bmp280 datasheet

Bosch
Sensortec | BMP280 Data sheet
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Modifications reserved | Data subject to change without notice
Document number: BST-BMP280-DS001-23 Revision_1.23_11202
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3.9 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. Data readout is done by starting a burst read from 0xF7 to
0xFC. The data are read out in an unsigned 20-bit format both for pressure and for temperature. It is
strongly recommended to use the BMP280 API, available from Bosch Sensortec, for readout and
compensation. For details on memory map and interfaces, please consult chapters 3.12 and 5
respectively.
The timing for data readout in forced mode should be done so that the maximum measurement times
(see chapter 3.8.1) are respected. In normal mode, readout can be done at a speed similar to the
expected data output rate (see chapter 3.8.2). After the values of ‘ut’ and ‘up’ have been read, the
actual pressure and temperature need to be calculated using the compensation parameters stored in
the device. The procedure is elaborated in chapter 3.11.
3.10 Data register shadowing
In normal mode, measurement timing is not necessarily synchronized to readout. 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.
Reading across several data registers can therefore only be guaranteed to be consistent within one
measurement cycle if a single burst read command is used. 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 I2C case. After the
end of the burst read, all user data registers are updated at once.
3.11 Output compensation
The BMP280 output consists of the ADC output values. However, each sensing element behaves
differently, and actual pressure and temperature must be calculated using a set of calibration
parameters. The recommended calculation in chapter 3.11.3 uses fixed point arithmetic. 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.
3.11.1 Computational requirements
The table below shows the number of clock cycles needed for compensation calculations on a 32 bit
Cortex-M3 micro controller with GCC optimization level –O2. This controller does not contain a floating
point unit, so all floating-point calculations are emulated. Floating point is only recommended for PC
applications where an FPU is present.