On an invensense IMU with fast sampling we need to lower the slave
sample rate for slave sensors such as the built-in AK8963 compass on a
MPU9250.
The slave rate is set as a multiple of the main rate, so it needs to be
much lower for fast sampling. If we leave it high then it greatly
impacts on IMU sample rate.
Without this change a MPU9250 with fast sampling and a compass enabled
will give a gyro rate of around 7200Hz. With this change it gets 7760Hz
FIFO sensors produce data at a well known rate, but samples come in
bunches, so we can't use the system clock to calculate deltaT.
non-FIFO sensors produce data when we sample them, but that rate is
less regular due to timing jitter.
For FIFO sensors this changes makes us use a learned sample rate,
which allows for different clock speeds on sensor and system board.
For non-FIFO sensors we use the system clock to measure deltaT
the overall effect is a fix for sensors that produce samples at other
than the claimed datasheet rate.
Andrew Tridgell
Lucas De Marchi
murata
mirkix
Randy Mackay