I have a question about MPU9250 SPI access. The schematic tells that the interrupt pin, namely DRDY, is connected to the MCU, but I did not find the relevant code in the firmware using the interrupt functionality. May I ask how MPU9250 is accessed via SPI? I use pixracer.
The interrupt pin is currently not used for that driver. Rather the MPU is internally configured to update at 1kHz and is then polled at 1kHz intervals for changes. So there may be some latency between when the data is measured and when it is read, but that is fairly negligible at this speed.
See https://github.com/PX4/Firmware/blob/master/src/drivers/mpu9250/mpu9250.cpp#L1088 for where the polling timer is initially triggered.