DocID025978 Rev 1

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AN4450

Software

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Terminology is very delicate for altitude. There is not one definition of altitude:

1.

Absolute pressure sensor measures the air pressure at the sensing point. The base

rule of thumb is that the pressure drops by 1 hPa every 8.3 meters.

2.

The “sea level pressure average” is 1013.25 hPa (standard altitude measurement done

in most digital barometer wrist watches)

3.

The “instant sea level pressure” is weather and location dependent and can vary by

more than 5 hPa (compare SFO and SIN airports on the web)

4.

Also beware that pressure altitude may be different than GPS altitude, which may also

be different than density altitude.

Aircraft flying between airports use the ISA altitude which is based on yearly/earth-wide sea

level pressure average value (as shown above) to avoid plane collision; this is the altimeter

reference.

When aircraft approach an airport, local equivalent sea level and airfield temperature are

wirelessly shared from airport to plane to compute local instant airfield altitude. This is

known as QNH, using METAR data.

6.5

SW filtering & internal FIFO

If higher precision is required, or if the air pressure/flow is unstable, a SW filter could be

implemented on the sensor measurements.

The internal FIFO buffer and HW filter can also be used, as indicated in previous sections;

this is strongly suggested since it reduces the level of pressure noise and in the meantime

decreases the power consumption.

For indoor navigation when sub meter detection is required, a special recursive filter could

be used. Tailoring the SW filter for the sensor characteristic and the application requirement

is usually recommended. Sensor fusion is the most advanced filtering which uses all

available information and uses them to reduce the positioning noise.

6.6

Reset the device to power-on configuration

If there are any doubts regarding the register content, there is the possibility to reset the

configuration of the device at the power-on condition setting both BOOT bit and SWRESET

bit in CTRL_REG2 (0x21).

The bit[2] SWRESET resets all the non-trimming registers to their reset values and stops

any internal state machines.

The bit[7] BOOT can be used, when the device is enabled, to reinitialize the volatile

registers with the content of the internal non-volatile trimming memory. These values are

factory trimmed and are different for every device. They permit good behavior of the device

and normally they should not be changed. If, for any reason, the content of the trimming

registers is modified, it is sufficient to use the BOOT bit to restore the correct values.