The FRDM-KL05Z board comes with a built in MMA8451Q accelerometer with I2C interface. In general I2C isn't the best idea for ultra low power demo, because it is so slow. We need to run the processor at least 100 to 200 usec to get the accelerometer readout (6 Bytes + overhead).
Then i connected a LIS331HH SPI/I2C accelerometer and implemented SPI at 24 MHz/4 to get the readout within 12 usec, which is a lot better. I can get this down to about 8 usec with 20 MHz/2. But the SPI accelerometer has much more noise (650 versus 130 or 100 ug/sqrt(Hz)). This means the onboard accelerometer of the FRDM-KL05Z is a lot better concerning noise.
Is this because it is I2C? Can we use the MMA8451Q with SPI? Is there/will there be a SPI version of the MMA8451Q that serves better for ultra low power applications?