NXP Semiconductors UM11227 - Measurement Firmware

NXP Semiconductors
UM11227
NTM88 family of tire pressure monitor sensors
UM11227 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2020. All rights reserved.
User manual Rev. 6 — 24 April 2020
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(10)
Where:
V
DD0
= the voltage determined with the external load resistor connected to V
SS
V
DD1
= the voltage determined with the external load resistor connected to V
DD
R
LOAD
= the resistance of the external load resistance in ohms
R
BATT
= the implied battery impedance in ohms
NXP recommends this calculation be performed with a reasonable current load on the
battery of approximately 3 mA (R
LOAD
approximately 1000 ohms).
10.26.6 Measurement firmware
The firmware library routines for making measurements are comprised of two function
calls as described in the firmware user guide. Each measurement is a combination of a
"read" that returns the raw ADC output data and a "comp" routine which compensates
that raw reading based on information contained in the Universal Uncompensated
Measurement Array (UUMA) assigned in RAM memory.
The read routines fill specific locations in the UUMA with raw data; but the compensation
routines depend what is already present in the UUMA as shown in the data flow in
Figure 59.
The user, therefore, has the option to decide how often each measurement (and its
component terms) is made. The resulting power consumption is then the sum of using
these components are defined in the product data sheet.
A typical flow for a compensated pressure measurement would be:
1. Call the TPMS_READ_PRESSURE routine which yields a raw pressure value and fills
the UUMA with this data.
2. Call the TPMS_READ_TEMPERATURE routine which yields a raw temperature value
and fills the UUMA with this data.
3. Call the TPMS_READ_VOLTAGE routine which yields a raw voltage value and fills
the UUMA with this data.
4. Call the TPMS_COMP_PRESSURE routine which then takes the raw pressure,
temperature and voltage values from the UUMA and compensates to provide a true
pressure reading to the accuracy as specified in the product data sheet.