AN4504 datasheet(37/41 Pages) STMICROELECTRONICS

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AN4504

First-in first-out (FIFO) buffer

Figure 20. Bypass-to-Stream mode

Bypass-to-stream can be used in order to start the acquisition when the configured interrupt

is generated.

6.4

Watermark

The watermark is a configurable flag that can be used to generate a specific interrupt in

order to know when the FIFO buffer contains at least the number of samples defined as the

watermark level. The user can select the desired level in a range from 0 to 31 using the

WTM[4:0] field in the FIFO control register while the FIFO source register FSS[4:0] always

contains the number of samples stored in FIFO. If FSS[4:0] is greater than WTM[4:0], the

WTM bit is set high in the FIFO source register, on the contrary, WTM is driven low when the

FSS[4:0] field becomes lower than WTM[4:0]. FSS[4:0] increases by one step at the ODR

frequency and decreases by one step every time that a sample set reading is performed by

the user.

Figure 21. Watermark behavior - WTM[4:0] = 10 (hex)

In Figure 21, the first row indicates the FSS[4:0] value, the second row indicates the relative

FIFO slot and last row shows the incremental FIFO data. Assuming WTM[4:0] = 10 (hex),

the WTM flag changes from “0” to “1” when the eleventh FIFO slot is filled (F10). Figure 22

shows that the WTM flag goes low when the FIFO content is less than WTM[4:0] which

means that nine unread sample sets remain in FIFO. The watermark flag (WTM) can be

enabled to generate a dedicated interrupt on the DRDY/INT2 pin by setting the I2_WTM bit

high in CTRL_REG3.

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