Ultra low power accelerometer

[dieterteuchert]

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?

[TomasVaverka · Accepted solution]

Dieter,

Regarding your last question, we are planning to launch such a part in June with full production. It will be basically the same device as the MMA8451Q, but now including the SPI.

Have you considered using the embedded FIFO buffer on the MMA8451Q? It is very useful for system power savings and minimizing traffic across the I2C bus. Look at our AN4073 for more information on the embedded FIFO buffer.

Regards,

Tomas

[Paul_Tian]

Hi, Dieter

The I2C interface in KL series is designed to operate up to 400 kbit/s with maximum bus loading and timing. The I2C device is capable of operating at higher baud rates, up to a maximum of clock/20, with reduced bus loading.

I find a g-sensor with SPI interface for your reference. Please check if MMA745xL series meet your requirement. You can find some material from link.

http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=MMA745xL&fpsp=1&tab=Documentation_Tab

Hope my reply can help you.

Best Regards

Paul

[dieterteuchert]

Thanks for your response. I could not find a noise spec of the MMA745xL. It has a consumption of 400 uA and 10 bits of resolution.

As far as i understand, with a Kinetis KL device onboard i can as well try an analog accelerometer. For example with a LIS344ALH, at 50 Hz x 2 msec usage, the current consumption could be as low as 70 uA plus the consumption of 150 ADC conversions per second in the KL15. I mean from the low power point of view the analog transfer of acceleration measurements could be the best solution.

And the LIS344ALH has a noise spec of 50 ug/sqrt(Hz), which is even better than the MMA8451Q.

[Paul_Tian]

Hi, Dieter

Yes, I think you can use KL15 and analog sensor for your application. Thanks.

Best Regards

Paul