Asset tracker accelerometer unit Kelvin/second?

Hardware
1

Wondering what the sampling rate of the accelerometer in the Asset Tracker is.

// Default to 5kHz low-power sampling
accel.setDataRate(LIS3DH_DATARATE_LOWPOWER_5KHZ); // = 0xb1001

From the datasheet exactly "ODR<3:0>
0b0111 = Normal / low power mode (400 Hz)
0b1000 = Low power mode (1.6 KHz)
0b1001 = Normal (1.25 kHz) / low power mode (5 KHz)"
source: https://cdn-shop.adafruit.com/datasheets/LIS3DH.pdf pp.30 same revision as ST

Normal mode using 11uA samples at 1.25kHz
Low power mode using 6uA samples at 5kHz :confused:

Thanks

2

In case it is unclear, KHz stands for kilohertz, which is a unit for measuring how many thousands of times a second something happens.

I’m not sure how you got Kelvin (temperature) per second as a unit for an accelerometer.

I don’t know the sampling rate of the accelerometer used in the Asset tracker.

3

My thoughts exactly, many times it is auto-captalized, but since the datasheet has both kHz and KHz in the same line, something else must be going on.

SI: Kelvin (K), kilo (k)

4

I think that data sheet is just messed up.

The capital-K problem is corrected here in the Application Note for that part:

2 Likes
5

I just checked the docs, and here is all it says about the accelerometer:

The shield also has an on-board accelerometer, the LIS3DH. It's extremely low power so won't chew up your energy budget. The accel communicates over SPI, so it takes up A2, A3, A4, and A5 as marked on the silkscreen of the shield. A configurable interrupt from the LIS3DH is connected to the Electron's "wake" (WKP) pin, so you should be able to make a project where the Electron and GPS stay in deep sleep until it's hit hard enough to cross a threshold you set on the accelerometer.

Its mentioned as being "extremely low power" so I assume it is running in one of the low power modes but I am unsure which one it is.

@ScruffR Do you know anything about the accelerometer or can you direct @makerken to someone who does?

6

Great find @bko! Still a little puzzled as why a higher data rate uses less power.

Thanks everyone.

7

Actually not, KHz is a typo - it should be kHz - but I'd not go to assume the capital K stands for Kelvin either, otherwise we would have Kelvin * Byte (even less would I change a multiplication for a division ;-))

But I've no idea about the reasons for the power consumption question at 5kHz.
The only thing I understand from the datasheet that it's rather the other way round. Not 5kHz cause the lower power consumption, but the low power mode causes the setting ODR0:3 = 0b1001 to mean 5kHz while in normal power mode the same setting stands for 1.25kHz.
Which mode (normal or low power) is controlled with the LPen bit (0x20:3)

1 Like
8

For current project it seems quick enough.

It can be configured to 1Hz, I wonder the power savings and responsiveness at that rate. I’d have to need it, and get a ucurrent gold.

Thanks, one datasheet mistake is easy enough to make, but two seems like I was missing something. A search for ‘kHz or KHz’ had someone saying Kelvin which I thought funny.