Data Sheet

Bosch Sensortec | BME280 Data sheet
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Modifications reserved | Data subject to change without notice
Document number: BST-BME280-DS002-15
Revision_1.6_092018
4. Data readout
To read out data after a conversion, it is strongly recommended to use a burst read and not address
every register individually. This will prevent a possible mix-up of bytes belonging to different
measurements and reduce interface traffic. Note that in I²C mode, even when pressure was not
measured, reading the unused registers is faster than reading temperature and humidity data
separately.
Data readout is done by starting a burst read from 0xF7 to 0xFC (temperature and pressure) or from
0xF7 to 0xFE (temperature, pressure and humidity). The data are read out in an unsigned 20-bit
format both for pressure and for temperature and in an unsigned 16-bit format for humidity. It is
strongly recommended to use the BME280 API, available from Bosch Sensortec, for readout and
compensation. For details on memory map and interfaces, please consult chapters 5 and 6
respectively.
After the uncompensated values for pressure, temperature and humidity ‘ut’, ‘up’ and ‘uh’ have been
read, the actual humidity, pressure and temperature needs to be calculated using the compensation
parameters stored in the device. The procedure is elaborated in chapter 4.2.
4.1 Data register shadowing
In normal mode, the timing of measurements is not necessarily synchronized to the readout by the
user. This means that new measurement results may become available while the user is reading the
results from the previous measurement. In this case, shadowing is performed in order to guarantee
data consistency. Shadowing will only work if all data registers are read in a single burst read.
Therefore, the user must use burst reads if he does not synchronize data readout with the
measurement cycle. Using several independent read commands may result in inconsistent data.
If a new measurement is finished and the data registers are still being read, the new measurement
results are transferred into shadow data registers. The content of shadow registers is transferred into
data registers as soon as the user ends the burst read, even if not all data registers were read.
The end of the burst read is marked by the rising edge of CSB pin in SPI case or by the recognition of
a stop condition in I
2
C case. After the end of the burst read, all user data registers are updated at
once.
4.2 Output compensation
The BME280 output consists of the ADC output values. However, each sensing element behaves
differently. Therefore, the actual pressure and temperature must be calculated using a set of
calibration parameters. In this chapter, the method to read out the trimming values will be given. The
recommended calculation uses fixed point arithmetic and is given in chapter 4.2.3.
In high-level languages like Matlab™ or LabVIEW™, fixed-point code may not be well supported. In
this case the floating-point code in appendix 8.1 can be used as an alternative.
For 8-bit micro controllers, the variable size may be limited. In this case a simplified 32 bit integer code
with reduced accuracy is given in appendix 8.2.
4.2.1 Computational requirements
In the table below an overview is given for the number of clock cycles needed for compensation on a
32 bit Cortex-M3 micro controller with GCC optimization level -O2. This controller does not feature a
floating point unit, thus all floating-point calculations are emulated. Floating point is only recommended
for PC application, where an FPU is present and these calculations are performed drastically faster.