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AN4450

Using FIFO modes

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5

Using FIFO modes

5.1

Effective use of the FIFO

The LPS25H embeds a 32-slot x 24 bit FIFO for pressure data coming from the registers

that normally feed PRESS_OUT (@ 0x28/0x29/0x2A). It allows lower frequency of serial

bus transactions and provides more time to collect all measurements taken.

The FIFO buffer is enabled by setting FIFO_EN to 1 (bit[6] in CTRL_REG2) and can work in

4 different main modes: Bypass Mode, FIFO Mode, Stream Mode and FIFO Mean Mode.

Each mode is selected by the 3 FIFO_MODE bits in FIFO_CTRL register.

Watermark level (P1_WTM), FIFO_empty (P1_EMPTY), FIFO_full (P1_Overrun) or

DATA_Ready (P1_DRDY) events can be enabled (in register CTRL_REG4 (0x23)) to

generate dedicated interrupts on the INT1 pin (provided that bit[1:0] of register CTRL_REG3

(0x22) are configured “00” to select Data Signal).

When FIFO is enabled temperature data must be read separately from pressure data. The

temperature readout is always referred to the last pressure data in the FIFO, as no FIFO is

available for temperature data. The register auto-increment “rounding” is then different

depending on FIFO_EN status, as shown in the picture below.

Figure 8. Auto-increment rounding depending on FIFO_EN

To switch FIFO on/off, the bypass mode must be used. Switching the FIFO on/off only using

the FIFO_en bit of register CTRL_REG2 would cause a wrong FIFO behavior.

5.1.1

Accessing the FIFO data

FIFO data is read through PRESS_OUT registers (0x28/0x29/0x2A). When FIFO is in

stream, trigger (i.e. state change on trigger) or FIFO mode, a read operation to the

PRESS_OUT registers provides the data stored in the FIFO. When in FIFO mean mode it is

not possible to read the data in the buffer, only the final averaged value will be available in

PRESS_OUT registers at the end of the process done with the selected speed (see

Section 5.1.5

).

Each time data is read from the FIFO, the oldest entry is placed in the PRESS_OUT

registers and both single read and multiple read operations can be used.