Using Sample Rate at MMA8491Q

[abdullah_cinar]

I want to wake my microcontroller up using the accelerometer. Do i need a host microcontroller to use the sample rate settings at mma8491? Because it is getting meanless for me. I want my microcontroller to sleep all the time. My accelerometer needs to measure wheter there is a movement or not; then if there is a movement, it must awaken my microcontroller.

[abdullah_cinar]

Hi Anthony,

Thanks for the reply! Yes i have already saw that price and ordered the mma8491q :smileyhappy: We want to inspect the garbage vans discharging of garbage containers. So actually a 45degree tilt detection is enough for us. We want to ignore the horizontal movement of the container(because normal people can push or pull the container too). It must just wake up when the container has been picked up by the garbage van,so the accelerometer must wake up with this movement and then needs to waken the host mcu, and then mcu implements its own processes. We want to use low power components as much as we can. I'm gonna think over the MMA8652FC, i can use it with the auto wake-up mode if we conclude that current consumption is enough for us.

Btw i know you are the new NXP but little confused when i see the site link :smileygrin:

[anthonyduhamel]

Hello Abdullah,

MMA8491Q does not include interrupt pins for mcu wake up.

I suggest you to use the MMA8652FC accelerometer instead of the MMA8491Q. To use sensor sample rate you're looking for a function we called DATA_READY interrupt. The sensor pulls-up (/pulls down) a pin to wake the mcu when a measurement occurs. 

For your application the best thing is to use the motion detection feature includes in the MMA8652FC .

The option is also using an interrupt pin to wake-up your mcu. You can configure thresholds thought I2C.

Anthony

[abdullah_cinar]

(Sorry i couldn't find the edit button so replied again.)

How can i use the tilt detection? That is enough for me too if i use it without the need of external mcu. I actually don't need interrupt to waken my mcu. Just changing the output level of one pin is enough for me. Is this possible with mma8491q? And what is the current consumption of this mode? Sorry i'm forcing this chip because i have already ordered the breakout board for this :/

[anthonyduhamel · Accepted solution]

Hello Abdullah,

MMA8491Q is a basic digital accelerometer but it cames with tilt detection feature.

So yes, there are 3 different pins for tilt detection.

You should be able to use this option without Mcu, but you have to clock the sensor EN pin to fit with the sensor operating specifications. Clocking the EN pin will refresh the g-measurements.

The current consumption depends on the measurement data rate. MMA8491Q datasheet notices 400to980nA/Hz in one-shot mode (1.8 to 68nA in standby mode).

Anthony

[abdullah_cinar]

I have found an accelerometer with motion activated wake up mode, 270nA current consumption. I don't know it is ethical to share the model number in here because it is not freescale's accelerometer. It's ADXL362 from analog devices. I have already ordered a breakout board for Freescale's MMA8491q but i realized i need wake-up mode. Is there any Freescale's accelerometer with wake-up mode and low current consumption like lower than 1microamps? MMA8652FC has 6.5 microAmps for ODR = 1.563 Hz.

[anthonyduhamel]

Hello Abdullah,

I'm sorry for that, but for the moment we don't have any ultra low-power accelerometer in our portfolio​. We will have a new one, but it is not on the market today.

ADXL362 is a pretty utlra low power device, that true, but look at the price....

... it is 7 times more expensive than ours.

Your side to decide. What is your project? Can you tell me more about it?

For your application, I'm not sure the tilt detection will be enough to detect motion.

Anthony

FYI: we are now the new NXP :smileywink:!

[abdullah_cinar]

By the way, can you clarify me, which operation mode do i need to look for the current consumption at the auto wake-up mode? I couldnt find it at the electrical characteristics. Does the system stands on the active mode when we enable the auto-wakeup?  MMA8450Q has wake-up mode too, but at the electrical characteristics, it says EN=1 at the low power mode. Do we need to drive the EN pin from an external source? or does it automatically wake up with motion detection?

Here is the capture:

[anthonyduhamel]

EN pin is to enable/disable I2C bus. You don't have to drive it. The current consumption depends on the selected ODR (CTRL_REG1).

For your application you can use the motion detection feature or the portrait/landscape function or the transient detection with a high pass filter . External interrupt pin can be configured through CTRL_REG4&5 registers, .

I suggest you to have a look on the datasheet and open a new thread/discussion if you have any other questions on this device.

Thanks,

Anthony

[abdullah_cinar]

Thank you so much Anthony, you helped a lot :smileyhappy: I will consider the information you gave at the development.

Best Regards.

[abdullah_cinar]

Hi Anthony,

Thank you for the information. Well,I guess i can use the tilt detection then. I'm gonna make a decision trying to generate a timer pulse from mcu and calculating the total current consumption. Actually i was planning to make mcu sleep and let the accelerometer waken the mcu up. But at that case i need an accelerometer which have motion detection. But they either are expensive or have higher current consumption. So i'm gonna wake my mcu at every 1 second then generate output for the en pin then go to sleep. This is a little meanless but i couldn't find any cheap and low power accelerometer. Do you know any?

[abdullah_cinar]

Thank you so much for the answer. Yeah i think i made a mistake, because of i'm a beginner, i tought if i use the sample rate i can measure the movement without the need of external mcu or clock. I can use MMA8652FC accelerometer then, but i need lower power consumption( like <1microamp). Is there any accelerometer for this?