade the accuracy of the measurements.
The FXTH870xD measures six physical parameters for use in the tire pressure monitoring application: pressure, temperature,
battery voltage, two external voltages and an optional X- and/or Z-axis acceleration. Each parameter is accessed in a different
manner and all use firmware subroutine calls as described in
Section 14 . These subroutines initialize some control bits within the
sensor measurement interface, SMI, and then place the MCU into the STOP4 mode until the measurement is completed with an
interrupt back to the MCU.
The accuracy, power consumption and timing specified for any measurement given in the electrical specifications in Section 17
are only guaranteed if the user obtains a reading using the specified firmware subroutine call in Section 14 .
The FXTH870xD uses a 6-channel, 10-bit analog-to-digital converter (ADC10) module. The ADC10 module is an analog-to-digital
converter using a successive approximation register (SAR) architecture with sample and hold. Capture of pressure and
acceleration sensor readings is controlled by the sensor measurement interface (SMI) and capture of temperature and voltage
readings are controlled by the MCU.
When making measurements of the various analog voltages the individual blocks. will first be powered up long enough to stabilize
their outputs before a conversion is started. The ADC channels are connected in hardware. Conversions are started and ended
synchronously with the sampling of the voltages.
The accuracy, power consumption and timing specifications given in the electrical specifications in Section 17 are based on using
the assigned firmware subroutines in Section 14 to make these measurements and convert them into an 8-bit, 9-bit or 10-bit
transfer function. These measurement accuracy specifications cannot be guaranteed if the user creates custom software routines
to convert these measurements.
10.1 Pressure Measurement
The pressure measurement consists of an interface to a pressure sensing element. Control bits on the MCU operate the SMI to
power up the P-Cell and capture a voltage which is converted by the ADC10. The resulting pressure transfer equation for the
100-450 kPa range:
Eqn. 1
The transfer equation of the 100-900 kPa range is:
Eqn. 2
Due to calibration routines and parameters stored in the FXTH870xD, the pressure range is selected at production and cannot
be changed in the field.
NOTE
Lack of change of the pressure measurement over time may indicate the package pressure
port to be blocked or the internal section of the sensor to be contaminated. User application
should maintain either locally or at the system data receiver a record of pressure
measurements along with temperature and/or accelerometer measurements, and possibly
identify the pressure port as blocked or contaminated if no changes are recorded over time.
Table 53. ADC10 Channel Assignments
ADC10 Channel Input Select Firmware Call(s) Characteristic
AD0
Pressure Sensor TPMS_READ_COMP_PRESSURE P
CODE
Optional X-axis Acceleration Sensor TPMS_READ_COMP_ACCEL_X A
XCODE
Optional Z-axis Acceleration Sensor TPMS_READ_COMP_ACCEL_Z A
ZCODE
AD1 Temperature Sensor TPMS_READ_COMP_TEMP_8 T
CODE
AD2 Bandgap Reference TPMS_READ_COMP_VOLTAGE V
CODE
AD3 GPIO PTA0 TPMS_READ_V0 G0
CODE
AD4 GPIO PTA1 TPMS_READ_V1 G1
CODE
AD5 V
REG
Monitor TPMS_WIRE_CHECK
P

P
450
P
CODE

100

P
450
–
 
+ =
P

P
900
P
CODE

100

P
900
–
 
+ =