Datasheet

BMP280 Digital Pressure Sensor

Page 20

BST-BMP280-DS001-19 | Revision 1.19 | January 2018

Bosch Sensortec

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Note: Specifications within this document are subject to change without notice. Not intended for publication.

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.

Table 16: Computational requirements for compensation formulas