LSM303AGR Datasheet, PDF (22/68 Pages) STMicroelectronics – ultra-low-power 3D accelerometer and 3D magnetometer

Functionality
LSM303AGR
4.1.2
4.1.3
Magnetometer offset cancellation
The offset cancellation is the result of performing a set and reset in the magnetic sensor.
The offset cancellation technique is defined as follows:
H_out
=
H_{-----n----–-----}H_{----n----–---1-}
2
where H_n and H_n-1 are two consecutive magnetic field measurements, one after a set pulse,
the other after a reset pulse.
Considering a magnetic offset (H_off), the two magnetic field measurements are:
 Set: H_n = H + H_off
 Reset: H_n-1 = –H + H_off
The offset is cancelled according the offset cancellation technique:
H_out
=
H_{-----n----–-----}H_{----n----–---1-}
2
=
2_----H_------+-----H_{----o---f--f---+-----–---}H_-----o---f-f
2
=
H
In the LSM303AGR the offset cancellation is enabled by setting the bit OFF_CANC = 1 in
CFG_REG_B_M (61h).
If the offset cancellation is disabled, a set of the magnetic sensor is performed anyway.
The set pulse frequency can be configured by setting the Set_FREQ bit in CFG_REG_B_M
(61h).
Magnetometer interrupt
In the LSM303AGR the magnetometer interrupt signal generation is based on the
comparison between data and a programmable threshold.
To enable the interrupt function, in INT_CTRL_REG_M register (63h) the "IEN" bit must be
set to '1'.
In the LSM303AGR the user can select the axis/axes in which the interrupt function can be
enabled. In order to do this, the XIEN, YIEN, and ZIEN bits in INT_CTRL_REG_M (63h)
need be set properly.
The threshold value can be programmed by setting the INT_THS_L_REG_M (65h) and
INT_THS_H_REG_M (66h) registers.
The threshold is expressed in absolute value as a 15-bit unsigned number. The threshold
has the same sensitivity as the magnetic data.
When magnetic data exceeds the positive or the negative threshold, the interrupt signal is
generated and the information about the type of interrupt is stored in the
INT_SOURCE_REG_M (64h) register. In particular, when magnetic data exceeds the
positive threshold the P_TH_S_axis bit is set to '1', while if data exceeds the negative
threshold the N_TH_S_axis bit is set to '1'. If magnetic data lay between the positive and the
negative thresholds, no interrupt signal is released.
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