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Bosch Sensortec | BMP280 Data sheet

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Document number: BST-BMP280-DS001-26 Revision_1.26_102021

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.

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