Datasheet

DocID17530 Rev 2 23/54
LIS3DH Digital main blocks
54
5.1.3 Stream mode
In Stream mode the FIFO continues filling data from the X, Y, and Z accelerometer channels
until the buffer is full (a set of 32 samples stored) at which point the FIFO buffer index
restarts from the beginning and older data is replaced by the current data. The oldest values
continue to be overwritten until a read operation frees the FIFO slots.
An overrun interrupt can be enabled, I1_OVERRUN = '1' in the CTRL_REG3 (22h) register,
in order to read the entire contents of the FIFO at once. If, in the application, it is mandatory
not to lose data and it is not possible to read at least one sample for each axis within one
ODR period, a watermark interrupt can be enabled in order to read partially the FIFO and
leave memory slots free for incoming data.
Setting the FTH [4:0] bit in the FIFO_CTRL_REG (2Eh) register to an N value, the number
of X, Y and Z data samples that should be read at the rise of the watermark interrupt is up to
(N+1).
5.1.4 Stream-to-FIFO mode
In Stream-to-FIFO mode, data from the X, Y and Z accelerometer channels are collected in
a combination of Stream mode and FIFO mode. The FIFO buffer starts operating in Stream
mode and switches to FIFO mode when the selected interrupt occurs.
The FIFO operating mode changes according to the INT1 pin value if the TR bit is set to ‘0’
in the FIFO_CTRL_REG (2Eh) register or the INT2 pin value if the TR bit is set to‘1’ in the
FIFO_CTRL_REG (2Eh) register.
When the interrupt pin is selected and the interrupt event is configured on the corresponding
pin, the FIFO operates in Stream mode if the pin value is equal to ‘0’ and it operates in FIFO
mode if the pin value is equal to ‘1’. Switching modes is dynamically performed according to
the pin value.
Stream-to-FIFO can be used in order to analyze the sampling history that generates an
interrupt. The standard operation is to read the contents of FIFO when the FIFO mode is
triggered and the FIFO buffer is full and stopped.
5.1.5 Retrieving data from FIFO
FIFO data is read from OUT_X_L (28h), OUT_X_H (29h), OUT_Y_L (2Ah), OUT_Y_H
(2Bh) and OUT_Z_L (2Ch), OUT_Z_H (2Dh). When the FIFO is in Stream, Stream-to-FIFO
or FIFO mode, a read operation from the OUT_X_L (28h), OUT_X_H (29h), OUT_Y_L
(2Ah), OUT_Y_H (2Bh) or OUT_Z_L (2Ch), OUT_Z_H (2Dh) registers provides the data
stored in the FIFO. Each time data is read from the FIFO, the oldest X, Y and Z data are
placed in the OUT_X_L (28h), OUT_X_H (29h), OUT_Y_L (2Ah), OUT_Y_H (2Bh) and
OUT_Z_L (2Ch), OUT_Z_H (2Dh) registers and both single read and read burst operations
can be used.
The address to be read is automatically updated by the device and it rolls back to 0x28
when register 0x2D is reached. In order to read all FIFO levels in a multiple byte read,
192 bytes (6 output registers of 32 levels) have to be read.